Fuel from ash

Abstract

Coal ash, which also consists of fly ash, is a very fine granular solid residue obtained as a by-product of coal combustion. The invention relates to the surprising result that a fuel can be obtained from coal ash. The process involves a pneumatic separation of the coal ash at relatively low temperatures. The coal ash is separated into at least two size fractions. The size fractions obtained by the process of this invention include at least one with lower carbon and another containing increased levels of carbon in the range of 50% by weight and a heating value in the range of 4000 to 6000 Btu/lb.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a fuel, a process to produce the fuel from coal ash and a method of burning the fuel. The process to produce the fuel from coal ash is based on a pneumatic separation where the coal ash is separated into at least two size fractions. The size fractions obtained by the process of this invention include; a major fine fraction containing lower carbon and a minor coarse fraction containing increased levels of carbon in sufficient quantity, to be used as a fuel.

[0003] 2. Description of the Prior Art

[0004] Coal ash is a very fine granular solid residue obtained as a by-product of coal combustion. Due to its abundance and cementitious properties, coal ash is widely used in the production of concrete. Coal ash is made up of various categories and comprises fly ash. Fly ash is a specific type of coal ash collected from the combustion gases of coal fired heaters and coal burning power plants. Fly ash is further segregated into several groups which are classified in large part, by the coal source the fly ash is derived from.

[0005] High carbon levels in coal ash adversely affect the durability of concrete, particularly to freeze thaw cycles therefore concrete producers have commonly, tried to reduce carbon to low levels in coal ash and fly ash used for concrete. 3% LOI (Loss-on-Ignition) is considered to be the maximum carbon level permitted to produce a concrete of good durability.

[0006] Due to this low carbon requirement in concrete the prior art in the field of carbon/coal ash and particularly fly ash, has been primarily concerned with the reduction of carbon from coal ash and chiefly by thermal processes. Therefore, it is well known that the carbon in coal ash burns or at least volatilizes at higher temperatures and particularly in the presence of oxygen.

[0007] Cochran in U.S. Pat. No. 5,160,539 teaches a method and low carbon product obtained in an bubbling fluid bed produced by the introduction of air and whose operating conditions are between 1300 and 1800° F. The product of this invention is a useful pozzolanic material. Similarly, U.S. Pat. No. 5,555,821 by Martinez discloses a stainless steel apparatus and a process for removing unburned carbon from fly ash. The fly ash is heated from 800 to 1200° C. in a double auger where an oxygen containing gas is injected, cooled and then finally recovered. The oxygen is said to accelerate the burning of the carbon in the fly ash and the process is said to achieve carbon levels of 0.7% in the fly ash.

[0008] U.S. Pat. No. 5,996,808 by Levy et al, discloses an inclined fluidized bed reactor with sound wave agitation which enhances the fluidization of the fine fly ash for processing fly ash into various fractions. According to Levy et al, the lighter and coarser fractions are separated, with the coarse fraction containing more carbon. The object of the Levy invention is to reduce the carbon levels to between 3 and 5% in the fly ash.

SUMMARY OF THE INVENTION

[0009] One object of this invention is to provide a process for producing a fuel from coal ash having an initial amount of carbon. This process comprising the pneumatic separation of coal ash into at least two size fractions;

[0010] the separation taking place in a solid-gas contactor in association with a classifier means wherein,

[0011] the two size fractions comprise and are separated into

[0012] a major fine fraction, and

[0013] a minor coarse fraction, being the fuel; and containing at least 45% of the initial carbon amount, and

[0014] a recovery of the fuel.

[0015] Another object of the invention is a fuel derived from coal ash having an initial amount of carbon. The fuel is produced by a pneumatic separation of the coal ash wherein the coal ash is separated into at least two fractions;

[0016] the separation taking place in a solid-gas contactor in association with a classifier means wherein,

[0017] the two size fractions comprise and are separated into

[0018] a major fine fraction, and

[0019] a minor coarse fraction, being the fuel; and containing at least 45% of the initial carbon amount, and

[0020] a recovery of the fuel.

[0021] Yet another object of the invention is a method of burning a fuel derived from coal ash. The coal ash having an initial carbon amount and the fuel being produced by, the pneumatic separation of coal ash into at least two size fractions;

[0022] the separation taking place in a solid-gas contactor in association with a classifier means wherein,

[0023] the two size fractions comprise and are separated into a major fine fraction, and

[0024] a minor coarse fraction, being the fuel; and

[0025] containing at least 45% of the initial carbon amount, and

[0026] a recovery of the fuel; and

[0027] a conveyance of the fuel to a combustion system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings:

[0029] FIG. 1 is a process flow diagram for producing the fuel derived from coal ash with a single classifier means illustrated.

[0030] FIG. 2 is a process flow diagram for producing fuel derived from coal ash, where the fuel is classified and removed from the solid gas contactor.

[0031] FIG. 3 is a process flow diagram wherein there are multiple particle size classification steps and intermediate products.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0032] FIG. 1, illustrates the process of the separation used to obtain the coal ash derived fuel. The coal ash is introduced into the process via the coal ash inlet (10) near the bottom of the circulating solid gas contactor (100). The coal ash to be added to the solid gas contactor, according to this invention, should contain at least 5% initial carbon amount (% LOI), and preferably at least 10% initial carbon and most preferably 14% initial carbon. It must also be noted, that nearly half of the carbon initially present in the coal ash is found in the coarse fraction (>=63 &mgr;m). This fraction contains at least 45%(w/w) of the initial carbon, and preferably more than 50%(w/w) of the initial carbon.

[0033] The type of solid gas contactor represented in FIG. 1 is, a fluidized bed, a circulating fluidized bed, a transport vessel or a classifying transport vessel. These four equipment types would usually be referred to as a fluidized bed reactor, a circulating fluidized bed reactor, a transport reactor or a classifying transport reactor, but because there is only particle classification without a reaction taking place, the word “reactor” has been either omitted or replaced with “vessel”.

[0034] Gas (22), usually air, is introduced at the bottom of the contactor, via a blower (400) and is distributed evenly in the bottom of the vessel. The gas distributor in the solid-gas contactor may be a perforated plate, although no plate is needed if the solid gas contactor is a transport vessel or classifying transport vessel.

[0035] It should be noted that the gas used as the means of separation of the coal ash is typically ambient air (20) and no particular pretreatment other than that required for the efficient operation of the blower is required. However, in some cases, the coal ash may be damp with moisture, the gas (22) may be heated to temperatures where the surface moisture is removed while the carbon content is unaffected. The maximum, gas temperature would be 200° C. but preferably temperatures around 150° C. are used. This heating can be accomplished through combustion of a fuel, or some other hot gas source, with the mixing of the hot gases (21) and ambient air used to attain the relatively low temperatures required. This mixing would occur near the intake of the blower (400) which would be designed to handle the higher temperatures.

[0036] The velocity of the gas in the contactor is such, that the coal ash is elutriated almost completely in the gas. The blower is design to be of a sufficient size (pressure and flowrate) to perform this elutriation.

[0037] The gas stream with the suspended coal ash particles entrained (24) leaves the contactor and enters a size classifier, which is represented in the flowsheet as one cyclone (200) but can also be a bank of multiple cyclones arranged in parallel. The size classifier is designed to separate the coarse fraction containing carbon from the fine fraction (28).

[0038] The coarse fraction leaving the bottom of the cyclone is split into two streams. The major portion of the stream (26) is returned to the bottom of the solid gas contactor, while the minor portion (30) is the coal ash fuel product. The fuel is recovered in a silo from where it can be transferred towards it eventual utilization. The coal ash fuel obtained is found to have a carbon content of between 40% and 60% and a thermal value between 4000 and 6000 Btu/lb.

[0039] Stream (26) is returned to the solid gas contactor to increase the solids loading in the contactor, which improves the efficiency of separation in the cyclone.

[0040] The fine particle size fraction leaving the cyclone (28) is treated in a dust collector (300) where almost all of the fine fraction is collected (32). The gas leaving the dust collector is typically drawn away by a fan (not shown on the diagram) and depending on the collection efficiency of the dust collector, the gases are exhausted to the atmosphere.

[0041] FIG. 2 is very similar to FIG. 1 but represents an embodiment of the invention where the solid gas contactor has a gas velocity that is reduced in the contactor itself by an increase in the cross sectional area of the contactor or other means. The lower gas velocity will be such that the coarse particles will no longer be elutriated into the classifier and can be collected. The vessel used is a circulating fluidized bed, or a classifying transport vessel and most preferably a toroidal (Torbed™) vessel.

[0042] We note that the elutriating gas (22) can once again be optionally heated by the addition of hot gases (21) at the intake of the blower (400).

[0043] The elutriated portion of the dust once again enters the cyclone system (24) where two fractions are separated. The fine fraction (28) going towards the dust collector.

[0044] The return coarse solid stream (26) from the classifier underflow, once again serves to increase the solids loading in the contactor. The coarse particle stream from the cyclone (26) can be split to obtain another size fraction represented in FIG. 2 as stream (31).

[0045] Furthermore, only one classifier has been represented in FIG. 1 and FIG. 2, but clearly several can be placed in series which would in turn produce multiple size fractions with varying granulometries and carbon %. A process flow diagram of such an arrangement in presented in FIG. 3. This arrangement would be of interest to reduce the size of the dust collector which tend to be more costly than cyclones, and where specific intermediate coarse coal ash size fractions are found to contain high carbon. The main limitation on the number of cyclones would be the size of the blower required, and total equipment capital cost.

[0046] In FIG. 3, we see that multiple size fractions (nn) can be recovered from (nnn) classifiers. Clearly such an arrangement can also accommodate a circulating fluidized bed, or a classifying transport vessel similar to that represented in FIG. 2, with its additional product fuel stream.

[0047] The fuel derived from fly ash thus obtained can be transported towards a combustion system that is suitable for the combustion of fine combustible solids. The fuel is meant to be used in a same way as coal and in installations that include; coal fired heaters, coal fired boilers, cement kilns and a coal burning power generating stations. The means of transporting the fuel include pneumatic or mechanical means.

[0048] Although we have considered the fuel as the coarse fraction its granulometry may be sufficiently fine that dust explosions become a hazard if transported dry and dust clouds are formed. Some combustion systems may require a larger size fuel feed, such as briquettes, to ensure that the fuel be burned in a safe manner. This may mean that the coal ash fuel requires some further treatment before use. Therefore, if the incinerator, power plant or kiln using the fuel, does not have the means to feed this type of fine fuel into its combustion system, an agglomeration step to produce a fuel briquette may be required before the fuel is fed to combustion. This agglomeration or densification step, will increase the fuel value by at least 15% (becoming 4600 Btu/lb. to 6900 Btu/lb) and reduce the difficulties of handling a combustible powder.

EXAMPLE 1

[0049] Pilot tests were conducted on the fly ash from the Trenton Power Plant, whose coal source is a bituminous coal. The granulometry of the fly ash, as well as, the % LOI of the Trenton fly ash are presented in Table 1. The trials were conducted using a pilot scale toroidal vessel (Torbed™) which is a pilot size classifying vessel. 1 TABLE 1 Fly Ash Size Distribution versus Initial % LOI in the sample (Fly Ash from the Trenton Power Plant) Particle Size weight %/ Total % LOI/ Distribution (microns) Initial % LOI fraction fraction >=125 &mgr;m 56.7%  7%  4.0% >=63 &mgr;m 48.5%  9%  4.4% >=45 &mgr;m 29.7%  9%  2.7%  <45 &mgr;m  5.5%  75%  4.1% Totals  100% 100% 15.1%

[0050] The Torbed™ used in the testing, gave the possibility of removing various size fractions of coal ash directly from the vessel. The air temperature of 20° C. was measured at the inlet of the blower. The two coarse fractions, (>=125 &mgr;m and >=63 &mgr;m) were removed separately and then re-combined. These two fractions make up 16.0% of the total ash while accounting for 55% of the total carbon in the coal ash, this is a surprising feature of the invention. The thermal value of the combined fraction was measured at 5372 Btu/lb, and the percentage of carbon in the final fuel product obtained is 52.1% weight percentage. The thermal value of the coal ash fuel obtained, places it in the range typically considered that of lignite coal which is between 4000 and 8300 Btu/lb.

[0051] It should be noted that although a fine fraction (<45 &mgr;m) is also obtained by this process which is lower in carbon than that prior to treatment, the carbon level is not reduced sufficiently to be considered a good source of pozzolanic material for use in concrete. Further treatment of this fine fraction is required if it is to be used in concrete.

EXAMPLE 2

[0052] Similar pilot tests were conducted on fly ash from a Florida power plant which is also derived from a bituminous type of coal. The two fly ash coarse fractions were once again removed and combined, and resulted in a fuel with a weight fraction of 45.4% carbon and a heating value of 5051 Btu/lb. Here it was observed that in a 15%(w/w) of the coal ash, 50.9%(w/w) of the total carbon of the original ash is found. The Florida fly ash properties are presented in Table 2. 2 TABLE 2 Fly Ash Size Distribution versus Initial % LOI in the sample (Fly Ash from the Florida Power Plant) Particle Size weight %/ Total % LOI/ Distribution (microns) Initial % LOI fraction fraction >=125 &mgr;m 58.6  6.5%  3.8%  >=63 &mgr;m 35.3  8.5%  3.0%  >=45 &mgr;m 22.1 10.0%  2.2%  <45 &mgr;m  5.8 75.0%  4.4% Totals 100%   100% 13.37%

[0053] Further tests were conducted with various other coal ashes. The test using a coal ash from a lignite coal source resulted in a fuel product of 1824 Btu/lb for the same particle fraction.

[0054] While a test with a coal ash from a sub-bituminous coal source resulted in a fuel value of 4251 Btu/lb for the same size fraction of >=63 &mgr;m.

[0055] The pilot tests resulted in the selection of the preferred type coal ash source as bituminous coal with the preferred particle size cut at >=45 &mgr;m and preferably >=63 &mgr;m.

Claims

1. A process for producing a fuel from a coal ash having an initial carbon amount, comprising;

a pneumatic separation of the coal ash into at least two size fractions;

the separation taking place in a solid-gas contactor in association with a classifier means, wherein

the two size fractions comprise when separated, a major fine fraction, and

a minor coarse fraction, being the fuel, and containing at least 45% of the initial carbon fuel amount, and

a recovery of the fuel.

2. The process according to claim 1, wherein the initial carbon amount is at least 5% by weight.

3. The process according to claim 1, wherein the initial carbon amount is at least 10% by weight.

4. The process according to claim 1, wherein the initial carbon amount is at least 14% by weight.

5. The process according to claim 1, wherein the fine fraction has a particle size distribution of less than 45 &mgr;m and the coarse fraction has a particle size distribution of greater than or equal to 45 &mgr;m.

6. The process according to claim 1, wherein the coarse fraction has a particle size distribution greater than or equal to 63 &mgr;m.

7. The process according to claim 1, wherein the fuel has a heating value between 4000 and 6000 Btu/lb.

8. The process according to claim 1, wherein the solid-gas contactor is selected from the group consisting of a fluidized bed, a circulating fluidized bed, a transport vessel and a classifying transport vessel.

9. The process according to claim 8, wherein the classifying transport vessel is a Torbed™ toroidal vessel.

10. The process according to claim 1, wherein the separation is conducted at ambient temperature or up to a temperature of 200° C.

11. The process according to claim 1, wherein the classifying means is a cyclone or a bank of cyclones.

12. The process according to claim 11, wherein the classifying means may be placed in series to obtain a number of different size fractions.

13. A fuel which is derived from a coal ash having an initial carbon amount and produced by,

a pneumatic separation of the coal ash wherein the coal ash is separated into at least two fractions;

the separation taking place in a solid-gas contactor in association with a classifier means,

wherein the two size fractions comprising, a major fine fraction, and

a minor coarse fraction, being the fuel, and containing at least 45% of the initial carbon fuel amount, and

a recovery of the fuel.

14. The process according to claim 13, w wherein the initial carbon amount is at least 5% by weight.

15. The process according to claim 13, wherein the initial carbon amount is at least 10% by weight.

16. The process according to claim 13, wherein the initial carbon amount is at least 14% by weight.

17. A fuel according to claim 13, wherein the fine fraction has a particle size distribution of less than 45 &mgr;m and the coarse fraction has a particle size distribution of greater than or equal to 45 &mgr;m.

18. The fuel according to claim 13, wherein the coarse fraction has a particle size distribution of greater than or equal to 63 &mgr;m.

19. The fuel according to claim 13, wherein the fuel has a heating value between 4000 and 6000 Btu/lb.

20. The fuel according to claim 13, wherein the circulating solid-gas contactor is selected from the group consisting of a fluidized bed, a circulating fluidized bed, a transport vessel and a classifying transport vessel.

21. The fuel according to claim 20, wherein the classifying transport vessel is a Torbed™ toroidal vessel.

22. The fuel according to claim 13, wherein the separation is conducted at ambient temperature or up to a temperature of 200° C.

23. The fuel according to claim 13, wherein the classifying means is a cyclone or a bank of cyclones.

24. The fuel according to claim 13, wherein the classifying means may be placed in series to obtain a number of different size fractions.

25. A method of burning a fuel derived from a coal ash having an initial carbon amount and produced by,

a pneumatic separation of the coal ash wherein the coal ash is separated into at least two fractions,

the separation taking place in a solid-gas contactor in association with a classifier means,

wherein the two size fractions comprising, a major fine fraction, and

a minor coarse fraction, being the fuel, and containing at least 45% of the initial carbon fuel amount, and

a recovery of the fuel; and

a conveyance of the fuel to a combustion system.

26. The method according to claim 25, wherein before the conveyance of the fuel to the combustion system there is an optional agglomeration step.

27. The method according to claim 26, wherein the agglomeration step is a densification of the fuel into briquettes.

28. The method according to claim 27, wherein the combustion system is selected from the group consisting of a coal fired heater, a coal fired boiler, a cement kiln and a coal burning power generating station.