METHOD AND APPARATUS FOR THE IMPROVED COMBUSTION OF BIOMASS FUELS

Abstract

A cylindrical furnace having a vertical axis controls combustion. Solid fuel, particulates, and gases inside the furnace rotate around the axis, inducing radial stratification using centrifugal forces. Fuel and particulates drag on the wall of the cylinder, slipping in and out of suspension, thereby increasing particle residence times. The solid particles comprise combustible fuel particles, and non-combustible ash and contaminants. Control of the temperature of non-combustible particles and the wall surface prevents these non-combustible particles from adhering to, and building up on, the furnace wall. It is also advantageous to control the gas temperature leaving the furnace to minimize temperature-driven corrosion of downstream heat-exchange surfaces. Method and apparatuses are described to control the gas, non-combustible particle, and wall temperatures. The furnace can be integrated into a stand-alone boiler or as a combustor in which a portion of the pyrolysis gas from the combusting fuel is burned in a separate vessel.

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to solid fuel furnaces, and is particularly applicable to furnaces that burn biomass fuels.

BACKGROUND OF THE INVENTION

In many power production, agricultural, and industrial process applications, biomass fuels are burned to produce steam, heat, and power. The fuel is typically burned in a furnace that is part of a boiler. In that case the hot gas from the furnace will typically pass across heat absorbing tubes, filled with water or steam, such as a screen, superheater, generating bank, and economizer. These tube bundles make up the convective section of the boiler. The furnace walls are typically constructed of water-filled tubes but absorb heat predominantly through radiation. The biomass fuels may include, but are not limited to bark, biological sludge, clippings (branches), forest residues, fiber line rejects (knots and screen rejects), urban waste (construction debris, pallets), agricultural waste (straw, rice husks, palm oil shells), sugar cane bagasse and pith, and sugar cane trash (tops and leaves). Biomass fuels are difficult to burn because they are often very wet (50% or higher moisture content), may have low specific density, may contain abrasive contaminants and corrosive chemicals, and may require significant processing or treatment to facilitate combustion.

In a typical furnace, it is desirable to create turbulent mixing of the combustion air, fuel, and pyrolysis gases but the turbulence is often insufficient to effectively burn the fuel. For example, if a fuel has a combination of high moisture content and low specific density (the density of individual fuel particles), the fuel can become entrained in the gas flow and exit the furnace before it has time to burn completely. If the fuel cannot be burned quickly, the combustion may continue into undesirable areas of the furnace or boiler. Light and wet fuels such as sugar cane bagasse are readily entrained in the gas flow and may exit the furnace before they burn completely. This can cause overly high temperatures in the convective sections of the boiler or even further downstream, and in the worst cases, combustion may continue into the economizer or beyond. The gases in an industrial furnace typically have a residence time of 2-3 seconds (the duration that air and combustion gases remain within the volume of the furnace) and, if the fuel is entrained in the gas flow, that is often not enough time to complete the combustion of wet and light fuels.

Grate-fired boilers are conventionally used for burning biomass fuels. These boilers may have a traveling grate, vibrating grate, or fixed grate (sloping, horizontal, or stepped). The fuel is injected or dumped onto the grate and combustion air is blown up through the grate (under-grate air or sometimes called primary air) to burn the fuel. In some of these boilers the under-grate air may constitute up to 80% of the total combustion air as a lot of air is needed to cool the grate. In many grate-fired boilers the fuel does not burn uniformly across the grate due to variations in fuel distribution, moisture content, size distribution, air distribution, etc. Where the fuel does burn, the fuel depth on the grate is reduced and the under-grate air, taking the path of least resistance, bypasses the deeper fuel and further increases the combustion in the shallow areas thereby exacerbating the poor distribution of combustion across the grate. Because of this mechanism the surface area of the grate is poorly utilized and a lot of fuel often goes unburned and wasted, reducing the efficiency of the boiler and creating disposal challenges of the ash.

Bubbling fluidized bed boilers (BFBs) are also commonly used for burning biomass fuels. In a BFB, a sand bed is typically fluidized using primary air and fuel is dumped into or injected over the bed. The sand acts as a heat sink and mixing medium and ideally, the fuel is distributed uniformly through the bed and mixes with the primary air, dries, and burns. BFB boilers, while being advantageous for burning wet fuels have significant drawbacks. They can be finicky to operate as the temperature and fluidization of the bed must be carefully controlled or the sand may overheat and sinter or glassify in the worst cases. BFBs are also susceptible to erosion caused by the entrainment of fine sand particles in the gas stream. The entrained particles can, in effect, sandblast the convective tube surfaces causing premature degradation and failure.

A common feature of most boilers used to burn biomass fuels is the large amount of under-grate or primary air required, as described above. To efficiently burn any fuel, it is desirable to minimize the excess air (the amount of air used above the stoichiometric requirement) but this is dependent on good mixing of the air and fuel. Over-fired air (OFA) systems are commonly used to improve the mixing and combustion in biomass boilers. OFA systems inject combustion air at one or more elevations above the grate or fluidized bed to complete the mixing of the fuel (entrained fuel particles and pyrolysis gases). If a boiler uses a high percentage of primary air, however, there may not be enough air available for good mixing at the OFA levels.

Alternative boiler designs have been developed that require much less primary air, for example, the stepped-floor grate as described in U.S. Pat. No. 8,707,876, the V-cell grate system described in U.S. Pat. No. 9,140,446, and the cylindrical power boiler described in U.S. Patent Publication No. 2016/0195260. U.S. Pat. No. 8,707,876, U.S. Pat. No. 9,140,446, and U.S. Patent Publication No. 2016/0195260 are owned by Applicant and are all hereby incorporated by reference. These systems are designed, at least in part, to enable the operation of a boiler with less primary air thereby freeing up more air to be injected above the fuel bed where it can more effectively improve the combustion of the fuel so that the boiler can operate more efficiently.

The power boiler described in U.S. Patent Publication No. 2016/0195260 is particularly useful for burning wet and light fuels such as sugar cane bagasse, pith, and trash. Sugar cane is the largest agricultural crop grown in the world (by area planted) and generates a tremendous amount of waste biomass. Bagasse and pith are commonly burned in outdated grate fired boilers that suffer from the deficiencies described above. Sugar cane trash, the left-over leaves and tops of the plants, is only minimally burned because the chlorine content of the leaves can cause corrosion of the high temperature superheater tubes in conventional boilers. Historically, most of the cane trash was burned in the fields but this practice is being curtailed around the world to reduce air pollution. This has created a dilemma for the sugar cane growers but also an opportunity to utilize an abundant, renewable fuel for power production.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved furnace for burning biomass fuels.

A furnace configured for burning biomass fuel includes a cylindrical enclosure with a vertical axis, the top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate location for further combustion. Solid fuel particles and air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which noncombustible particles are present, in which said gases are induced to rotate about the axis of said cylindrical enclosure. Said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and the average residence time of said particles in said cylindrical enclosure is greater than the average residence time of said gases in said cylindrical enclosure. The air-to-fuel ratio is controlled at least at one combustion air injection elevation to control a temperature above the elevation of said combustion air injection.

The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more thorough understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a sectional elevation of a furnace depicting the control elements

FIG. 2 shows is a sectional plan view through the furnace showing the spatial relationship of some of the described features. The arrows in FIG. 2 show the direction of flow of fuel and gas.

FIG. 3 is a flow chart showing a process of combusting biofuels.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Chloride corrosion in biomass-fired power boilers is a temperature driven phenomenon with higher temperatures accelerating corrosion rates. For this reason, superheater tubes are more susceptible to chloride corrosion as they operate at higher temperatures. Also, unburned carbon (unburned fuel) leaving the furnace may be trapped in the convective sections and burn there. This can create a high temperature, localized reducing environment that can accelerate corrosion. Another problem with conventional boilers is sintering of ash and inorganic contaminants (predominantly sand) in the boiler. Sintering of these materials is also a temperature driven phenomenon and their agglomeration can adversely affect the operation of the boiler. For example, if a boiler has a cylindrical furnace as described in U.S. Patent Publication No. 2016/0195260, and operates by spinning the gases inside the furnace to push entrained particles to the perimeter, a build-up of ash and/or sand on the wall can impede the circulation of gas in the furnace.

Complete combustion and good temperature control throughout the furnace facilitates operation of furnaces that depend on the interaction of the gas flow and the wall(s) of the furnace and also facilitates efficient combustion of biomass fuels, especially sugar cane trash and other chlorine-containing fuels. Good control in turn requires reducing the primary air so that enough air is available in the OFA system to regulate the combustion throughout the furnace. One preferred embodiment utilizes a cylindrical furnace (US application 2016/0195260) with a V-Cell floor (U.S. Pat. No. 9,140,446) and an automated control system to ensure complete combustion, to control gas temperature, and prevent excessive buildup on the furnace wall and corrosion of boiler tubes.

FIG. 1 is a sectional elevation of a furnace depicting the control elements. Solid lines with arrows indicate process connections transporting fuel or gases and the arrows indicate the direction of flow. Dashed lines with arrows indicate control connections and the arrows indicate the direction of the flow of information.

Referring to FIG. 1, furnace 1 has a cylindrical form about vertical axis 2. The furnace bottom 3 is shaped as a truncated cone which is terminated at a lower end at a floor which may comprise a gate 25, and the furnace top 4 is disposed to conduct hot gas to downstream heat transfer surfaces or a tandem vessel for further combustion. The heat transfer surfaces may be integrated with the furnace to form a stand-alone boiler, or the gas may be conducted to a separate boiler for further combustion or to a separate heat recovery steam generator. Fuel is stored in fuel bin 5 and delivered through fuel chute 6 and feeder means 7 and is then pneumatically conveyed to the furnace through injection pipe 8. The motive force for the fuel injection is provided by flue gas fan 9 and booster fan 10, supplying recirculated flue gas to pneumatically inject the fuel. The fuel may be injected into the furnace at multiple locations and elevations. Flue gas flow control damper 11 controls the flow of flue gas to feeder means 7. Combustion air fan 12 supplies air through distribution duct 13 to OFA (Over Fired Air) nozzle duct 14 through air flow control damper 15. Flue gas fan 9 supplies flue gas through distribution duct 16 to OFA nozzle duct 14 through flue gas flow control damper 17. OFA Nozzle duct 14 is connected to injection nozzle 18, that is fitted with pressure control damper 19. Although one injection nozzle 18 is shown, additional similar nozzles may be suitably located at multiple locations and elevations around furnace 1. Fuel bed air duct 20 is connected to furnace bottom 3 and supplies combustion air through air flow control damper 21 or flue gas through flue gas flow control damper 22, or a mixture of the two, to cooling plenum 23. The air, flue gas, or mixture, then flows down through cooling plenum 23 to upper ash chute 24, then up through movable grate 25 as shown by arrows 26. Movable grate 25 is perforated to allow gas to pass therethrough. Agitation steam pipe 27 supplies steam through steam flow control valve 28 to an array of nozzles 29 around the inside of furnace bottom 3. Upper ash chute 24 is closed at the bottom 3 by upper ash damper 30, thus forcing the air and/or gas flowing into upper ash chute 24 from cooling plenum 23 to flow up through ash grate 25. Lower ash damper 31 creates an air lock between itself and upper ash damper 30. Ash grate 25 is opened temporarily to discharge ash into ash chute 24, then returns to its closed position. Upper ash damper 30 then opens temporarily to discharge the ash to lower ash chute 32, then returns to its closed position. Lower ash damper 31 then opens temporarily to discharge the ash from the furnace. During normal operation, upper ash damper 30 and lower ash damper 31 are never open at the same time, therefore the air and/or gas flowing from cooling plenum 23 is always forced to flow up through movable grate 25 and cannot leak out of the furnace.

Fuel is fed from fuel bin 5 to fuel chutes 6 by fuel feed screw 33, powered by electric motor 34, and the speed of motor 34 is controlled by VFD (variable frequency drive) 35. One fuel feed system is shown comprising items 6, 7, 8, 33, 34, and 35, but multiple similar fuel feed systems may be used. Flue gas fan 9, booster fan 10, and combustion air fan 12 are powered by motors 36, and the speed of flue gas fan 9, booster fan 10, and combustion air fan 12 are controlled by VFDs 37, 38, and 39 respectively. Flue gas flows from flue gas fan 9 through duct 40 to booster fan 10, then from booster fan 10 through duct 41 to flue gas flow control damper 11.

A gas cooling means is shown at 81 and employs a water spray to reduce the gas temperature. Control valve 82 is regulated to inject the desired amount of water. Multiple water sprays may be suitably located within the furnace. For example, the gas temperature tends to be hottest just downstream of each injection nozzle 18 where combustion air is injected. A water spray can be located adjacent to each nozzle to control the peak gas temperature. Other ways to cool the gas can also be used. For example, steam injection can be used in lieu of, or in conjunction with, a water spray but a water spray will absorb more heat due to the latent heat of vaporization.

Ash extraction duct 42 is located in the upper half of furnace 1 and pulls a portion of the ash-laden flue gas out of the furnace. Cyclone 43 separates the ash from the flue gas and the gas is returned to mix with the remaining flue gas downstream, as shown at 44. The ash is retained in cyclone hopper 45 until it is periodically discharged through air lock valves 46. The ash may be discharged to a disposal means or may be collected for analysis to determine, for example, the chlorine content or unburned carbon content. An elevated unburned carbon content of the ash may indicate the combustion needs to be improved and the chlorine content can be used to determine an optimum operating temperature to minimize corrosion and plugging of downstream convective sections. Analyzing the content of the extracted fly ash may be automated or may be an offline endeavor, but temperature element 47 and temperature transmitter 48 can be used to continuously monitor the temperature of the extracted ash. This will provide feedback to furnace controller 49 to adjust the combustion in the boiler to, for example, keep the ash temperature below a predetermined set point. One ash extraction system comprising items 42-48 is shown but multiple similar ash extraction systems may be used.

Temperature element 50 and temperature transmitter 51 may be used as part of temperature measuring system 52 to measure the average furnace exit gas temperature (FEGT) across the width of the furnace 53. An acoustic pyrometer may be suitably employed as measuring system 52. Temperature transmitter 51 will send a feedback signal to furnace controller 49 to adjust the combustion in the boiler to maintain the FEGT within a predetermined or calculated range. One temperature measuring system comprising items 50-52 is shown but multiple similar temperature measurement systems may be used. Temperature element 50, and other temperature elements, may be any appropriate type of temperature sensor.

FIG. 2 shows is a sectional plan view through the furnace showing the spatial relationship of some of the described features. The arrows in FIG. 2 show the direction of flow of fuel and gas. Referring to FIG. 2, fuel enters the furnace through at least one injection pipe 8. Flue gas is used to convey the fuel and inject it into the boiler and enters with the fuel. Air, flue gas, or a combination thereof, is injected through at least one injection nozzle 18. Injection pipes 8 and injection nozzles 18 are aligned tangentially with imaginary circle 54 inside the furnace. One imaginary circle is shown but injection pipes 8 and injection nozzles 18 may be tangential to different imaginary circles. Similar arrangements of injection pipes 8 and injection nozzles 18 can be replicated at multiple elevations in the furnace. Utilizing injection pipes 8 and injection nozzles 18 in this manner will induce the fuel, air, and combustion gases to rotate about vertical axis 2 inside the furnace. The rotation will impart a centrifugal force to the fuel and gases, separating the denser fuel and denser gas (generally cooler) to move to the perimeter of the furnace and the less dense gas (generally hotter) to be displaced to the center. This will cause the cooler combustion air to mix with the fuel around the perimeter and the fuel particles will drag on the furnace wall where they will slip in and out of suspension in the gas. This will cause the average retention time of the fuel to be greater than the average retention time of the gas thereby giving the fuel more time to be burned completely.

As the biomass fuel burns, the residual inorganic ash can be abrasive and biomass fuels typically contain significant amounts of contaminating sand, both of which can become entrained in the swirling gas flow and abrade the wall tubes of the furnace. Left unchecked this abrasion will quickly erode the tubes causing the furnace or boiler to shut down. To prevent erosion of the wall tubes, a common practice is to line the furnace with a high-temperature abrasion-resistant refractory. In addition to protecting the tubes, the refractory also acts as a heat sink and helps to stabilize the combustion in the furnace by alternatively absorbing and releasing heat as the combustion in the furnace naturally fluctuates. The refractory lining has inherent disadvantages, however, in that it impedes the radiant heat transfer to the water-filled wall tubes and the elevated temperature of the refractory surface can cause the ash and sand circulating with the gas to stick to the furnace wall. The cylindrical furnace depends on the rotation of the internal gases to increase the retention time of the fuel and to strip contaminating ash and sand out of suspension to minimize the erosion and pluggage of downstream tube banks. If sufficient material accumulates on the furnace wall, the necessary circulation of the gases will be impeded, and the furnace will not operate properly.

The stickiness of the ash and sand are determined by their melting behavior and that in turn is influenced by their chemical makeup. The chlorine content of the ash, for example, acts as a eutectic with higher chlorine content depressing the melting temperature of the ash. The melting behavior is characterized by four stages: first melting, sticky range, radical deformation, and complete melting, with the temperature increasing from the first to the last stages respectively. For the ash and sand or other significant inorganic contaminants, laboratory tests will be conducted to determine the different melting stage temperatures and that data will be used as variables in the programming of furnace controller 49. Similarly, a sintering temperature may be determined for the ash or sand with the sintering temperature indicating a sticky temperature. Some furnaces are designed to operate at temperatures above the radical deformation temperature of the ash. These are called slagging furnaces and at those temperatures the ash is fluid enough to run off under its own weight. In biomass burning furnaces, however, especially those burning fuels with high chlorine content, elevated temperatures can increase the chlorine-induced corrosion of the tube surfaces. To minimize the potential for chlorine induced corrosion, to minimize the excrescent material build-up on the furnace wall, and to minimize the pluggage of any convective tube banks, the furnace is designed to operate below the sticky range of the ash and contaminating sand. This requires strict control of the combustion in the boiler.

Referring again to FIG. 1, furnace 1 is lined with refractory 55 and at least one thermowell 56 is imbedded in refractory lining 55. In some embodiments, multiple thermowells are located at suitable locations around the perimeter of the furnace and at multiple elevations within the furnace. Temperature element 57 (typically a thermocouple) will collect temperature data and temperature transmitter 58 will send the data to furnace controller 49. Furnace bottom 3 is lined with refractory 59 but refractory lining 59 may have different physical characteristics than refractory lining 55. At least one thermowell 60 is imbedded in refractory lining 59, but in some embodiments, multiple thermowells are suitably located within furnace bottom 3. Temperature element 61 (typically a thermocouple) will collect temperature data and temperature transmitter 62 will send the data to furnace controller 49.

Furnace controller 49 uses a distributed control system, or a programmable logic controller, or another computer-based system to collect and analyze input data and send out control signals to various control elements around the furnace. FIG. 1 does not show all the inputs and outputs to and from furnace controller 49, only some of those associated with controlling the combustion temperature. Many control scenarios are possible; several are described below:

Control Scenario #1: Thermowell 56 is located above injection nozzle 18. The combusting gases are circulating in the furnace but following a helical path upward, therefore the temperature is most suitably measured above the associated control element. If the temperature input from temperature transmitter 58 to furnace controller 49 is higher than desired, furnace controller 49 can decrease the combustion air flow through injection nozzle 18 by closing air flow control damper 15. This will reduce the air to fuel ratio at that elevation, the combustion will be retarded, and the refractory temperature measured at thermowell 56 will drop. Similarly, if the temperature is too low the air to fuel ratio can be increased. Thermowell 56 measures the temperature of the refractory in which it is embedded close to the interior surface of the refractory, preferably within 10 millimeters. The most accurate means to determine the gas temperature based on the temperature measurement at the thermowell is to independently measure the gas temperature using a hand-held pyrometer or by other means, and establish a correlation between the gas temperature and the refractory temperature. Alternately, a calculation can be made to determine the gas temperature based on the refractory temperature measurement, tube temperature, refractory properties, location of the thermowell, and thickness of the refractory. At the location of the thermowell, the refractory will be cooler than the combustion gas temperature as it is cooled by the water filled tubes surrounding the vessel, and the heat transfer from the gas to the refractory is imperfect. For example, typical sticky temperature for sugar cane trash is around 1000° Celsius. If the average gas temperature is 900° C., and the tube temperature is 227° C., and silicon carbide refractory is used with an average thickness of 75 mm, the surface temperature of the refractory can be calculated to be about 616° C. Once that is known, the temperature profile through the refractory can be established. For example, with the parameters given, the refractory temperature at the thermowell may be about 564° C. Now, the heat transfer to the refractory is predominantly driven by radiation and is generally a function of the difference between the gas temperature to the 4th power and the refractory surface temperature to the 4th power. So, any increase in the gas temperature will dramatically increase the heat flux to the refractory and the refractory temperature will rise accordingly. In this control scenario, therefore, if the temperature at the thermowell rises above 564° C., the control system will start to close the associated damper to retard the combustion to prevent the gas temperature from reaching the sticky point.

Control Scenario #2: Ash extraction duct 42 is located above injection nozzle 19. The temperature of the particulates extracted by extraction duct 42 is measured by temperature element 47 and transmitted to furnace controller 49 by temperature transmitter 48. If the measured temperature of the particulates is higher than the sticky temperature of the ash, for example, furnace controller 49 can reduce the combustion air flowing to injection nozzle 18 by closing air flow control damper 15. This will reduce the air-to-fuel ratio at and above the elevation of injection nozzle 19, retard the combustion in the furnace above that point, and reduce the temperature of the particulates captured by ash extraction duct 42.

In either Control Scenario #1 or #2 it may be desirous to maintain a certain total gas flow through injection nozzle 18 to maintain the rotation of the gas inside the furnace. In that case, furnace controller 49 can adjust the air to fuel ratio at and above the elevation of injection nozzle 18 by opening (or closing) flue gas control damper 17 and simultaneously closing (or opening) air flow control damper 15.

In either Control Scenario #1 or #2, if the flow through injection nozzle 18 changes, the position of pressure control damper 19 can be adjusted to increase or decrease the upstream gas pressure in injection nozzle 18. For example, if the flow decreases, pressure control damper 19 can be closed to increase the pressure. This will increase the gas velocity leaving injection nozzle 18 and maintain the momentum transfer from the injected gas to the rotating gas inside furnace 3. In this case pressure transmitter 63 will provide a feedback signal to furnace controller 49.

Temperature measuring system 52 is employed to measure the average gas temperature leaving furnace 1. Temperature transmitter 51 sends the measured temperature to furnace controller 49 that in turn controls the air to fuel ratio in furnace 1 by adjusting air flow control damper 15, flue gas control damper 17, and/or pressure control damper 19.

If multiple gas injection systems, similar to items 14, 15, 17, 18, and 19, are installed around the boiler, they may be controlled in parallel or individually to control the combustion temperature locally or throughout the furnace while maintaining the desired rotation of the combusting gas in the furnace.

The temperature measured at 56 by temperature element 57 is sent to furnace controller 49 where it is compared against a desired value. If the temperature at 56 is found to be too high, furnace controller 49 may open valve 82 and inject a flow of water into the furnace. Furnace controller 49 can turn the flow of water on or off, or regulate the flow, to control the temperature at 56. Water spray 81 can be used to control a local temperature such as at 56, or an average gas temperature, such as that measured by measurement system 52. In the latter case, multiple water sprays 81 may be suitably employed.

Using temperature element 57 to measure the internal temperature of refractory lining 55 is a practical, reliable, and well-established procedure, but to avoid buildup of ash and contaminants on the furnace wall, it is necessary to know the surface temperature of refractory lining 55. This may be calculated given the temperature measured by temperature element 57, thickness of refractory lining 55, the location of thermowell 56, the temperature of the furnace wall tubes 64, and the thermal conductivity of refractory lining 55. It is also advantageous to measure the temperature of the gas close to the surface of refractory lining 55. The gas temperature is difficult to calculate accurately from the refractory temperature, but it can be measured directly by a variety of means. Gas pyrometer 65 may comprise an optical pyrometer, a laser pyrometer, a suction pyrometer, a thermocouple exposed to the circulating gas, or other technology. Gas pyrometer 65 is ideally located above injection nozzle 18 to measure the gas temperature slightly downstream from injection nozzle 18. Temperature transmitter 66 sends the temperature data to furnace controller 49. FIG. 1 shows one gas pyrometer, but multiple pyrometers may be installed at different elevations and locations around the furnace.

Additional instrumentation may also be used to control the operation of the furnace. Optical camera 67 can be used to ascertain the depth, movement, and combustion of the fuel bed at the bottom 3 of furnace 1, or it can be used to determine the rotational velocity of the gases in the furnace. Feedback can be through manual observation by a human operator or by a machine vision system providing feedback to furnace controller 49. Multiple optical cameras may be used to monitor separate aspects simultaneously. Flow elements 68, 70, 72, and 74, and flow transmitters 69, 71, 73, and 75 can be used to measure and transmit combustion air and flue gas flow data to furnace controller 49. Pressure transmitters 76 can be used to measure and transmit the gas pressures in bed air duct 20 to furnace controller 49. Furnace controller 49 can use that data to adjust injection nozzle damper 19 to maintain the desired injection velocity. Pressure transmitters 77, 78, and 79 can transmit the discharge pressures from the combustion air fan, flue gas fan, and booster fan, respectively, to furnace controller 49. Furnace controller 49 can use that data to control the speed of combustion air fan 12, flue gas fan 9, and booster fan 10 to maintain their desired discharge pressures. Pressure transmitters 76 and 80 can provide feedback to furnace controller 49 to control the pressure in bed air duct 20 and steam pipe 27 respectively.

Furnace controller 49 may be programmed to operate the boiler as autonomously as possible but some human intervention may be required. For example, periodic observations by human operators can fill in gaps in operational data not obtainable by an automated system. There also may be times when field instrumentation is offline and human operators must fill in. Human observations may therefore form part of a feedback loop to furnace controller 49.

The furnace is limited to use in a boiler and can be used in other application in which a solid fuel is burned, such as in a gasifier, in which gaseous combustion products are removed from the furnace and burned in a separate location. While several embodiments are disclosed, one skilled in the art will recognize that many other embodiments are possible within the scope of the invention.

FIG. 3 is a flow chart 300 showing a process of combusting biofuels. In block 302, a mixture of solid fuel particles and combustion air is injected into a cylindrical furnace, such as a furnace described in FIGS. 1 and 2. The fuel-air mixture from block 302 is then burned in block 304. The configuration of air and fuel injectors shown in FIG. 2 then induces rotation of the fuel-air mixture around an axis of the cylindrical furnace in block 306. In block 308, radial centrifugal forces occurring due to the rotation of the fuel-air mixture induce a radial stratification wherein the heavier solid particles tend to concentrate near the wall of the cylindrical furnace while the lighter gases remain nearer the axis. In block 310, the particles which are concentrated near the wall of the cylindrical furnace tend to rotate slower and remain within the furnace longer than the combustion gases. In block 312, a temperature at or near an air injection location is measured and then in block 314, this measured temperature is compared to a desired temperature range. Block 316 is entered if the temperature is above the desired range (i.e., T too high)—the flow of combustion air is then reduced to decrease the amount of combustion and thereby lower the temperature at and above the air injection location. Block 318 is entered if the temperature is below the desired range (i.e., T too low)—the flow of combustion air is then increased to increase the amount of combustion and thereby raise the temperature at and above the air injection location. If block 314 indicates that the temperature is within the desired range, then block 312 is re-entered to continue temperature measurements.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments described herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

A preferred method or apparatus of the present invention has many novel aspects, and because the invention can be embodied in different methods or apparatuses for different purposes, not every aspect need be present in every embodiment. Moreover, many of the aspects of the described embodiments may be separately patentable. The invention has broad applicability and can provide many benefits as described and shown in the examples above. The embodiments will vary greatly depending upon the specific application, and not every embodiment will provide all of the benefits and meet all of the objectives that are achievable by the invention.

It should be recognized that embodiments of the present invention can be implemented via computer hardware, a combination of both hardware and software, or by computer instructions stored in a non-transitory computer-readable memory. The methods can be implemented in computer programs using standard programming techniques—including a non-transitory computer-readable storage medium configured with a computer program, where the storage medium so configured causes a computer to operate in a specific and predefined manner—according to the methods and figures described in this Specification. Each program may be implemented in a high-level procedural or object-oriented programming language to communicate with a computer system. However, the programs can be implemented in assembly or machine language, if desired. In any case, the language can be a compiled or interpreted language. Moreover, the program can run on dedicated integrated circuits programmed for that purpose.

Further, methodologies may be implemented in any type of computing platform, including but not limited to, personal computers, mini-computers, main-frames, workstations, networked or distributed computing environments, computer platforms separate, integral to, or in communication with charged particle tools or other imaging devices, and the like. Aspects of the present invention may be implemented in machine readable code stored on a non-transitory storage medium or device, whether removable or integral to the computing platform, such as a hard disc, optical read and/or write storage mediums, RAM, ROM, and the like, so that it is readable by a programmable computer, for configuring and operating the computer when the storage media or device is read by the computer to perform the procedures described herein. Moreover, machine-readable code, or portions thereof, may be transmitted over a wired or wireless network. The invention described herein includes these and other various types of non-transitory computer-readable storage media when such media contain instructions or programs for implementing the steps described above in conjunction with a microprocessor or other data processor. The invention also includes the computer itself when programmed according to the methods and techniques described herein.

Computer programs can be applied to input data to perform the functions described herein and thereby transform the input data to generate output data. The output information is applied to one or more output devices such as a display monitor. In preferred embodiments of the present invention, the transformed data represents physical and tangible objects, including producing a particular visual depiction of the physical and tangible objects on a display.

Whenever the terms “automatic,” “automated,” or similar terms are used herein, those terms will be understood to include manual initiation of the automatic or automated process or step.

In the description and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . .” To the extent that any term is not specially defined in this specification, the intent is that the term is to be given its plain and ordinary meaning. The accompanying drawings are intended to aid in understanding the present invention and, unless otherwise indicated, are not drawn to scale.

The various features described herein may be used in any functional combination or sub-combination, and not merely those combinations described in the embodiments herein. As such, this disclosure should be interpreted as providing written description of any such combination or sub-combination.

The following are additional enumerated embodiments according to the present disclosure.

A first embodiment, which is a furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which noncombustible particles are present, in which said gases are induced to rotate around the vertical axis and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, and in which an air-to-fuel ratio is controlled at least at one combustion air injection elevation to control a temperature above the elevation of said combustion air injection.

A second embodiment, which is the furnace of the first embodiment in which at least one melting temperature has been determined for at least one composition of said noncombustible particles and said melting temperature is used to determine a desired range of said controllable temperature.

A third embodiment, which is the furnace of the second embodiment in which said cylindrical enclosure is internally lined with refractory and said controllable temperature is the temperature of said refractory measured at least at one location within said cylindrical enclosure.

A fourth embodiment, which is the furnace of the second embodiment in which said controllable temperature is a furnace exit gas temperature measured at least at one location at an exit of said cylindrical enclosure.

A fifth embodiment, which is the furnace of the second embodiment in which said controllable temperature is a temperature of said noncombustible particles extracted from said cylindrical furnace.

A sixth embodiment, which is the furnace of the second embodiment in which said controllable temperature is a temperature of said gas measured within a cylindrical height of said furnace.

A seventh embodiment, which is the furnace of the first embodiment in which said air-to-fuel ratio is controlled by regulating a quantity of said combustion air injected at least at one of said air injection elevations.

An eighth embodiment, which is the furnace of the seventh embodiment in which an injection velocity of said combustion air is controlled independently of a flow of said combustion air.

A ninth embodiment, which is the furnace of the first embodiment in which said air-to-fuel ratio is controlled by diluting said combustion air with oxygen-depleted gas taken from a furnace flue gas.

A tenth embodiment, which is the furnace of the ninth embodiment in which an injection velocity of said diluted combustion air and said furnace flue gas is controlled independently of a flow of said diluted combustion air and said furnace flue gas.

An eleventh embodiment, which is a furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a bottom of said enclosure being conically shaped and truncated at a floor, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which noncombustible particles are present, in which said gases are induced to rotate around the vertical axis and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and tan average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, and in which an air-to-fuel ratio is controlled at least at one combustion air injection location within said conically shaped bottom to control a temperature within said conically shaped bottom.

A twelfth embodiment, which is the furnace of the eleventh embodiment in which at least one melting temperature has been determined for at least one composition of said noncombustible particles and said melting temperature is used to determine a desired range of said controllable temperature.

A thirteenth embodiment, which is the furnace of the twelfth embodiment in which said conically shaped bottom is internally lined with refractory and said controllable temperature is the temperature of said refractory measured at least at one location within said conically shaped bottom.

A fourteenth embodiment, which is the furnace of the twelfth embodiment in which said controllable temperature is a gas temperature measured at least at one location above a top of said conically shaped bottom but below a lowest level of fuel injection.

A fifteenth embodiment, which is the furnace of the twelfth embodiment in which said controllable temperature is a temperature of said noncombustible particles extracted from said conically shaped bottom.

A sixteenth embodiment, which is the furnace of the twelfth embodiment in which said controllable temperature is a temperature of said gas measured within said conically shaped bottom.

A seventeenth embodiment, which is the furnace of the eleventh embodiment in which said air-to-fuel ratio is controlled by regulating a quantity of said combustion air injected at least at one of said combustion air injection locations.

An eighteenth embodiment, which is the furnace of the seventeenth embodiment in which the injection velocity of said combustion air is controlled independently of the flow of said combustion air.

A nineteenth embodiment, which is the furnace of the eleventh embodiment in which said air-to-fuel ratio is controlled by diluting said combustion air with oxygen-depleted gas taken from a furnace flue gas.

A twentieth embodiment, which is the furnace of the nineteenth embodiment in which an injection velocity of said diluted combustion air and said furnace flue gas is controlled independently of a flow of said diluted combustion air and said furnace flue gas.

A twenty-first embodiment, which is the furnace of the first embodiment in which said separate means for further combustion is an existing boiler.

A twenty-second embodiment, which is the furnace of the first embodiment in which said furnace is integrated into a stand-alone boiler.

A twenty-third embodiment, which is the furnace of the first embodiment in which said heat absorbing surfaces are contained in a separate heat recovery steam generator.

A twenty-fourth embodiment, which is a furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which noncombustible particles are present, in which said gases are induced to rotate around the vertical axis and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, and in which a momentum of said injected combustion air and fuel is controlled, either together or separately, to control a rotation of said rotating gases.

A twenty-fifth embodiment, which is the furnace of the twenty-fourth embodiment in which furnace flue gas is also injected, either separately or as a dilutant to said combustion air or as transport media for said fuel, and in which a momentum of said injected furnace flue gas is controlled, either separately or together with said combustion air and fuel, to control a rotation of said rotating gases.

A twenty-sixth embodiment, which is a furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which noncombustible particles are present, in which said gases are induced to rotate about the axis of said cylindrical enclosure and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, and in which a rotational velocity of said rotating gases is measured at least at one location within said cylindrical enclosure, and in which said rotational velocity measurement is used as an input to a control loop to regulate said rotational velocity.

A twenty-seventh embodiment, which is a furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a bottom of said enclosure conically shaped and truncated at a floor, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which a portion of said injected fuel at least partially fills said bottom, in which noncombustible particles are present, in which said gases are induced to rotate around the vertical axis and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, in which said fuel residing in said bottom is induced to rotate about the vertical axis, and in which an injection of combustion air, furnace flue gas, or steam, is regulated to control a rotational velocity of said rotating fuel.

A twenty-eighth embodiment, which is the furnace of the twenty-seventh embodiment in which the rotational velocity of said rotating fuel is measured at least at one location within said conically shaped bottom, and in which said velocity measurement is used as an input to a control loop to regulate said rotational velocity.

A twenty-ninth embodiment, which is the methods and apparatuses described above.

A thirtieth embodiment, which is a method of operating a furnace, comprising injecting solid fuel particles and combustion air into a cylindrical furnace; burning said fuel and releasing gaseous products of combustion and noncombustible particles, inducing said gases to rotate around a vertical axis of said cylindrical furnace, wherein said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and wherein an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical furnace; and controlling an air-to-fuel ratio at least at one combustion air injection elevation to control a temperature above the elevation of said combustion air injection.

A thirty-first embodiment, which is the method of the thirtieth embodiment further comprising determining at least one melting temperature for at least one composition of said noncombustible particles and using said melting temperature to determine a desired range of said controllable temperature.

A thirty-second embodiment, which is the method of the thirty-first embodiment in which said cylindrical furnace is internally lined with refractory and controlling the controllable temperature is a temperature of said refractory measured at least at one location within said cylindrical furnace.

A thirty-third embodiment, which is the method of the thirty-first embodiment in which said controllable temperature is a furnace exit gas temperature measured at least at one location at an exit of said cylindrical furnace.

A thirty-fourth embodiment, which is the method of the thirty-first embodiment in which said controllable temperature is a temperature of said noncombustible particles extracted from said cylindrical furnace.

A thirty-fifth embodiment, which is the method of the thirty-first embodiment in which said controllable temperature is a temperature of said gas measured within a height of said cylindrical furnace.

A thirty-sixth embodiment, which is the method of the thirtieth embodiment in which said air-to-fuel ratio is controlled by regulating a quantity of said combustion air injected at least at one of said combustion air injection elevations.

A thirty-seventh embodiment, which is the method of the thirtieth embodiment in which said air-to-fuel ratio is controlled by diluting said combustion air with oxygen-depleted gas taken from a furnace flue gas.

A thirty-eighth embodiment, which is the method of the thirty-seventh embodiment in which an injection velocity of said combustion air is controlled independently of a flow of said combustion air.

A thirty-ninth embodiment, which is the method of the thirty-eighth embodiment in which the injection velocity of said diluted combustion air and said furnace flue gas is controlled independently of the flow of said diluted combustion air and said furnace flue gas.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments described herein without departing from the scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

Claims

1. A furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which noncombustible particles are present, in which said gases are induced to rotate around the vertical axis and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, and in which an air-to-fuel ratio is controlled at least at one combustion air injection elevation to control a temperature above the elevation of said combustion air injection.

2. The furnace of claim 1 in which at least one melting temperature has been determined for at least one composition of said noncombustible particles and said melting temperature is used to determine a desired range of said controllable temperature.

3. The furnace of claim 2 in which said cylindrical enclosure is internally lined with refractory and said controllable temperature is the temperature of said refractory measured at least at one location within said cylindrical enclosure.

4. The furnace of claim 2 in which said controllable temperature is a furnace exit gas temperature measured at least at one location at an exit of said cylindrical enclosure.

5. The furnace of claim 2 in which said controllable temperature is a temperature of said noncombustible particles extracted from said cylindrical furnace.

6. The furnace of claim 2 in which said controllable temperature is a temperature of said gas measured within a cylindrical height of said furnace.

7. The furnace of claim 1 in which said air-to-fuel ratio is controlled by regulating a quantity of said combustion air injected at least at one of said air injection elevations.

8. The furnace of claim 7 in which an injection velocity of said combustion air is controlled independently of a flow of said combustion air.

9. The furnace of claim 1 in which said air-to-fuel ratio is controlled by diluting said combustion air with oxygen-depleted gas taken from a furnace flue gas.

10. The furnace of claim 9 in which an injection velocity of said diluted combustion air and said furnace flue gas is controlled independently of a flow of said diluted combustion air and said furnace flue gas.

11. A furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a bottom of said enclosure being conically shaped and truncated at a floor, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which noncombustible particles are present, in which said gases are induced to rotate around the vertical axis and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and tan average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, and in which an air-to-fuel ratio is controlled at least at one combustion air injection location within said conically shaped bottom to control a temperature within said conically shaped bottom.

12. The furnace of claim 11 in which at least one melting temperature has been determined for at least one composition of said noncombustible particles and said melting temperature is used to determine a desired range of said controllable temperature.

13. The furnace of claim 12 in which said conically shaped bottom is internally lined with refractory and said controllable temperature is the temperature of said refractory measured at least at one location within said conically shaped bottom.

14. The furnace of claim 12 in which said controllable temperature is a gas temperature measured at least at one location above a top of said conically shaped bottom but below a lowest level of fuel injection.

15. The furnace of claim 12 in which said controllable temperature is a temperature of said noncombustible particles extracted from said conically shaped bottom.

16. The furnace of claim 12 in which said controllable temperature is a temperature of said gas measured within said conically shaped bottom.

17. The furnace of claim 11 in which said air-to-fuel ratio is controlled by regulating a quantity of said combustion air injected at least at one of said combustion air injection locations.

18. The furnace of claim 17 in which the injection velocity of said combustion air is controlled independently of the flow of said combustion air.

19. The furnace of claim 11 in which said air-to-fuel ratio is controlled by diluting said combustion air with oxygen-depleted gas taken from a furnace flue gas.

20. The furnace of claim 19 in which an injection velocity of said diluted combustion air and said furnace flue gas is controlled independently of a flow of said diluted combustion air and said furnace flue gas.

21. The furnace of claim 1 in which said separate means for further combustion is an existing boiler.

22. The furnace of claim 1 in which said furnace is integrated into a stand-alone boiler.

23. The furnace of claim 1 in which said heat absorbing surfaces are contained in a separate heat recovery steam generator.

24. A furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which noncombustible particles are present, in which said gases are induced to rotate around the vertical axis and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, and in which a momentum of said injected combustion air and fuel is controlled, either together or separately, to control a rotation of said rotating gases.

25. The furnace of claim 24 in which furnace flue gas is also injected, either separately or as a dilutant to said combustion air or as transport media for said fuel, and in which a momentum of said injected furnace flue gas is controlled, either separately or together with said combustion air and fuel, to control a rotation of said rotating gases.

26. A furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which noncombustible particles are present, in which said gases are induced to rotate about the axis of said cylindrical enclosure and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, and in which a rotational velocity of said rotating gases is measured at least at one location within said cylindrical enclosure, and in which said rotational velocity measurement is used as an input to a control loop to regulate said rotational velocity.

27. A furnace burning biomass fuel defined by a cylindrical enclosure with a vertical axis, a bottom of said enclosure conically shaped and truncated at a floor, a top of said enclosure disposed to conduct hot gases to heat-absorbing surfaces or to a separate means for further combustion, in which solid fuel particles and combustion air are injected into said furnace and said fuel burns releasing gaseous products of combustion, in which a portion of said injected fuel at least partially fills said bottom, in which noncombustible particles are present, in which said gases are induced to rotate around the vertical axis and said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical enclosure, in which said fuel residing in said bottom is induced to rotate about the vertical axis, and in which an injection of combustion air, furnace flue gas, or steam, is regulated to control a rotational velocity of said rotating fuel.

28. The furnace of claim 27 in which the rotational velocity of said rotating fuel is measured at least at one location within said conically shaped bottom, and in which said velocity measurement is used as an input to a control loop to regulate said rotational velocity.

29. (canceled)

30. A method of operating a furnace, comprising:

injecting solid fuel particles and combustion air into a cylindrical furnace;

burning said fuel and releasing gaseous products of combustion and noncombustible particles,

inducing said gases to rotate around a vertical axis of said cylindrical furnace, wherein said rotation entrains at least some of said fuel and noncombustible particles to rotate with said gases, and wherein an average residence time of said particles in said cylindrical enclosure is greater than an average residence time of said gases in said cylindrical furnace; and

controlling an air-to-fuel ratio at least at one combustion air injection elevation to control a temperature above the elevation of said combustion air injection.

31. The method of claim 30 further comprising determining at least one melting temperature for at least one composition of said noncombustible particles and using said melting temperature to determine a desired range of said controllable temperature.

32. The method of claim 31 in which said cylindrical furnace is internally lined with refractory and controlling the controllable temperature is a temperature of said refractory measured at least at one location within said cylindrical furnace.

33. The method of claim 31 in which said controllable temperature is a furnace exit gas temperature measured at least at one location at an exit of said cylindrical furnace.

34. The method of claim 31 in which said controllable temperature is a temperature of said noncombustible particles extracted from said cylindrical furnace.

35. The method of claim 31 in which said controllable temperature is a temperature of said gas measured within a height of said cylindrical furnace.

36. The method of claim 30 in which said air-to-fuel ratio is controlled by regulating a quantity of said combustion air injected at least at one of said combustion air injection elevations.

37. The method of claim 30 in which said air-to-fuel ratio is controlled by diluting said combustion air with oxygen-depleted gas taken from a furnace flue gas.

38. The method of claim 37 in which an injection velocity of said combustion air is controlled independently of a flow of said combustion air.

39. The method of claim 38 in which the injection velocity of said diluted combustion air and said furnace flue gas is controlled independently of the flow of said diluted combustion air and said furnace flue gas.