Process and system for calcination of high solids kraft paper pulp mill lime mud
The invention features methods and systems for calcining wet calcium carbonate lime mud produced in a re-causticizing manufacturing operation, for instance, Kraft pulp mill lime mud (“lime mud”) and converting it to re-burned lime by (a) feeding wet lime mud obtained from a re-causticizing manufacturing process into a flash dryer and then feeding the dried lime mud into a bubbling fluid bed calciner thermally linked by moving media heat transfer (MMHT) utilizing solid particulate media to a circulating fluid bed combustor wherein the MMHT provides heat input for calcination and drying; (b) recycling the media being from said calciner to said combustor wherein said combustor receives mill WWTP sludge, or precipitated lignins, or biomass, or NCGs as fuels to re-heat the solid particulate media; and (c) recovering calcined “soft-burned” lime mud from the fluid bed calciner. Steam and heated boiler feed-water are also generated and exported to the mill's steam distribution and generation system as well as hot process water for use in the mill's boiler house and manufacturing operation. The system for calcining calcium carbonate lime mud produced from a re-causticizing manufacturing operation and converting it to re-burned lime comprises a calciner and a combustor linked by a moving media heat transfer MMHT system or apparatus. The MMHT system or apparatus thermally links separate fluid bed combustion (exothermic) and calcination (endothermic) stages. The system further comprises a flash dryer or spray dryer that utilizes exhausted heat from the calcination stage.
This invention relates generally to an improved process for calcining or “reburning” calcium carbonate “high solids lime mud” as produced in, for instance, a Kraft or other alkali-based paper pulp manufacturing operation and converting it to high-quality re-burned lime while using only low cost biomass and biomass derived waste water treatment plant (WWTP) sludge, non-condensable waste mill gas (NCG), or low cost solid fuels such as coal, petroleum coke, etc. The biomass and biomass related fuels are also “carbon-neutral” with regards to their global environmental impact.
BACKGROUND OF INVENTIONThe Kraft pulp and paper industry is a major energy consumer, with the majority of that need being met by low cost, carbon-neutral, biomass and biomass related fuels. The conventional lime mud calcination process has, however, not easily been converted to biomass fuels and remains a conspicuous consumer of high cost, greenhouse gas emitting fossil fuels. In the United States, there are 150 Kraft pulp mills with a typical 1000 air dried tons per day (adtpd) bleached pulp mill requiring 320 tpd of calcined lime mud with an annual natural gas and oil consumption of approximately 625 billion Btus. At 2007 energy prices this is approximately US $4.0 million per year, per mill, or $600 million for all US mills.
The benefit of significantly reduced fossil fuel consumption with associated operating cost reduction and replacement of greenhouse gas emitting fossil fuels with carbon-neutral fuels are the key economic drivers for a new Kraft paper pulp mill lime mud calcination process as described with this invention.
In the Kraft paper pulping process, cellulosic wood chips are mixed with aqueous cooking liquor (a.k.a. “white liquor”) composed primarily of sodium hydroxide (NaOH), sodium sulfide (Na2S), sodium carbonate (Na2CO3) and sodium sulfite (Na2SO3). This mixing occurs in a “digester” vessel at a temperature and pressure satisfactory to separate the cellulosic fiber from the natural lignins that bind such fibers.
The liberated fiber is separated from the resultant “black liquor” and is subsequently washed, bleached (or remains unbleached) and is eventually transformed into numerous paper grades.
The separated black liquor contains, aside from the original white liquor chemicals, lignins and other organic matter that previously bound the cellulosic fiber. In order to recover and recycle these costly pulping chemicals, as well as produce valuable pulp mill steam and power from the contained organic lignins, the black liquor is concentrated in multiple-effect evaporators and delivered as a concentrated fuel to a “chemical recovery boiler.”
This recovery boiler combusts the organics under unique oxidizing/reducing conditions to both produce high-pressure steam and a molten inorganic ash (“smelt”) consisting primarily of Na2S and Na2CO3. The co-produced high-pressure steam is subsequently exhausted via a steam turbine/generator to produce mill power and lower pressure mill process steams.
The smelt is drained from the recovery boiler and quenched in water to create “green liquor.” This green liquor is subsequently clarified and filtered to remove insoluble impurities whereupon it is delivered to the “slakers” to initiate conversion of the dissolved Na2CO3 into NaOH required in the white liquor. This slaking process utilizes calcium oxide CaO (a.k.a. reburned lime) to convert Na2CO3 into NaOH via the following two consecutive reactions:
CaO(s)+H2O→Ca(OH)2(s) 1)
Na2CO3(aq)+Ca(OH)2(s)→2NaOH(aq)+CaCO3(s) 2)
The slaker product slurry, consisting of all the chemicals involved in reactions 1 and 2, is fed to subsequent re-causticizers where reaction 2 nearly proceeds to completion with some residual Na2CO3 remaining in the white liquor. The resultant white liquor mix of NaOH, Na2S, Na2CO3, and Na2SO3 is physically separated from the precipitated calcium carbonate (CaCO3) and recycled to the digester to initiate the pulping process.
The precipitated CaCO3 (also known as lime mud) is further water washed and filtered to recover as much white liquor as economically possible before being fed to a calciner which converts the mud into re-burned lime, or “calcine”, (CaO and impurities) for recycle to the slakers. During the washing/filtering process, trace amounts of residual Na2S are air oxidized into more stable sodium thiosulfate (Na2S2O3) to reduce noxious total reduced sulfur (TRS) compounds which can be created in and emitted by the calciner from un-converted Na2S.
The highly endothermic lime mud calcination reaction typically occurs in a rotary kiln, although fluidized bed calciners have also been utilized. Use of an external lime mud flash drying (LMD) process, when combined with the rotary kiln, is the current “state-of-the-art” optimized energy consuming lime mud calcination process.
The first fluidized bed (“FluoSolids”) lime mud calcination process was commercially introduced in 1963. It initially gave significant competition to rotary kilns due to its relatively lower fuel consumption, higher product quality, and compactness. It fell into disuse, however, as rotary kiln/LMD technology re-captured the fuel economy lead and FluoSolids installations experienced operability issues and the inability to economically operate at the high unit capacities required by a “world-class” Kraft pulp mill.
The kiln's primary endothermic (TR=25° C.) calcination reaction is:
CaCO3(s)→CaO(s)+CO2(g), +42.5 Kcal/gm mole (891,764 Kcal/metric ton CaO) 3)
The rotary kiln calcines the mud between 1000° C. (1832° F.) and 1200° C. (2192° F.) and at CO2 partial pressures well below the atmospheric pressure equilibrium concentration for these temperatures. This produces a re-burned lime having the best physiochemical properties suitable for subsequent slaking and efficient re-causticizing.
Due to the high calcination temperatures, and to not contaminate and/or upset the recausticizing process with inorganic impurities, either high-cost oil and/or natural gas fuels are always utilized as rotary kiln fuel. Low-cost solid fuels such as raw biomass (tree trimmings, sawdust, etc.), mill WWTP sludge and coal, etc. are not used due to their contaminating ash content. Wet mill WWTP sludge and raw biomass have the added disadvantage of lower adiabatic flame temperature
Accordingly, while many energy-intensive pulp mill operations have converted to low-cost waste and biomass fuels for steam and power production since the 1970s, the rotary kiln remains a conspicuous consumer of premium liquid and gaseous fuels. While advances have been made to reduce this premium fuel consumption, it still remains between 1.4 (with LMD) and 1.7 million Kcal/metric ton calcine dependent on initial mud moisture content, calciner capacity, fuel type, product lime availability, and installed energy conservation features.
Due to technology limitations, attaining future significant fossil fuel consumption/cost reductions in the rotary kiln/LMD calcination process appears difficult. There is, however, wasted energy within the rotary kiln/LMD calcination process that could be recovered with the proper technical approach. At higher lime mud solids concentration the calciner's exit gas temperature increases. If a counter-current heat transfer process (such as a rotary kiln) were thermally balanced the exit gas temperature would remain constant as fuel input was reduced to compensate for the decreased water input.
Such energy efficiency, however, is not possible with the rotary kiln/LMD calcination process since a very large non-variable fuel amount is required to provide the constant endothermic calcination heat-of-reaction enthalpy (at 25° C.) and also heat reaction products (CaO and CO2) to the calcination temperature. This non-variable fuel input has associated gaseous fuel combustion products from which heat is recovered via counter-current contact with dried lime mud solids in the kiln pre-heat section using densely packed hanging chains as heat transfer surface. In this manner, dried lime mud is pre-heated before it enters the following kiln calcination stage. This reduces fuel consumption.
The reduced temperature gaseous combustion products (and released CO2) leave the kiln pre-heat section and enter the kiln drying section where these gases' enthalpy content evaporates incoming lime mud water content. Older kilns also have chains within the kiln drying section to improve gas-to-water heat transfer. Newer kilns with an LMD do not have drying section chains and are easier to control and operate. As previously stated, as lime mud solids content increases the need for drying enthalpy decreases. The following kiln pre-heat section, however, has insufficient chain heat transfer ability to absorb available energy from the combustion products and CO2 associated with the aforementioned non-variable fuel component and transfer it into the dried solids entering from the drying zone. This unabsorbed combustion products and CO2 enthalpy exits the system as higher LMD outlet gas temperature when increasingly higher solids lime mud is used. Over the last thirty years, improvements in lime mud filtration and washing have increased filter cake solids content from 70% to over 85%, resulting in significant fuel savings and improved white liquor recovery. Unfortunately, the current rotary kiln/LMD technology is limited in the ability to economically respond to this fuel saving opportunity and will become less fuel-efficient as filter cake solids content further increases.
The less utilized fluidized bed calcination process never had a solids pre-heat section, and wastefully dissipated this excess heat via a water spray cooler to control lime mud flash dryer exit temperature. Designs have been proposed to address this dilemma by insertion of a waste heat boiler in place of the spray cooler step, but this approach will likely not be commercialized because of the high surface fouling characteristics of calciner exit gas caused by the presence of low eutectic melting point Na2CO3/Na2SO4 mixtures.
It would, therefore, be beneficial to provide a process whereby fuel combustion products could be separated from gaseous calcination reaction products (CO2) such that the excess heat contained in the combustion products could be viably extracted as process steam without the undesired influence of heat transfer fouling mixtures such as Na2CO3/Na2SO4. This is not possible within the body of a rotary kiln, however the disclosed invention, with separated combustion and calcination stages, addresses this need.
Concurrent with these needed fuel reduction efforts, all mills must control the amount and toxicity of gaseous, liquid, and solid wastes expelled. Many of these emissions have been reduced or eliminated thanks to better manufacturing practices but WWTP sludge (cellulosic, organic, and inorganic matter from waste water treatment) remains a costly disposal problem since it must ultimately be placed in a landfill. As previously discussed, WWTP sludge cannot be used in existing rotary kiln representing a lost opportunity to conserve fossil fuels.
Safe disposal of non-condensable waste mill gas (NCGs), has typically required that they be combusted in the recovery or power boiler, or more likely, the rotary kiln lime mud calciner. While NCG combustion in rotary kilns has been widely practiced there are operability problems (kiln deposit “ringing”, SO2 “blow-through”, etc.) that persist at most mills. Accordingly, there is an increasing trend to install standalone NCG incinerator/boilers that raise steam and scrub sulfurous emissions. These incinerator/boilers, however, are not always available when NCGs are produced so a back-up disposal means is desirable.
Numerous advances have been previously made related to various aspects of lime mud and limestone calcination. U.S. Pat. No. 2,212,446 teaches limestone calcination in a 100% steam atmosphere (a claim of the disclosed invention) using an indirect heated rotary calciner. U.S. Pat. No. 2,700,592 teaches using moving media heat transfer (MMHT) between an endothermic fluidized bed process and an exothermic fluidized bed sulfide ore roasting process. U.S. Pat. No. 2,738,182 teaches fluidized bed calcination of Kraft pulp mill lime mud including recycling finely ground calcine product into a calciner bed to control agglomeration. U.S. Pat. No. 3,961,903 teaches a spray dryer to dry lime mud using multiple hearth calciner off-gases as the drying medium prior to feeding the dried mud to the calciner. U.S. Pat. No. 3,991,172 teaches direct combustion products calcination of fine limestone by passing it through a fluidized bed of a “granular heat carrier medium”. U.S. Pat. No. 4,321,239 teaches using multiple spray dryers to dry lime mud using multiple hearth calciner off-gases as the drying medium prior to feeding the dried mud to a calciner. U.S. Pat. No. 4,389,381 teaches using MMHT by passing fine limestone through an inert heat carrier contained in an endothermic fluidized bed and using a coal fueled exothermic fluidized bed to re-heat the heat carrier. Ash is separated from the re-heated heat carrier prior to calcination. Calcination is accomplished in an air atmosphere of unspecified composition. U.S. Pat. No. 4,606,722 teaches a solid fuel gasified external to a rotary kiln lime mud calciner with the syngas being used as calciner fuel. A vitrified gasifier ash is mixed with calcine and removed in the slaker. U.S. Pat. No. 4,631,025 teaches direct injection of a solid fuel (petroleum coke) into a fluidized bed lime mud calciner. U.S. Pat. No. 4,707,350 teaches calcination of fine limestone in an electrically heated fluid bed calciner fluidized in a 100% CO2 atmosphere with recovered CO2 as the fluidizing gas. U.S. Pat. No. 4,760,650 teaches indirect steam heated drying of lime mud in a steam atmosphere prior to feeding the dried lime mud into a fluid bed calciner. The steam is generated from calciner off-gas. U.S. Pat. No. 5,110,289 uses a separate flash dryer to dry Kraft pulp mill lime mud using rotary calciner off-gases as the drying medium. U.S. Pat. No. 5,230,880 teaches calcination of fine limestone in an electrically heated fluid bed calciner fluidized in an air atmosphere. The fine limestone is passed through a bed of coarser calcined limestone particles that act as a heat transfer media between the fine limestone and the electric heaters. U.S. Pat. No. 5,354,375 describes a lime mud calcination process using a shaft kiln to process pelletized lime mud in a counter-current fashion using direct firing of oil or natural gas fuel. U.S. Pat. No. 5,378,319 describes a lime mud calcination process using an electrically heated microwave belt oven to process lime mud in a counter-current fashion using a counter-current air sweep. U.S. Pat. No. 5,644,996 teaches a technique to cool freeboard gases in a fluidized bed lime mud calciner to below 500° C. (932° F.) to minimize freeboard scaling when the calciner fluid bed is between 875° C. (1607° F.) and 1000° C. (1832° F.). The injected coolant is the entire amount of wet lime mud. U.S. Pat. No. 5,653,948 teaches an indirectly heated fluid bed calciner using electricity or oil/gas firing to calcine very fine limestone particles. The limestone is injected beneath a coarser limestone bed that acts as the heat transfer medium. U.S. Pat. No. 5,711,802, teaches a technique to reduce the LMD inlet gas temperature from a rotary kiln lime mud calciner to between 400° C. (752° F.) and 600° C. (1112° F.); eliminates dryer scaling and reduces kiln dust carry-over. United States Patent Application Publication No. 2006/0039853 teaches a process to separate CO2 from utility boiler stack gases with an “activated” CaO sorbent and then separately re-generating the sorbent and recovering the CO2 in a steam blanketed vacuum calciner.
SUMMARY OF INVENTIONIn a first aspect, the present invention provides a process for calcining calcium carbonate lime mud produced from a mill manufacturing operation and converting it to reburned lime, comprising:
(a) feeding lime mud obtained from the manufacturing operation to a bubbling fluid bed calciner thermally linked by moving media heat transfer (MMHT) having a solid particulate media to a circulating fluid bed combustor wherein the MMHT provides heat input for calcination;
(b) recycling the solid particulate media being from said calciner to said combustor; and
(c) recovering calcined lime mud from the fluid bed calciner.
In some embodiments, the process features further after step a) using calcination gases to dry wet lime mud from the manufacturing process in a flash dryer. In still further embodiments, the process features after step a) recovering excess energy from the process as steam and/or hot water. The dry lime may be obtained from a flash dryer, and the combustor may be provided WWTP sludge, biomass, precipitated lignins or NCGs as fuel.
An air and steam mixture may be provided as a diluent gas in the calciner fluidization atmosphere, and steam there from may catalyze the calcination reaction. A mixture of air, CO2, steam and calcined lime mud may be produced from said calciner. In some embodiments, the method features providing the mixture of air, CO2, and steam and calcined lime mud to a separator to separate coarse calcined lime mud and providing the hot air/CO2/steam mixture and residual fine particle calcined lime mud to a flash dryer. In other embodiments, the method features quenching the hot air, CO2, and steam mixture and residual fine particle calcined lime mud with cooled calcine product to a lower temperature in a second cyclone separator before directing the hot air, CO2, and steam mixture and residual fine particle calcined lime mud to a flash dryer.
In preferred embodiments, the process features additionally providing wet lime mud sufficiently near the fluid bed calciner and flash dryer such that exiting gases from the calciner substantially dry the wet lime mud and the resulting relatively dry lime mud is fed to the calciner.
In yet other embodiments, the method makes use of a calciner and a flash dryer operating in countercurrent gas/solids flow wherein wet lime mud is dried by exiting calciner gases and the resulting dry mud is fed to the calciner. In some embodiments, the fluid bed calciner is a bubbling fluidized bed calciner wherein the bed may comprise reheated solid particulate media returning from the circulating fluid bed combustor. In some embodiments, the method features feeding a makeup media for said moving media to said circulating fluid bed combustor. The makeup media may be alumina, silica, mullite or other solid, inert materials noted for strong thermal cycling and mechanical strength characteristics. In additional embodiments, the method features providing a sorbent limestone to said circulating fluid bed combustor. Such sorbent limestone may be useful to neutralize fuel derived SO2. In yet other embodiments, the combustor may have a second fuel source.
In still further embodiments, the method features generating either low-pressure saturated steam or high pressure superheated steam by heat exchange with hot combustion products. These features may allow generating low pressure saturated steam or high-pressure super heated steam for export to a steam system of a mill. In addition, in some instances, the methods and systems may feature heating air present in the combustor and calciner and internal process service air by heat exchange with combustion products. Further, the methods and systems may include recovering heat from calcine streams to generate hot boiler feedwater or hot process water.
Additionally, the method may include mixing the wet lime mud feed to the flash dryer with H2O2, to convert sodium sulfide contained in said lime mud into Na2SO4. Aqueous solutions of Na2CO3, or Na2SO4 may also be mixed to change the Na2CO3/Na2SO4 ratio within the lime mud to a higher melting point. This may be particularly effective to mitigate calciner scaling and fouling and unwanted gaseous emissions. Still further, the method may include injecting the dry lime mud feed to the calciner at the base of the fluidized media bed of the calciner. This may help to maximize dry lime mud particle residence time thereby ensuring thorough calcination.
In a second aspect, the present invention provides a system for calcining calcium carbonate lime mud produced from a mill manufacturing operation and converting it to re-burned lime comprising a bubbling fluid bed calciner thermally linked by a MMHT to a second circulating fluid bed combustor and wherein the MMHT provides heat input for calcining the lime mud. The system comprises a calciner and a combustor linked by a MMHT system or apparatus. The MMHT system or apparatus thermally links separate fluid bed combustion (exothermic) and calcination (endothermic) stages. In some embodiments, the system further comprises a flash dryer or a spray dryer.
Unless otherwise specified, as used herein, the following terms mean the following:
By “lime mud” is meant a water-wet lime mud produced as a fine precipitated calcium carbonate particle reaction product in a re-causticizing manufacturing step. Often sodium hydroxide is made from the reaction of calcium oxide and sodium carbonate.
By “waste water treatment (WWTP) sludge” is meant a primary fuel such as water-wet sludge as produced in a Kraft paper pulp mill's waste-water treatment plant (WWTP). This sludge contains organic, and inorganic, materials that may be rejected from various pulping and paper making steps. The energy content may be in the form of organic compounds, primarily cellulosic fiber and rejected lignins.
By “biomass” is meant a positive value fuel and may consist of bark, field trimmings, etc. derived from a mill's raw fiber feedstock (trees). The term also includes precipitated lignins.
By “non-condensable gases (NCGs)” is meant a noxious and explosive mix of mercaptans, hydrogen sulfide, and methanol collected from various mill processes, e.g. digestion and black liquor evaporation. NCGs may be combusted in a recovery boiler, or an incinerator/boiler, both to eliminate the nuisance and to recover energy and valuable sulfur.
By “calcination” is meant a high temperature endothermic (heat is added to drive a chemical reaction) industrial thermal process to thermally dissociate inorganic carbonates (i.e., calcium and magnesium carbonates, a.k.a., limestones) and hydroxides (i.e., aluminum and magnesium hydroxides) into the reactive, solid calcium, magnesium, or aluminum oxides and liberated gaseous reaction products, water vapor and/or carbon dioxide. For example, lime mud may be first dried and then dissociated into carbon dioxide and calcium oxide with the latter being recycled to a preceding re-causticizing manufacturing operation. The gaseous, liberated carbon dioxide, is that which originated with the sodium carbonate.
By “calciner” is meant a chamber or apparatus for conducting a calcination reaction. A “calciner” may be fueled with oil, natural gas, or in some cases, coal or biomass, and, dependent on the feedstock's physical state. A “calciner” may be a rotary kiln, flash calciner or bubbling fluid bed calciner as manufactured by FL Smidth, a multiple hearth furnace as manufactured by MHF Services, or a shaft kiln as manufactured by Metso Minerals.
By “bubbling fluid bed calciner” is meant a calciner that uses a solid particulate heat transfer media (e.g., silica, alumina, mullite, etc.) that is suspended (i.e., fluidized) in an upward flowing stream of steam. The upward gas velocity may allow mixing solids with steam and evolved carbon dioxide “bubbles,” but is usually not high enough to transport the media out of the bubbling bed but still allowing the calcine particle to be transported out of the bubbling media bed
By “combustion” is meant an oxidative combustion (i.e., exothermic) reaction to release thermal energy contained in solid fuels (WWTP sludges, biomass, coal, coke, etc.) by mixing the fuels with excess air (the oxygen source). “Combustion” is a widely used high temperature industrial thermal process used to create recoverable energy from fuel combustion products (carbon dioxide, nitrogen, and water vapor). The extracted energy may then be used for a final endothermic purpose, i.e., calcination, heating fluids, drying, raising steam, etc.
By “combustor” is meant a chamber or apparatus that conducts a combustion reaction under controlled conditions and permits the controlled extraction of liberated heat energy for useful process purposes. There are numerous industrial combustors available. The optimal choice is determined by fuel type and the desired end use of the extracted energy, i.e., steam production, hot water production, gases (air, etc.) heating, process heat transfer fluid heating, or solid particulate moving heat transfer media heating.
By “moving media heat transfer (MMHT)” is meant a means or process for transferring exothermic heat generated by solid fuel combustion process in one vessel to an endothermic calcination reaction in another vessel. For example, typical calcination processes utilize in situ (in the same vessel as calcination) combustion. Many of these calcination processes use ash-free, higher cost liquid and gaseous fuels (oil, natural gas) since ashes contained in much lower cost solids fuels would contaminate the calcine. MMHT allows low cost fuel use while avoiding calcine contamination with ash. By using MMHT, a calciner's endothermic reaction heat may be transported to the calciner by a solid particulate media that freely flows back-and-forth between a combustor and a calciner. Many low cost solid fuels are effectively combusted in a circulating fluid bed combustor, and many calcination reactions effectively occur in bubbling fluidized bed reactors. MMHT provides a fluidizable solid particulate heat transfer media that freely flows between separated steps. Thereby, MMHT provides for extracting released heat in a combustor and then transferring that same heat into a calciner. In exemplary MMHT systems, the solid particulate heat transfer media has a larger size particle than the calcine's such that the calcine passes through the fluidized media bed, fully calcined, without cross-contamination, and the calciner step operating temperature is lower than the combustor's such that the contained media heat can be transferred to the calciner and then re-heated in the higher temperature combustor. In other exemplary systems, MMHT is facilitated by steam catalysis which lowers the calcination temperature thereby creating an effective temperature differential between the combustor and calciner. Conversely, MMHT facilitates steam catalysis use by permitting flexible calcination atmosphere control by using a separate combustor and calciner
By “circulating fluid bed combustor” is meant a combustor design featuring a circulating fluid bed combustor comprised of two fluidized media sections. A lower bubbling fluid bed section contains large particle heat transfer media fluidized by incoming fluidization air. Fine particle media and fuel may be injected into this large media bed. The air velocity is normally insufficient to transport large media particles out of this bed, but sufficient to vertically transport fine particle media and fuel ash media out of it into a second section, known as the transport column. Fine media within this latter section absorbs much of the released heat. Some of the fine media contacting the transport section's wall may circulate back to the lower bed. This design approach is very similar to that used for circulating fluid bed boilers as manufactured by Foster Wheeler.
By “flash dryer” is meant a convective dryer that may be fed with filter cakes, sludges, or fibrous materials. The feed may be introduced proximate to a drying gas introduction point such that dried solids and spent drying gases may move in a co-current fashion in a vertical transport column. Such a co-current flow profile may limit the inlet gas temperature dependent on the heat sensitivity of the feed material. Flash dryers include those manufactured by Alstom.
The present invention provides high reactivity “soft-burned” calcined lime product that is required in the pulp mill's chemical recovery circuit and is produced in an efficient energy cost manner by utilizing readily available low quality and low cost fuels.
The present methods and systems utilize two particularly relevant technologies, namely, (1) MMHT and (2) steam catalyzed, controlled atmosphere calcination. The methods of the present invention rely heavily on MMHT to thermally link separate fluid bed combustion (exothermic) and calcination (endothermic) stages. This permits use of low cost mill waste and biomass fuels without contaminating calcine with fuel ash. It also results in high quality reburned lime by not commingling ash with the reburned lime created by the calcination reaction.
By using MMHT, unlike the rotary kiln/LMD process, required enthalpy for the lime mud calcination and drying steps is not generated in situ within the calciner. Instead, a separate circulating fluid bed combustor burns low value, mill (WWTP) sludge with readily available, higher quality biomass (bark, tree trimmings, sawdust, etc.) and noxious NCG fuels to heat circulating inert, solid particulate media. This high temperature media, when separated from ash contaminates, is then transported to the lower operating temperature bubbling fluid bed calciner where it surrenders its stored heat to satisfy the calciner and preceding lime mud dryer's endothermic heat needs. The cooled media exiting the calciner is then returned to the combustor for reheating.
WWTP sludge is not used in a rotary kiln/LMD lime mud calciner given its high ash and moisture content. While ash contamination alone is a major impediment, WWTP sludge's high moisture content precludes creation of the high adiabatic flame temperatures required in a rotary kiln for effective heat transfer and flame stability.
A strong energy balance relationship exists, however, which, when combined with fluid bed combustion and MMHT, justifies using WWTP sludge in a calcination process provided there is a significant operating temperature differential between the combuster and calciner media beds. A typical bleached Kraft pulp mill requires 290 mtpd of lime/1000 mtpd of air-dried (AD) paper pulp. Additionally, a typical mill will landfill approximately 100 mtpd dry basis WWTP sludge per 1000 mtpd of AD pulp. Therefore, the dry basis WWTP sludge to lime mud mass ratio, on an equivalent basis, is 0.345 dry basis mtpd WWTP sludge/mtpd calcine.
A typical, modern rotary kiln/LMD has a high heat value (HHV) fuel oil consumption of 1.4 million Kcal/metric ton calcine. Wet (58% water) mill WWTP sludge has a typical HHV of 2159 Kcal/kg. Therefore, given the aforementioned mass ratio, the total energy available in wet WWTP sludge per ton of calcine is 1.77 million Kcal, or a significant portion of the required net calciner heat load even when considering that heat value lost to evaporating WWTP sludge associated water.
The most common application of MMHT is fluid catalytic cracking in which a liquid refined crude oil feedstock is thermally “cracked” into various useful vapor fractions (naphtha, gasoline) in a fluid bed reactor utilizing a recirculating solid catalyst to both transfer heat and drive the cracking reactions.
The methods described herein also use a steam/air mixture to control calciner CO2 partial pressure while also catalyzing the reaction allowing it to proceed at a lower temperature than that found with just air-based calcination. The physiochemical aspect of catalyzation has not previously been proposed commercially. Lowering the calcination temperature facilitates MMHT use by creating a satisfactory temperature differential between the hot media and the calcination temperature, thereby permitting rapid heat transfer to occur. Conversely, MMHT facilitates the flexible adjustment of steam/air mixtures and temperatures in the calciner by not allowing gaseous combustion products to mix with the calcination atmosphere.
The favorable effect of steam on calcium carbonate's calcination reaction rate has been infrequently researched with the first significant works by Bischoff [“Kinetics of Thermal Dissociation of Dolomite and Limestone in Various Gas Flows”, Zeitschrift für Anorganische Chemie., vol. 262, 1950] and Macintire/Stansel [“Steam Catalysis in Calcination of Dolomite and Limestone Fines”, Industrial and Engineering Chemistry, vol. 45, no. 7, July 1953].
More recently, detailed works by Burnham/Stubblefield/Campbell [“Effects of Gas Environment on Mineral Reactions in Colorado Oil Shale”, Fuel, vol. 59, December 1980], Weisweiler/Hoffman [“Effect of Water Vapour on the Calcination of Limestone in a Fluidized Bed Reactor”, Zement-Kalk-Gips (36, Jahrgang), nr. 10, 1983], Khraisha/Dugwell [“Effect of Water Vapour on the Calcination of Limestone and Raw Meal in a Suspension Reactor”, Transactions of the Institute of Chemical Engineers, vol. 69, part A, January 1991], and Wang/Thompson [“The Effects of Steam and Carbon Dioxide on Calcite Decomposition using Dynamic X-Ray Diffraction”, Chemical Engineering Science, vol. 50, no. 9, 1995] have provided greater insight.
These researchers conclude that using a 100% steam atmosphere or steam/air atmospheres of certain ratios result in two key impacts during limestone calcination; (1), an approximate 50° C. to 60° C. (90° F. to 108° F.) lowering of the equilibrium CO2 dissociation temperature relative to that for 100% air or N2 and (2), a limited catalytic effect, primarily at lower temperatures. This effect diminishes after a maximum steam concentration is attained and/or as temperature increases.
In the calciner, hot media is introduced along with injected dry lime mud. Also, a steam catalyst and sufficient fluidizing air to control calciner temperature and CO2 partial pressure are injected. All these components are thoroughly mixed in the back-mixed, bubbling fluid bed calciner. Reaction products consisting of calcined lime mud particles and the gaseous steam/air/CO2 mixture exiting the fluid bed calciner are cyclone separated before the largely cleaned, hot, steam/air/CO2 mixture reports to the lime mud dryer. Reduced temperature media is gravity discharged from the fluid bed calciner and returned to the combustor to renew the heating cycle.
To inhibit calcine agglomeration and calciner surface scaling due to the presence of low melting point mixtures of Na2CO3 and Na2SO4, a portion of the hot calcine product is recycled to the calciner to create more nucleation sites, thereby reducing the potential for calcine particle agglomeration and calciner surface scaling. To also minimize introduction of low melting point Na2CO3/Na2SO4 mixtures prior to the calciner, hydrogen peroxide (H2O2) is added to wet lime mud (prior to feeding same to the flash or spray dryer) to convert residual sodium sulphide (Na2S) to Na2SO4. This creates a higher melting point Na2CO3/Na2SO4 mixture or small amounts of Na2SO4 or Na2CO3 solutions may be added at the same point.
The calcination and lime mud drying steps in the present invention are process decoupled from the combustion step. This feature of the present invention provides the opportunity for substantial process control within the calciner since the calcination atmosphere can be carefully modeled without the influence of fuel combustion products. The exit temperature and humidity of the lime mud dryer may also be optimized for maximum energy efficiency by balancing its enthalpy needs with the calciner atmosphere's exiting temperature and volume. The calcination temperature can be varied between 760° C. (1400° F.) and 854° C. (1570° F.) by altering the media circulation rate between calciner and combustor. Considering this separation of unit operations, the CO2 partial pressure exiting the calciner can also be varied between 25% and 90%, but typically 85%, of the dissociation equilibrium CO2 partial pressure for calcination within a given steam/air atmosphere. Further, the amount of catalyst steam entering the calciner can be varied between 5% and 20% of the total calciner exiting gas volume due to this separation of unit operations.
Within certain combinations of calcination reaction CO2 partial pressure and temperatures the calcination reaction rate may be significantly depressed. To ensure complete calcination, the dry lime mud is injected into the base of the calciner, beneath the bubbling media bed, yielding a “hindering effect’ of the larger media particle bubbling bed on an upward flowing smaller lime mud particle thereby providing enhanced residence time. The dense media bed acts as a physical barrier to prevent un-calcined mud particles from exiting the calciner too quickly. See Talukdar/Mathur, “Residence Time Studies of Fine Particles Circulating through a Fluidized Bed of Coarse Solids”. Department of Engineering, University of New Hampshire. Presented at AIChE 1995 Annual Meeting.
The total enthalpy of exiting calcination gases may be sufficient to efficiently dry the incoming lime mud in a flash dryer at an inlet gas temperature not exceeding 593° C. (1100° F.) while maintaining a flash dryer exit temperature at no less than 88° C. (190° F.) and no greater than 104° C. (220° F.) dependent on the entering total lime mud solids content, but with the need to maintain a dried particle moisture content at no greater than 2% by weight. This drying gas inlet temperature control is necessary to prevent lime mud agglomeration in the flash dryer, mechanical damage to the flash dryer and is accomplished by quenching calciner hot exit gases with re-cycled, cold calcine product from the calcine cooler and returning re-heated calcine to the cooler.
Fuel ash and combustion products are not mixed with solid and gaseous reaction products in the present methods and systems and unlike the rotary kiln/LMD process, high cost liquid/gaseous fossil fuels are not used. Decoupling also permits the relatively clean hot, gaseous combustion products to transfer sensible heat into combustion air preheating and steam generation using conventional heat exchanger designs without the unwanted heat transfer fouling influence of the Na2CO3/Na2SO4 low melting point mixtures potentially found in the calcination stage.
MMHT allows cost effective equipment design. The large exhaust gas volume from the rotary kiln/LMD lime mud calciner largely derives from fuel combustion products and not the commingled CO2 calcination reaction product. Decoupling fuel combustion from the fluid bed calciner reduces calciner exit gas volume thereby significantly increasing the calcine production rate per unit fluid bed area at the same superficial fluidization velocity. Further, the separated circulating fluid bed combustor is free to operate at a much higher fluidization velocity than the separated bubbling fluid bed calciner. Accordingly, the present methods and systems allow a compact fluid bed calciner with product throughput rates at least equivalent to, and perhaps greater than the largest rotary kiln/LMD systems but without the land usage penalty.
Hydrocarbon cracking and pyrolysis processes typically use MMHT to transfer heat from a solid to a gaseous stream whereas the present methods and systems are designed to transfer heat from one fluidized solid to another. The media used for MMHT is selected for its excellent thermal stability and resistance to mechanical decrepitation. Inert materials such as alumina, silica, and corundum are several examples. The selected media's size distribution and specific gravity are such so as to allow vertical transport (with fine ashes) at superficial gas velocities between 3.1 and 6.1 meters/sec (10 to 20 feet/second) in the combustor while also developing a bubbling, dense fluid bed in the calciner at superficial gas velocities less than 1.5 meters/second (5 feet/second). The small, dried lime mud particles have a transport velocity well beneath 1.5 m/s, allowing them to transit through the bubbling media bed, absorbing heat from the media, and undergoing calcination before exiting with the steam/air/CO2 gas mixture.
MMHT combined with the associated separation of the combustion and calcination processes allows using of variable steam/air mixtures as calciner atmosphere diluent and fluidizing gas instead of fuel combustion products, or just air. Variable steam and dilution controls CO2 partial pressure that impacts calcination reaction rate. Steam also catalyzes the calcination reaction such that it proceeds at a lower temperature than if the fluidization gas was only air. The total volumetric amount of steam/air mixtures also control CO2 partial pressure such that the calcination reaction rate can occur at an acceptable level while resulting in a steam/air/CO2 mixture that provides sufficient enthalpy in the exiting calciner gases to efficiently dry incoming wet lime mud fed to the flash or spray dryer.
The present invention provides an integrated process system having five (5) separate, but interconnected, pyro-processing and heat exchange unit operations; lime mud drying, bubbling fluid bed dry lime mud calcination, bubbling fluid bed indirect calcine cooling, direct media heating within a circulating fluid bed combustor, and combustion products heat recovery and steam generation. For energy economy reasons, the calciner and dryer operate in countercurrent gas/solids flow with wet lime mud being dried by exiting calciner gases and the resultant dry mud then being fed to the calciner.
Hot combustion products 63 exiting fluid bed combustor 15 enter the heat recovery means 78 which is provided with ambient calciner fluidization air 83, ambient combustion air at 101, and re-heated boiler feedwater 96 from calcine cooler 119. Recovered heat is created in the form of superheated high pressure steam 81 exported to the mill, pre-heated combustion air 55, pre-heated calcination fluidization air 88 and internal service air 87 leave the heat recovery means 78.
Calciner 112 receives calciner fluidization gas 114 comprised of hot air 88 from a heat recovery means 78 and catalyst steam 113 supplied from the mill. The calciner off-gas 116 consisting of hot CO2, steam, air and calcine products, proceeds to hot cyclone separator 117. Hot calcine 118 passes to calcine cooler 119 and some residual un-calcined lime mud and calcine is recycled at 150 to calciner 112. Steam, air, CO2 and some residual calcine 120 from the hot cyclone 117 pass directly to a flash dryer 125 but may be first passed to temper cyclone 124 (
Fluid bed calcine cooler 119 is seen to have as indirect cooling inputs boiler feed water 152 from the mill, cool water 156 from the mill, and fluidization air 143. Its outputs include reheated boiler feed water which is fed 96 to heat recovery means 78 and also in part returned 153 to the mill; a hot water return at 158, cooled calcine product at 167 to the mill; and cooled calcine recycle at 121 to the temper cyclone 124. Reheated calcine 126 leaving the temper cyclone 124 and hot calcine 118 from hot cyclone 117 are both returned to the fluid bed calcine cooler 119.
Cooled combustion products 104 from heat recovery means 78 are sent to a bag house filter 105 at which dry ash 106 is separated from combustion products 108 and added to dry ash 71 from combustor 15 and are then subsequently disposed. Cleaned, cool combustion products at 108 are sent to atmosphere. A more detailed diagram is provided in
The primary fuel, wet mill WWTP sludge, is delivered [1] to indirectly steam heated dryer (2) that uses low-pressure, saturated steam [3] provided by the mill. Sweep air stream [4] acts to carry the evaporated water and prevent condensation. Condensed steam [5] is returned to the mill. The dryer exit gas [6] reports to the fabric filter (7) where it is separated into clean gas air/water mixture [8] and captured dryer carryover solids [9].
Dried mill WWTP sludge, [10] is mixed with dryer carryover solids [9] and jointly fed [11] to silo (12). The combined streams [13] are pneumatically injected [14] into fluid bed combustor lower section, (15) via pressurized air [16] provided by blower (17).
Clean dryer exit gas [8] reports to a direct contact water cooler (18) using mill process water [19] as the coolant. Condensed hot water [20] is returned to the mill's process hot water system. Cooled, saturated exhaust air [21] is pressurized and delivered via blower (22) to the fluid bed combustor's secondary combustion air inlet stream, [23].
Under normal operating conditions, there is a close balance between mill WWTP sludge supply and the ultimate calciner heat load. Should there be a heat supply shortage, dried, precipitated lignins from the mill's recovery boiler area can be added directly [24] to silo (12) and jointly injected [14] with dried mill WWTP sludge into fluid bed combustor lower section, (15) via pressurized air [16] provided by blower (17).
Should the mill not have available precipitated lignins, fuel silos (25) and (26), provide increasingly higher cost fuels. Silo (25) stores wet biomass fuel, the preferred secondary fuel due to its low cost and availability in Kraft pulp paper mills. The biomass is delivered via [27] to a chipper/shredder (28) after which it [29] is mechanically conveyed [30] to the fluid bed combustor lower section (15) via screw feeder (31) or other appropriate feeding device.
A silo (26) stores costlier fossil fuels such as high sulfur coal, petroleum coke, shredded tires, waste coal, etc. should biomass be unavailable in sufficient quantities to satisfy the system's total energy requirements. They are delivered [32] to a pulverizer (33), pulverized into [34] and then pneumatically conveyed [35] into the lower fluid bed combustor section [15] using transport air [16] provided by blower (17). The fuel is pulverized in (33) so as to insure rapid combustion and subsequent through ash separation from hot heat transfer media.
The silo (36) contains makeup media to replace that destroyed by cyclic handling when utilizing MMHT. The media can be alumina, silica, mullite or other solid, inert materials noted for strong thermal cycling and mechanical strength characteristics. It is gravity delivered via [37] to the cooled media return “J” valve, (38). Other fuels injected into the fluid bed combustor lower section (15) are all collected Kraft mill NCGs and fuel oil or natural gas, [39]. The NCGs provide a noticeable heat input and can be safely disposed of in an environmentally sound manner during normal NCG incinerator downtime periods. Oil or natural gas is used for rapid temperature trimming, and startup.
To neutralize emitted SO2 from solid fuel, NCG, and fuel oil combustion, sorbent limestone is added to the lower fluid bed combustor lower section, (15) at a molar Ca/S ratio between 1.0 and 2.5. This technique is well known to those familiar with fluid bed combustor design. Market quality limestone is stored in silo (40) and conveyed [41] to pulverizer (42) prior to pneumatic injection [44] into lower fluid bed combustor bed section (15) using transport air [16] provided by blower (17). The limestone is pulverized so as to ensure rapid SO2 sorption and subsequent complete separation from hot heat transfer media.
Lastly, separated combustor ash [45] from storage silo (46) is pneumatically conveyed [47] into the fluid bed combustor lower section, (15) by transport air [16] provided by blower (17). This ash may be recycled to ensure complete fuel carbon content combustion. This technique is well known to those familiar with fluid bed combustor design.
Fuel Combustion and Media HeatingWWTP sludge and precipitated lignins [14], other fuels [30, 35, and 39], limestone [44] and ash (47) are injected into the fluid bed combustor lower section (15) of a refractory-lined circulating fluid bed combustor which may be comprised of eight sections; (15), (48), (49), (50), (51), (52), (53), and (54).
Pressurized combustion air, between 1.14 and 1.36 bar (16.5 to 19.7 psia) and pre-heated to between 149° C. and 204° C. (300° F. to 400° F.) is introduced via stream [55] into the cylindrical or rectangular combustor plenum, (54). The amount of air introduced is less than the stoichiometric amount required for full combustion of all fuels entering cylindrical or rectangular combustor section (15) so as to ensure reducing conditions within this combustor section. A gas distribution grid (nozzle or orifice plate) mechanically separates combustor sections (15) and (54). All these techniques are well known to those familiar with fluid bed combustor design. The fluid bed combustor lower section (15) is a dense bubbling bed made primarily of large media particles. These large media particles are sized to not elutriate when the cross-sectional gas combustion product gas velocity within the fluid bed combustor lower section (15) is 6.1 meters per second (20 feet per second). Introduced fuels are gasified and partially combusted in this sub-stoichiometric combustion section, their released heat being absorbed by 788° C. to 882° C. (1450° F. to 1620° F.) cooled media (56) comprised of that from the calciner via stream [57], combustor bed return (58), and makeup via stream (37).
Returned cool media [56] enters fluid bed combustor lower section (15) via a “J” valve (38, a.k.a. loop-seal) fluidized with preheated air [60] at 1.57 bar (22.7 psia) and 538° C. (1000° F.).
As large clinkered ash particles increase in volume in fluid bed combustor lower section (15), they, and some media are gravity discharged [58] via a high-temperature “cone” valve (61) and are externally separated with large media being returned to fluid bed combustor lower section (15) via cooled media stream [56]. Such cone valve designs are well known to those familiar with fluid bed combustor and calciner design techniques.
Pressurized secondary combustion air [62], between 1.15 and 1.22 bar (16.7 to 17.7 psia) and pre-heated to between 149° C. and 204° C. (300° F. to 400° F.) is provided to complete fuel burnout and circulating media heating in cylindrical or rectangular combustor section (48).
This secondary combustion air introduction technique is widely used with circulating fluid bed boilers. Total excess oxygen exiting fluid bed combustor transport section (48) will be between 10% and 35% above that required for stoichiometric combustion and is dependent on a given fuel's properties and combustion characteristics. The combustor equilibrium temperature for ash, media, and gas under these circumstances, is between 843° C. and 927° C. (1550° F. and 1700° F.).
Due to the high superficial gas velocity of combustion products in fluid bed combustor transport section (48), ash particles and circulating media are vertically transported together into cylindrical combustor freeboard section (49) in excess of the media's transport velocity. This velocity will not exceed 6.1 mps (20 fps).
Combustor freeboard section (49) is a cylindrical expanded upper chamber that acts to disengage reheated circulating media at between 843° C. and 927° C. (1550° F. to 1700° F.) from ash. Its cross-sectional area is such that the gaseous combustion products exiting fluid bed combustor transport section (48) are rapidly expanded to a lower velocity. This lower gas velocity is less than the circulating media particles' vertical transport velocity of 6.1 mps (20 fps) but much greater than the fine ash particles' vertical transport velocity.
In this manner, entrained ash exits combustor freeboard section (49) via stream [63] with fuel combustion products while reheated, largely ash-free media drops by gravity into combustor storage section (50) that is an integral hot media storage hopper. The volume of combustor storage section (50) is such that it can store hot media when the calcination step requires only 25% of the combustor's heat release capacity.
Depending on calciner enthalpy needs, the reheated circulating media is withdrawn at an appropriate controlled rate from combustor storage section (50) via multiple discharge ports, the flow through each discharge port is being externally controlled by multiple high-temperature cone valves (51). The number of discharge ports and valves will be between 4 (four) and 24 (twenty four), the exact amount a function of calcine production capacity and related fluid bed cross-sectional area.
Reheated circulating media discharged from combustor storage section (50) via multiple cone valves (51) may contain some entrained fine ash. This ash, if returned to the calciner with reheated circulating media, may eventually contaminate the Kraft mill's recausticizing circuit. Therefore, the reheated media/ash mix will first enter a “cone cap and slope” stripper (52) where the ash contaminated media flows downward, by gravity, over a series of cone caps and slopes (see detail “A” on the process flow diagram). Air (64) at 1.57 bar (22.7 psia) and 538° C. (1000° F.) flows upward through the stripper, separating the ash from the downward flowing circulating media. Such stripper designs are well known to those familiar with oil refinery fluid catalytic cracking design techniques.
The ash/air mixture [65] is vented into the combustor freeboard section, (49). Cleaned hot circulating media [66] at 843° C. to 927° C. (1550° F. to 1700° F. is gravity discharged from the stripper (52) and reports to the fluid bed calcination section [112] via injectors [53] properly prepared to provide the calcination/drying steps net endothermic enthalpy need.
Ash and un-sulfated limestone particles enter hot cyclone (67) via [63] where most of the incoming solids are separated from hot gaseous combustion products at 843° C. to 927° C. (1550° F. to 1700° F.). The cyclone solids underflow [68] enters silo (46) where it is split via high-temperature valves via two streams. One stream [45] is the previously mentioned ash recycle flow and the second [69] enters a small water-cooled rotating disc cooler (70). Cooled solids [71] at less than 93° C. (200° F.) then exit the system at this point.
The combustor is pre-heated on initial start-up by ambient temperature primary combustion air [72] heated to no greater than 816° C. (1500° F.) via oil or natural gas [39] in a direct-fired heater (73). When the fluidized media bed in fluid bed combustor lower section (15) reaches a suitable temperature, premium fuel (oil, gas, or coal) is injected [39] directly into the bed to elevate its temperature to that needed for autogenic fuel combustion. Premium fuel use [39] is then gradually disengaged as mill WWTP sludge, NCG, biomass, and/or fossil fuel feed commence.
Combustion Products Heat Recovery and Steam GenerationSolids and hot combustion products exiting the combustor are separated in a cyclone with ash exiting the system via a conventional rotary cooler. Largely cleaned, hot combustion products then enter multiple convective heat exchangers that, in counter-current series; generates high pressure superheated steam for export to the mill's steam loop; preheated calciner fluidization air; preheated internal process service air; and preheated primary and secondary combustion airs. Cooled combustion products exit to ambient via conventional gas clean-up devices.
Hot, ash laden combustion products [63] exiting combustor freeboard section (49) and entering cyclone (67) may be mixed with ammonia or urea [74] to reduce nitrogen oxide emissions with selective non-catalytic removal (SNCR) technology well known to those familiar with fluidized bed boiler design. Should the nitrogen oxide content exiting (49) be less than that required by law, then this step will not be necessary.
Hot, largely ash-free combustion products exiting cyclone (67) via [75] may be mixed with natural gas or oil [39] in incinerator (76) to increase the temperature to 982° C. (1800° F.) at a sufficient residence time in incinerator (76) such that any chlorinated organics (dioxins) can be destroyed. Should the dioxin content exiting combustor freeboard section (49) be less than that required by law, then this step will not be necessary.
Hot gases [77] exiting (76) enter gas/gas heat exchanger section (78) to superheat saturated steam [79] exiting steam drum (80) at between 125.2 bar (1815 psia) and 42.4 bar (615 psia). The super-heated steam temperature will be between 538° C. (1000° F.) and 399° C. (750° F.) when exported to the mill via stream [81]. The final superheated conditions are compatible with the mill's main steam loop.
Hot combustion products exiting gas/gas heat exchanger section (78) enter gas/gas heat exchanger section (82) to heat internal process service air to 538° C. (1000° F.). Ambient process service air is delivered to (82) via [83] by blower (84), or multiple blowers, at pressures greater than between 1.36 bar (19.7 psia) and 1.57 bar (22.7 psia), that higher pressure satisfactory to overcome system pressure drops prior to the specific end-use point. The exiting hot air is directed into five (5) flows; via [60] to the combustor cool media injection valve, (38); via [64] to the combustor hot media stripper (52); via [85] to the calciner cool media return stripper, (86); via [87] to the combustor hot media injection valve, (53); and via [88] to the fluid bed calciner inlet air plenum, (89).
Cooled combustion products exit gas/gas heat exchanger section (82) via [90] and enter forced-circulation boiler economizer/evaporator section (91). The steam/water mix [92] generated within economizer/evaporator section (91) tubular heat exchange surfaces enters steam drum (80) where saturated steam between 125.2 bar (1815 psia) and 42.4 bar (615 psia) is flashed and exits via stream [79] and reports to gas/gas heat exchanger section (78). Steam drum (80) saturated liquid underflow (93) is extracted and boosted to evaporation pressure by boiler circulation pump (94) being delivered via [95] to economizer/evaporator section (91). Pre-heated boiler feed-water makeup, originally from the mill's boiler-house, enters steam drum (80) via [96] having been pre-heated in second fluid bed cooler section (97).
Still further cooled combustion products leaving economizer/evaporator section (91) via [98] are split into two streams, one entering primary combustion air pre-heater section (99) and the second entering secondary combustion air pre-heater section (100).
Primary air pre-heater section (99) pre-heats primary combustion air delivered via [101] by primary combustion air blower (102) at between 1.15 bar and 1.36 bar (16.7 psia to 19.7 psia). The primary combustion air exiting primary combustion air pre-heater section (99) via [55] is heated to between 149° C. and 204° C. (300° F. to 400° F.) and then reports to combustor fluidizing air plenum (54).
Secondary air pre-heater section (100) pre-heats secondary combustion air delivered via [23] by secondary combustion air blower (103) at between 1.15 bar and 1.22 bar (16.7 psia to 17.7 psia). The secondary combustion air exiting (100) via [62] is heated to between 149° C. and 204° C. (300° F. to 400° F.) and then reports to fluid bed combustor transport section (48).
Finally, completely cooled combustion products exiting primary air pre-heater section (99) and secondary air pre-heater section (100) via [104] enter a fabric filter bag-house (105) where residual fine ash is separated from combustion products. Ash [106] exits the system, and particulate-free combustion products [108] are discharged to atmosphere via combustor exhaust fan (107).
Calcination of Dried Lime MudThe dried lime mud is calcined in a cylindrical air/steam fluidized “bubbling fluid bed” (BFB) calciner using inert hot media entering the calciner at a higher temperature than the calciner's operating bed temperature. Entering hot media releases its stored energy as the endothermic heat load required to calcine dried lime mud to “re-burned” lime product at the proper reaction conditions. Calciner fluidization gas composed of air and steam catalyst enters at a controlled amount to ensure that the gaseous CO2-air/steam reaction atmosphere is continually maintained at a CO2 partial pressure adequate to drive the calcination reaction. The steam acts as a catalyst to drive the calcination at a lower temperature than if the same reaction was conducted in a 100% air atmosphere. The CO2-air/steam reaction atmosphere exiting the calciner is directed to a flash dryer to evaporate water associated with lime mud provided by the Kraft pulp mill and has the proper enthalpy requirement to dry this wet incoming lime mud at the lowest possible dryer outlet temperature and maximum relative water saturation.
Gravity delivered to the fluid bed calciner feed injectors (53) is hot, stripped media [66] at a rate dependent on the calciner's and dryer's endothermic heat load requirement. The hot media is motivated through the injectors by pressurized hot air [87] delivered at 538° C. (1000° F.) and greater than 1.57 bar (22.7 psia). The injector is preferably an “L” valve design, but may also be of the “J” type. Such valve designs are well known to those familiar with fluidized bed design techniques.
Dried lime mud is transported [109] by a pressurized air/steam mixture to the calciner media injector, (53) at a point downstream of the hot air/hot media mixing point. Blower (110) pressurizes ambient air to ensure that the injection pressure into the cylindrical calciner bed is not less than 1.57 bar (22.7 psia). Mill steam at 2.07 bar (30 psia) is injected into [109] via [111].
The resulting air/steam/hot media/dry lime mud mixture in calciner media injector [53] is injected at a pressure not less than 1.57 bar (22.7 psia) into the base of cylindrical fluid bed calciner section (112) at a point directly above the calciner's circular gas distribution plate. The total number of injectors will vary from between four (4) and twenty-four (24) dependent on calcine production capacity and calciner distribution plate cross-sectional area. The steam/air ratio in the calciner media injector (53) internal path corresponds to a CO2 partial pressure no greater than 90% of the CO2 equilibrium partial pressure for the CaCO3 calcination reaction to proceed at the injector's maximum internal pressure and minimum temperature.
Dried lime mud [109] may also be injected via an air/steam mixture directly into the calciner bed, thereby bypassing calciner media injector (53) should it be necessary to avoid excessive scaling in the injectors.
Calciner fluidization air at 538° C. (1000° F.) is delivered [88] from heat exchanger (82). Prior to entering the calciner, catalyst steam [113] provided as mill saturated steam at 2.07 bar (30 psia), is injected into air stream [88] at an amount equal to between 2% and 20% of the total air/steam/CO2 mixture exiting the calciner freeboard section (115) by stream [116]. Further, the air/steam fluidization gas mixture [114] volumetric amount entering calciner air plenum (89) will be controlled so as to ensure the CO2 partial pressure in fluid bed calcination section (112) is no greater than 90% of the CO2 equilibrium partial pressure at fluid bed calcination section (112)'s maximum fluidized bed pressure and minimum fluidized bed temperature when also considering injected air amounts contained in calciner input streams [87], [109] and [150]. The air/steam fluidization gas mixture [114] is injected into the calciner plenum section (89) at a pressure adequate to fluidize fluid bed calcination section (112) at a fluidized bed height of no greater than 2.44 meters (8.0 feet). Lastly, the total enthalpy, when considering temperature and mass amounts, contained in all gases exiting in stream [116] will be sufficient to satisfy the lime mud drying step's requirement.
Fluid bed calcination section (112) is fluidized at the distributor plate top by the air/steam mixture [89] at a velocity greater than the circulating media's incipient fluidization velocity but less than its maximum transport velocity of 6.1 mps (20 fps) and always greater than the smaller calcined lime mud particle's transport velocity.
Fluid bed calcination section (112) expands in cylindrical cross-sectional area as CO2 is liberated by the calcination reaction and stops expanding at calciner freeboard section's (115) entry point. This increasing cross-sectional area ensures that the CO2/air/steam gas mixture exits fluid bed calcination section (112) and enters calciner freeboard section (15) at a velocity greater than the media's incipient fluidization velocity but less than its minimum transport velocity but always greater than the smaller calcined lime mud particle's transport velocity. This ensures that elutriated calcined lime mud is transported into calciner freeboard section (115) and media is disengaged from the calcine, falls back, and remains in fluid bed calcination section (112). Cooled media exits the calciner through a gravity discharge overflow port located at the interface of fluid bed calcination section (112) and calciner freeboard section (115), i.e., the top surface of the calciner bubbling fluid bed. Media discharged from fluid bed calcination section (112) may contain some entrained calcine. This calcine, if returned to the combustor with the media will create an economic loss. Therefore, the media/calcine mix enters a “cone cap and slope” stripper (86) where the media/calcine mix flows downward, by gravity, over a series of cone caps and slopes (see detail “A” on the process flow diagram). Hot air [85] at 538° C. (1000° F.) and less than 1.57 bar (22.7 psia) flows upward through the stripper, separating calcine from the downward flowing media. Such stripper designs are well known to those familiar with oil refinery fluid catalytic cracking design techniques. The stripped calcine and associated air enter calciner freeboard section (115).
Cool, stripped media exits “cone cap and slope” stripper (86) via discharge stream [57] and is combined with make-up media [37] and recovered media [58] with all reporting to the media return valve (38). Valve (38) returns media to fluid bed combustor lower section (15) as previously explained. Valve (38) is preferably a “J” type valve but may also be an “L” type valve. Such valve designs are well known to those familiar with fluidized bed design techniques.
The number of strippers (86) and valves (38) are not less than two (2) to ensure proper distribution of returned media into the fluid bed combustor lower section (15).
Calcined lime mud particles elutriated into the calciner freeboard section (115) represent the total calcine production rate. This calcine, along with the exiting CO2/air/steam gas mixture, exits calciner freeboard section (115) via stream [116] prior to entering hot cyclone (117). The calcined lime mud particle temperature in both fluid bed calcination section (112) and calciner freeboard section (115) will be not less than 760° C. (1400° F.) and not greater than 854° C. (1570° F.) and typically between 791° C. (1455° F.) and 800° C. (1472° F.). Hot media entering the fluid bed calcination section (112) via injectors (53) will be not less than 832° C. (1530° F.) and not greater than 927° C. (1700° F.). Cooled media entering media stripper (86) will be no less than 28° C. (50° F.) higher than the calcined lime mud particle entering calciner freeboard section (115). This will ensure rapid heat transfer between the incoming hot media and the incoming dried lime mud.
The entire cylindrical calciner vessel represented by sections (89), (112), and (115) will surround combustor cylindrical, or rectangular, section (48). The combustor storage section (50) will share a common floor/roof with calciner freeboard section (115). In this manner the calciner and combustor are integrated into a compact, vertical design to minimize land area requirements. All interior surfaces of the calciner and combustor will be refractory lined for abrasion resistance and thermal insulation purposes. Such designs are well known to those familiar with fluidized bed design techniques.
Lime Mud DryingThe hot CO2/air/steam and elutriated calcine product mixture exiting the calciner via [116] is largely cleaned of calcine in hot cyclone (117). Separated larger particle calcine, representing most of the calcine production rate, exits hot cyclone (117) via [118] where it enters the first first fluid bed cooler section, (119).
Exiting hot cyclone (117) via [120] is a hot CO2/air/steam and residual fine particle calcine mixture at 760° C. (1400° F.) to 854° C. (1570° F.). Intercepting stream [120] is stream [121] that is a pressurized dense phase mixture of ambient air and cooled calcine having first been discharged from fluid bed calcine cooler section (122) via [164] at approximately 93° C. to 121° C. (200° F. to 250° F.) with blower (166) providing injection/transport air via stream [165]. This mixing causes stream [120] to be tempered (“quenched”) to no greater than 593° C. (1100° F.) in the tempering cyclone, (124). Lowering the temperature ensures that flash dryer (125) operates without mechanical problems or excessive entering wet lime mud agglomeration.
Separated and re-heated calcine at a temperature somewhat less than 593° C. (1100° F.) exits tempering cyclone (124) via [126] where it enters the first fluid bed cooler section, (119).
The cooled CO2/air/steam and fine particle calcine mixture exits the tempering cyclone (124) at no greater than 593° C. (1100° F.) via stream [127]. Intercepting stream [127] is emergency quench air stream [128] and, if needed, emergency quench water stream [129] to ensure that the hot gases and residual fine particle calcine entering flash dryer (125) do not exceed 593° C. (1100° F.).
Prepared lime mud [130] exiting the pug-mill feeder (131) also enters flash dryer (125) at a point above the stream [127] entry point. This lime mud is dispersed into gas stream [127] by appropriate means to ensure that rapid water evaporation in flash dryer (125) occurs at a temperature no less than 88° C. (190° F.) and no greater than 104° C. (220° F.) dependent on the entering gas temperature, total lime mud solids content, and the need to maintain a dried particle moisture content at no greater than 2% by weight. At these conditions most, if not all, of the NaOH present in the entering lime mud will be converted to Na2CO3 due to the high CO2 content in flash dryer (125).
A very small portion of the CaO contained in the calcine entering flash dryer (125) via [127] is re-hydrated and re-carbonated to Ca(OH)2 and CaCO3 due to the CO2 and water vapor present in the dryer and is then commingled with dried lime mud which eventually reports to the calciner where it is re-calcined.
The dried lime mud and cooled CO2/air/water vapor mixture, exiting flash dryer (125) via exit duct [132] report to dryer cyclone (133). Dryer cyclone (133) separates entering larger dried lime mud particles that then exit the cyclone base via a rotary valve and duct [134] and then report to the dried lime mud storage silo, (135).
Gases exiting dryer cyclone (133), which contain the residual finest lime mud particles, enter a fabric bag-house filter, or electrostatic precipitator (136) via exit duct.
The separated dried lime mud particles exit (136) via a rotary valve and duct and then report to the dried lime mud storage silo, (135).
The collected dried lime mud in dried lime mud storage silo (135) is discharged from its conical silo base through a multiplicity of discharge valves (between 4 and 24 in number) but always a number equivalent to the number of calciner hot media/dried lime mud injectors, (53). A Blower (110) receives ambient air [138] and boosts it to a pressure satisfactory to transport the dried lime mud exiting from silo (135) to injectors (53) to ensure an injector discharge pressure entering the calciner (112) is no less than 1.57 bar (22.7 psia). Mill saturated steam at 2.07 bar (30 psia) is injected into [109] via [111] to obtain the proper steam catalyst proportion to that entering the calciner (112).
Dried lime mud free gases exiting bag-house filter or ESP (136) are extracted by an exhaust gas fan (138) that delivers exhaust this CO2/air/water vapor mixture gas to ambient atmosphere.
Lime Mud PreparationLime mud feed for the flash dryer (125) is prepared in a pug mill feeder (131) when lime mud filter cake [139] at up to 90% total solids content is mixed with a sufficient amount of liquid hydrogen peroxide (H2O2) [140] to fully oxidize trace, residual amounts of Na2S in the lime mud into sodium sulfate (Na2SO4). This conversion prevents the generation of reduced sulfur (TRS) compounds in the flash dryer or calciner and helps to mitigate scaling and fouling in the calciner by raising the Na2SO4/Na2CO3 mass ratio into a higher melting point region.
A small amount of sodium sulphate (Na2SO4) or sodium carbonate (Na2CO3) solution [141] may also be added to mitigate scaling and fouling in the calciner by raising the Na2SO4/Na2CO3 mass ratio into a higher melting point region. The total lime mud solids content exiting pug mill feeder (131) via stream [130] will be slightly reduced from the maximum 90% total solids present in lime mud filter cake [139] with the addition of liquid H2O2 and Na2SO4/Na2CO3 solution [141]. This exiting lime mud solids content in stream [130] should be maintained at no less than 85% to ensure proper feeding and dispersion into the flash dryer (125) and is accomplished by adding dried lime mud [142] to pug mill feeder (131) which has been re-cycled from dried lime mud storage silo (135) via mechanical means.
The lime mud feed rate to flash dryer (125) from pug mill feeder (131) is controlled to maintain a constant dried lime mud product moisture content considering the interrelationship of actual flash dryer inlet drying gas composition and temperature. Surge dried solids are stored in dried lime mud silo (135). This control technique is well known to those familiar with the practice of industrial flash drying.
Calcine Cooling and PelletizingHot calcine [118] discharged from cyclone (117) and reheated calcine [126] discharged from the tempering cyclone (124) report to rectangular first fluid bed cooler section (119) via dip-leg seals. First fluid bed cooler section (119) is fluidized with pressurized air [143] entering first fluid bed cooler plenum section (144) provided by cooler fluidization air blower (145). First fluid bed cooler section (119) fluidized bed is a “back-mix” fluidized bed design to ensure uniform mixing of fluidization air and entering hot calcine. Mixed, hot calcine exits the section (119) by flowing over a full bed width divider wall and enters second fluid bed cooler section (97).
A portion of hot calcine exits first fluid bed cooler section (119) via an overflow weir into a discharge port [147] at a controlled rate via a cone valve. The amount of calcine entering [147] is between 0% and 20% of the total calcine production rate. Calcine in [147] is transported by pressurized air [148] provided by blower (149) to the calciner via [150] where it is injected at a pressure no less than 1.57 bar (22.7 psia). Calcine recycling ensures complete lime mud calcination while providing extra nucleation sites to mitigate sodium salt fouling affects in the calciner fluid bed. Calcine entering the second fluid bed cooler section (97) is fluidized by ambient air [143] delivered by a blower (145) through second fluid bed cooler plenum section (151). The fluidized calcine flows down the rectangular bed around immersed tube bundles within the fluidized bed. Flowing through this tube bundle, counter-flow to the solids flow, is pressurized mill boiler feed-water entering via [152]. Boiler feed-water, now heated to a higher temperature, exits via stream [96] and reports to the inlet of the steam drum (80). Excess heated boiler feed-water also exits via [153] and reports back to the pulp mill's boiler-house. Cooled calcine exits second fluid bed cooler section (97) by flowing over a full bed width divider wall before entering third fluid bed cooler section (154).
Calcine entering third fluid bed cooler section (154) is fluidized by ambient air [143] delivered by a blower (145) through third fluid bed cooler section plenum (155). The fluidized calcine flows along the rectangular fluid bed path around immersed tube bundles within the fluidized bed. Fresh mill water [156] flows through this tube bundle, counter-flow to the solids flow, boosted in pressure by a pump (157). Hot water exits via stream [158] and reports to the mill hot water system or the re-causticizing circuit for dilution and washing purposes.
Instead of counter-flow pipe bundles, cross-flow plates or pipe bundles may also be utilized in fluid bed cooler sections (97) and (154), or another indirect heat transfer device known to those familiar with this type fluid bed cooler.
The gases in the fluid bed cooler freeboard are withdrawn by exhaust fan (146) via [159]. This exhaust stream reports to a fabric filter bag-house (160) and then exhausts to ambient via stream (161). Collected fine particulate calcine [163] reports with the bulk cooler calcine flow [123] to the inlet of the pelletizer (162). The cooled calcine at approximately 93° C. to 121° C. (200° F. to 250° F.) exits the third fluid bed cooler section (154) via an overflow weir (122) with the rate controlled by a rotary valve. The product calcine is highly reactive since the low-temperature calcination process creates a “soft-burned” product. Its fine particle size also makes it somewhat dusty. Therefore the calcine [123] is pelletized in pelletizer (162) to mitigate potential handling and safety problems before being transported via [167] to the mill re-causticizing circuit's slakers. A portion of the cooled calcine [123] is extracted at a controlled rate [164] by a rotary valve and is pneumatically transported via air [121] to the tempering cyclone (124)) inlet gas line [120]. The required air [165] is boosted in pressure by blower (166).
While the present invention has been exemplified by specific embodiments, it will be understood in view of the present disclosure that numerous variations upon the invention are now enabled to those skilled in the art. Accordingly, the invention is to be broadly construed and limited only by the scope and spirit of the present disclosure.
Claims
1. A method for calcining calcium carbonate lime mud from a manufacturing operation and converting it to reburned lime, comprising:
- (a) feeding lime mud to a fluid bed calciner wherein the fluid bed calciner is thermally linked by moving media heat transfer (MMHT) to a circulating fluid bed combustor by a heat transfer media wherein said media moves between said calciner and said combustor wherein the MMHT provides heat input for calcination;
- (b) recycling the heat transfer media from said calciner to said combustor wherein said combustor receives one or more fuels selected from group consisting of waste water treatment (WWTP) sludge, or biomass, or precipitated lignins and NCGs; and
- (c) recovering calcined lime mud from the fluid bed calciner.
2. A method in accordance with claim 1, wherein an air and steam mixture is provided to the fluid bed calciner as fluidization and diluent gas.
3. A method in accordance with claim 2, wherein a mixture of air, CO2, steam and calcined lime mud are produced from said calciner.
4. A method in accordance with claim 3, further comprising providing the mixture of air, CO2, and steam and calcined lime mud to a separator to separate coarse calcined lime mud and providing the mixture of hot air, CO2, and steam residual fine particle calcined lime mud to a flash dryer.
5. A method in accordance with claim 1, further comprising before (a) providing wet lime mud sufficiently near the fluid bed calciner and flash dryer such that the calciner and flash dryer operate in counter/current gas/solids flow where exiting calciner gases substantially dry the wet lime mud and the resulting dry lime mud is fed to the calciner.
6. A method in accordance with claim 4 further comprising quenching the hot air, CO2, and steam mixture and residual fine particle calcined lime mud with cooled calcine product to a lower temperature in a second cyclone separator before directing the hot air, CO2, and steam mixture and residual fine particle calcined lime mud to a flash dryer.
7. A method in accordance with claim 4, wherein the calciner and flash dryer operate in countercurrent gas/solids flow wherein wet lime mud is dried by exiting calciner gases and the resulting dry mud is fed to the calciner.
8. A method in accordance with claim 1, wherein the fluid bed calciner is a bubbling fluidized bed calciner and wherein the fluidized bed contains reheated media particles returning from the combustor.
9. A method in accordance with claim 1, further comprising providing a makeup media for said moving media to said circulating fluid bed combustor.
10. A method in accordance with claim 1, further comprising providing a sorbent limestone to said circulating fluid bed combustor.
11. A method in accordance with claim 1, further comprising providing a fossil fuel selected from a group consisting of coal, petroleum coke, waste coal, and shredded tires to said combustor.
12. A method in accordance with claim 1, further comprising generating steam by heat exchange with hot combustion products.
13. A method in accordance with claim 1 further comprising preheating air present in the combustor and calciner by heat exchange with combustion products.
14. A method in accordance with claim 1 further comprising recovering heat from the calcine.
15. A method in accordance with claim 6 further comprising mixing the wet lime mud feed with one or more selected from the group consisting of H2O2, Na2CO3, and Na2SO4.
16. A method in accordance with claim 1 further comprising injecting the dry lime mud to the fluid bed calciner at the base of the fluidized media bed of the fluid bed calciner.
17. A system for calcining calcium carbonate lime mud produced from a manufacturing operation and converting it to re-burned lime comprising a bubbling fluid bed calciner, a circulating fluid bed combustor and an apparatus or system adapted to thermally link the fluid bed calciner and the circulating fluid bed combustor.
18. A system according to claim 17 further comprising a dryer selected from the group consisting of a spray dryer and a flash dryer.
Type: Application
Filed: Aug 24, 2007
Publication Date: May 7, 2009
Inventor: Robert A. Rossi (North Bergen, NJ)
Application Number: 11/895,403
International Classification: D21C 11/06 (20060101); D21F 1/66 (20060101);
