FI110266B - A method for gasifying a carbonaceous fuel in a fluidized bed gasifier - Google Patents

Abstract

The invention relates to a process for the gasification of a carbonaceous fuel in a fluidized bed gasifier (1). In the process, a fuel (5), such as coal or peat, and a primary gasification gas (6, 7), such as air, are fed into the gasifier, and a product gas (10) is removed from the gasifier, the product gas being purified by separating fine-grained solid ingredients (16) therefrom. The solids fraction (16) which is separated from the product gas (16) and which contains unreacted carbon is fed together with a stoichiometric excess of air (27) into another fluidized bed reactor (20), which serves as an oxidizer, and the oxygen-containing gas (34) exiting from the oxidizer is directed to the fluidized bed gasifier (1) as a secondary gasification gas. By the increased oxidation, the carbon conversion in the process is increased and a residual ash with a low carbon content is produced.

Description

110266

A method for gasifying a carbonaceous fuel in a fluidized bed gasifier

The present invention relates to a process for gasifying a carbonaceous fuel in a fluidized bed gasifier, wherein the gasifier is supplied with fuel and the primary gasification gas is removed and the product gas is purified from the gasifier to be purified from the

Fuels with high volatile components can be gasified in 10 fluidized beds with high carbon conversion at a sufficient gasification temperature, typically above 900 ° C, and the cyclone separated solids from the product gas stream are recycled to the fluidized bed. In contrast, with less volatile components and less reactive fuels, such as coal and peat, the carbon conversion in the corresponding simple fluidization process is easily below 90%. 15 Due to the problems of ash melting and sintering and the quality of the product gas in the air gasification process, increasing the carbon conversion by raising the gasification temperature is practically impossible.

In the known U-GAS (cf. Mason, DM, Patel, JG, Fuel Processing Technology, 3 (1980), pp. 181-206) and KRW processes, the problem has been solved by creating another fluidized bed at the base of the gasification reactor. a more oxidizing high-temperature zone where the ash partially melts while the surrounding bed operates at a lower temperature. This resulted in a so-called. an agglomerating carburettor (cf. U.S. Patent No. 3,981,690), in which other bed may be selectively removed from the bottom of the reactor; [, * heavy agglomerates containing more ash. However, this method is best suited to oxygen / vapor gasification, whereby the amount of oxygen and vapor feed and the feed points can be selected to create the necessary zones. Air • •:. ' · · In water, the use of water vapor is limited by the reduction of the calorific value of the gas and, moreover, air makes it more difficult to create the necessary high temperature zone. Also this,, ·. the use of this method is confined to fuels with ash agglo ;;; The HTW process of U.S. Patent No. 86075 and prior to it; · Winkler Carburettor (Hebden, D., Stroud, H.J.F., Chemistry of Coal Utilization; Wiley & Sons, pp. 1694-1696) again: ': it was found difficult to achieve sufficient conversion of more than 95% in the carburettor despite dust recycling and high temperature oxygen / vapor gasification. Thus, the dust separated by the cyclone and the filter was considered to be superior to 2 110266 burns in a separate incinerator. Many different attempts to achieve better conversion already in the carburetor itself ended in no avail.

U.S. Pat. No. 4,828,581 is further known as the so-called. Battelle Columbus process using two parallel fluidized bed reactors by circulating hot Fri-5 material from the oxidation reactor to the gasification reactor and separating the product gases of each. The aim is to produce medium-heat product gas without the need for oxygen. The problem with this process is the implementation of the transfer of hot bed materials, which is difficult especially in pressurized applications.

Another common problem with fluidized bed gasifiers, especially with sulfur-containing fuels, is that, due to the reducing atmosphere, airborne dust contains compounds that are unstable in the environment, such as calcium sulfide, which should be oxidized before being released into the environment.

U.S. Patent No. 4,347,064 describes a gasification process using two fluidized bed reactors, each of which functions as a gasifier. The non-gaseous carbon residue of the first reaction step 15 is gasified in a second step at a higher temperature with a mixture of oxygen and water vapor introduced into the second fluidized bed reactor, wherein the oxygen content is at least 30 and usually 40-70% by volume. The publication also recommends feeding fresh fuel to the second gasification step, that is, to maintain the gasification reactions side by side. '.: 20 in a fluidized bed reactor.

...: It is an object of the invention to provide an improved fluidized bed gasification process for: · the above-mentioned less volatile carbonaceous fuels !!; can be gasified at high, more than 90% and, in preferred applications, even more than 95% carbon conversion, while reducing the '25 environmental problems caused by the solid residue in the gasification process. The process according to the invention is characterized in that said second fluidized bed reactor is used as an oxidant by supplying to it a stoichiometric excess of air so that the carbon contained in the supplied solid fraction '... · is completely converted and a gas containing excess oxygen is obtained. which is introduced into the fluidized bed gasifier as gasification gas.

: ..,: 30 In the process of the invention, the carbon that is not degassed in the fluidized bed gasifier is • converted to oxidized carbon monoxide and / or carbon dioxide, which is recovered. gasification, whereby the carbon conversion of the process is substantially increased. In addition to carbon, other environmentally harmful solid components separated from the product gas stream are oxidized before reaching the 3,110,266 ash fractions to be removed from the process. The resulting ash thus has a low carbon content and does not contain significant amounts of environmentally toxic components.

In addition, the fluidized bed oxidation of the solid according to the invention may undergo partial agglomeration of the ash which facilitates the separation of the ash from the gas stream returned to the carburettor 5.

The combustion heat released in the fluidized bed oxidation of the invention can be utilized by recovering it, with the control of the oxidized temperature, by heat recovery directly from the oxidizer or from the gasifier product gas. On the other hand, the carbon dioxide and water vapor contained in the secondary gasification gas 10 produced by the oxidation may partially be reduced to carbon monoxide and hydrogen in the gasifier, thereby contributing to the combustion value of the resulting product gas.

The air supplied to the fluidized bed oxidant as an oxidizing gas can be mixed with water vapor to control the oxidized temperature. The oxidant may have a temperature in the range of about 800 ° C to 900 ° C, most preferably 800 ° C to 850 ° C, sufficient for rapid oxidation of the carbon in the solid, and also the oxidized compounds in the solid, such as CaS, are quite oxidizable. Preferably, excess heat can be recovered from the oxidant by heat transfer to maintain the temperature within said optimum ranges.

If necessary, a sorbent may be added to the fluidized bed oxidant to enhance the sulfur retention and other additives, e.g. to prevent melting of the ash. Part of the acid ···. the ash remaining from the digestion can be removed from the bottom of the fluidized bed oxidant and a portion can be separated from the oxidant into the gas stream to the carburettor by a separate separator such as a cyclone. Due to its higher specific gravity and its possible partial agglomeration, the ash is now more readily distinguishable than the light ash particles containing a high proportion of porous carbon from the product gas stream from the gasifier.

In the process according to the invention, coal, coal, peat and solid biofuels, such as wood, are suitable for gasification. Air may be used as the primary gasification gas for the fluidized bed gasifier. The fluidized bed of the carburetor is preferably formed by a substantially stationary bubbling fluidised bed, but also by a circulating ·· *. the use of a bed reactor as a gasifier is possible. The floating particles of the bed are formed by a finely divided solid such as sand or lime. According to the invention, the secondary gasification gas supplied from the fluidized bed oxidant, which, in addition to the carbon oxides and water vapor, contains excess oxygen remaining in the oxidizing air used, is preferably supplied to the gasifier above the bubbling fluidized bed. The product gas from the carburettor may first be introduced into the cyclone, which has a coarse solids fraction separated from the gas stream, and returned to the fluidized bed of the carburettor, and thereafter through a heat recovery heat exchanger to the filter to provide a finer carbonated carboxylate.

The invention will now be described in more detail with reference, first, to the accompanying drawing, which is an example of an apparatus for applying the gasification process of the invention.

The apparatus according to the drawing comprises a vertical cylindrical reaction vessel 1 acting as a fluidized bed gasifier, provided at its lower end with a grate 2. Above the grate 2 is a substantially stationary bubbling fluidised bed 3 consisting of fine-coiled particles such as sand. In the figure, the addition of bed material 15 to reaction vessel 1 is indicated by arrow 4, fuel to be gasified by arrow 5, gasification gas to be introduced by arrows 6 and 7, e.g., arrow 6 denotes gasification air and arrow 7 to water vapor, and ash removal from reaction vessel 8.

From the space 9 above the fluidized bed 3, near the top of the reaction vessel, the conduit 10 leads to the product gas produced by gasification into a cyclone 11 which separates the gas stream. '.: Solid particles coarser than 20 backgrounds, which are returned via channel 12 to the gas from the cyclone 11, the product gas proceeds down the conduit 13 through the heat: · *: exchanger 14 to the particle separator 15, which is e.g. a ceramic filter. The finer solids fraction separated by the filter 15 exits into the channel 16, and; . the purified product gas continues down the duct 17 to the desired fuel use.

The passage 16 first passes the fine non-reactive carbonaceous solid separated by the filter 15 into a tank 18 from where the solids are fed either downstream of channel 19 directly into the fluidized bed reactor 20 or through channel 21 first to the intermediate vessel 22 and further to the fluidized bed some of the solids can be removed from the intermediate tank (arrow 23). The fluidized bed 20 of the fluidized bed II 30 consists of a vertical cylindrical vessel partially provided with a grate 24. Above the grate 24 is a bubbling fluidized bed 25 of large particles, e.g. sand, lime or lime, and upper space 26. The purpose of the fluidized bed oxidant 20 is to oxidize the carbon contained in the finely divided solids separated by the filter 15 from the product gas stream, as well as the other oxidizing components present 35. The supply of oxidizable gas, such as air, is indicated by arrow 27, optional water vapor supply by arrow 28, addition of bed material by arrow 29, possible small injection of steam or water to cool the resulting flue gas by arrow 30, and ash removal by arrow 31. and 33 to optimize the oxidation temperature, whereby excess heat can be transferred, e.g., for use in the steam circuit of the power plant.

The gas from the fluidized bed oxidant 20, which, in addition to carbon oxides and water vapor, may also contain oxygen due to the excess air used in the oxidation, is led through channel 34 to cyclone 35, which removes ash particles (arrow 36) from the gas stream 10 through the fluidized bed according to reaction space 9 above the fluidized bed 3.

In the process illustrated in the drawing, solid particulate fuel, e.g. coal, peat, wood waste or a mixture thereof, is pyrolysed in the carburettor 1 to form product gas with combustible components, e.g. carbon monoxide, hydrogen and hydrocarbons. 15 At the same time, residual carbon is produced, the gasification of which is slower, and thus the particles that are elutriated from the gasifier with the product gas still contain a large amount of inert carbon. Due to the high carbon content, the particles are relatively light, which results in only the largest particles, typically over 10-20 μιη: η, being separated from the product gas by cyclone 11. The particles separated by the cyclone 11 back into the fluidized bed 3. . The cartridges continue to react and finally grind so small that they are cycloidal; through. The process of the invention does not necessarily require the primary cycle of cyclone 11 and channel 12, but the implementation of the primary cycle '' results in a smaller fluidized bed oxidant 20, which is often costly. · Optimal reasons. It is also advantageous for the quality of the product gas produced that the carburettor has a high content of carbon and ash particles at the top which promotes the decomposition of the tar compounds and the formation of the desired product gas components, carbon monoxide and hydrogen.

• · ·

After cyclone 11, the product gas passes through a heat exchanger 14 whereby the temperature of the gas ·; · * is lowered from a typical gasification temperature (800-1000 ° C) to a level; M.p .: 30-300-550 ° C. At the same time, metal vapors (e.g., alkali metals) in the product gas are condensed and can be removed in connection with the removal of particles. Particulate matter removal. ·. for example, ceramic filters 15, whereby the product gas 17 leaving the process is clean and suitable, for example, as a gas turbine fuel. The separated '···' carbonaceous dust may be introduced directly into the fluidized bed oxidant 20 or 35, alternatively, in some driving situations, all or part of the filter-separated dust may be collected in the intermediate container 22 and subsequently fed to the oxidant.

6 110266 It is also possible to remove dust from the system via the dust extraction line 23, for example in the event of disturbances or shutdowns.

The conditions for the oxidized 20 are controlled by adjusting the air, water vapor and dust feed so that the carbon material contained in the dust is oxidized to carbon oxides. Because the oxidation reactions of the fine-5 split carbon material are significantly faster than the gasification reactions in the gasifier 1, complete oxidation of the carbon occurs within the oxidation time of a few seconds already available. Thanks to the fluidized bed, the oxidized temperature remains stable and heat and material transfer is efficient. Dust from the gasification process separated by the filter 15 contains not only carbon material 10 but also other substances of concern for further use or disposal of the dust, such as calcium sulfide and other environmentally unstable or toxic compounds. In the course of oxidation, these compounds also oxidize and the dust removed by cyclone 35 resembles that of a conventional incinerator. The oxidizer temperature is adjusted to the desired temperature (preferably about 800-900 ° C), either by using a generous amount of air and / or water vapor supply or by removing some of the heat generated by combustion with the optional mattress heat exchanger 32 for use in the steam circuit of the power plant. In the latter case, the oxidant can be used closer to the stoichiometric combustion air coefficient, whereby the oxygen content of the flue gas introduced into the gasifier is typically 1-5%. The performance of cyclone 35 20 is based on the fact that the ash particles leaving the oxidizer 20 are clearly heavier than the porous carbonaceous particles eluting from the carburizer 1.

: '· *; In addition, partial agglomeration of the ash may occur in the oxidizer.

The hot flue gas introduced from the oxidant 20 to the space 9 above the mattress 3 serves as a secondary gasification gas, the oxygen in which reacts with the rapidly flammable compounds in the gasifier. At high temperature, carbon dioxide and v · * water vapor also participate in the gasification reactions in the carburettor and

partly to carbon monoxide and hydrogen. When the oxidant is used below 850-880 ° C

At the temperature, the alkali metals also react mainly as condensed products t. (mainly to alkali sulphides) which can be removed by cyclone in conjunction with particle removal *, 30. If necessary, the flue gas leaving the oxidant can be cooled • «· ';;; 'With optional heat exchanger 33 or small steam or water injection 30.

. L The fluidized beds 3, 25 of the fluidized bed reactors 1, 20 are kept to the desired size by removing coarse ash 8, 31 from the bottom of the reactors as necessary and adding sand, lime or other selected mattress material to the mattress. A small change of mattress material-35 also avoids the accumulation of sticky ash agglomerates in the fluidized beds of the reactors.

7 110266

The performance of the process according to the invention has been demonstrated in experiments according to the following examples, in which bituminous coal and a coal-wood mixture have been gasified using equipment operating at a pressure of 0.34 MPa. When similar fuels were gasified without the oxidation reactor according to the invention, the conversion of elemental carbon contained in the fuel into gaseous products remained at a level of 75-90%, although very high gasification temperatures, which are problematic for ash sintering, were used.

Example 1

The only fuel used in the carburettor was bituminous coal. About 55 wt.% Of the carbonaceous dust separated by the heat filter was fed. The oxidized bed temperature was adjusted to about 940 ° C. The bed material of the oxidation reactor was a 1: 1 mixture of limestone and sand. The carbon content of the cyclone dust separated by the post-oxidation cyclone was 0.55%, while the filter-separated dust fed to the oxidant had carbon content of 65.1%. Thus, the conversion of the oxidized feed filter dust was almost complete. Since not all the dust was recycled, the total conversion of the fed coal remained at 92.4%. However, this too is more than 10 percentage points higher than the conversion achieved with the same coal in conventional gasification. During the period, after the hot filter, concentrations of the vaporous alkali metals (Na, K) and sulfur compounds (H2S, COS) contained in the product gas of the carburettor were measured. The total volatile alkali metals were less than 0.05 ppm (m) and H2S was 220 ppm (vol). ! and a COS of about 20 ppm (vol). In practice, this indicated that the recycling of dust containing sulfur and alkali metals did not increase the level of impurities contained in the filtered product gas with respect to alkali metals or sulfur, but were captured and removed from the cyclone dust.

«· · • · · · ·. *: *; Example 2

58% by weight of sawdust and 42% by weight of bituminous rock were used as fuel for the carburettor. * *: carbon. The carbonaceous dust separated by the carburettor heat filter was completely recycled to oxidized (100% recycle). Oxidized bed material. was the same as in Example 1 and the bed temperature was maintained at 895 ° C. Oxidized heart rate, ···. The carbon content of the ion-separated dust was 0.25 wt.%, While the carbon-separated content of the hot filter applied to the oxidant was 57.4 wt.%. Conversion of carbon to gas: 96% calculated by gas analysis and more than 99% based on the carbon content of the solids removed from the system. The corresponding conventional gas and wood gasification has achieved levels of 82-87%. During the test period measurements as in the previous experiment: vaporous alkali metals (total Na and K) less than 0.05 ppm (m), H2S 210 ppm (vol) and COS 20 ppm (vol).

Example 3 In this experiment, more hot filter dust was fed to the oxidant than process-5 generated during the trial period. This simulated a case where dust had previously been collected in a warehouse for later recycling. The fuel for the carburettor was 86% by weight of sawdust and 14% by weight of bituminous coal. The recycling rate of the filter dust was 112 wt%. The same sand / limestone mixture as in the previous examples was used as the bed material of the oxidant and the bed temperature of the oxidant was adjusted to 850 ° C. The carbon content of the cyclone-separated dust in the oxidant was 0.3 wt%, while the carbon content of the oxidised feed filter dust was 34.0 wt%. The conversion of carbon to gas calculated by solid flows was over 99%. The vaporous alkali metal contents measured after the carburettor product gas filter were less than 0.05 ppm (m), H2S 180 ppm (vol), and COS 20 ppm (vol).

No agglomeration in the oxidant bottom ash was observed during the test runs. After the test runs, the reactor was opened and examined for possible deposits. No deposits were observed. The high carbon dioxide content of the gas produced was a natural consequence of the high heat losses typical of a small-scale tester. In order to maintain the internal temperature levels of the reactors, more than • 120 of the fuel had to be burned. Key Process Variables for Example Experiments and • 1; · The gas properties measured are given in Table 1 below.

»« · · «• · 1 • t · • 1 • 1 1 t ·> · · i 1 · • · i t II» II »» · · 110266 9

table 1

Esimerkki__1__2__3

Pressure, MPa (abs.) __ 0.34__0.34__0.34

Carburettor Air Factor 0.35 0.40 0.36

Kokonaisilmakerroin__0,60__0,67__0,52

Carbon mass flow rate, g / s 4.7 2.3 1.1

Wood debris flow, g / s - 2.9 6.2

Carbon / Wood ratio,% by weight 100/0 42/58 14/86

Carburettor mattress feed, g / s 0.36 0.20 0.24

Input of oxidized mattress material, g / s 0 0 0

Gasification air supply, g / s 15.3 15.2 16.1 Steam supply to the gasifier, g / s 2.8 2.4

Air supply oxidized, g / s 10.7 10.5 7.6

Oxidation fluidization rate, m / s 0.33 0.33 0.22

Dry product gas composition by volume CO 8.8 6.5 8.7 CO2 13.3 15.4 13.9 H2 6.5 5.0 5.6 N2 (+ Ar) 70.7 71.9 69, 6 CH4 0.62 1.2 2.0 c2Hy <0.01 o, os 0.32 ·; H2S__0.02 0.02 0.02 ["H20 in wet gas, vol% __ 13.9__15.6__9.8

Lower calorific value for dry product gas, 2.2 2.0 3.1 v: MJ / m3n ____ · Cyclone dust carbon content, wt% __ 0.55__0.25__0.30 * *

Claims (9)

10 110266 A method for gasifying a carbonaceous fuel in a fluidized bed gasifier (1), comprising feeding fuel (5) and a primary gas (6, 7) to the gasifier and removing product gas (10) from the gasifier by separating 5 further feeding with an oxygen-containing gas to a second fluidized bed reactor (20), the gas of which is introduced into the fluidized bed gasifier (1) as a secondary gasification gas, characterized in that said second fluidized bed reactor (20) is used as an oxidant by supplying stoichiometr complete conversion to obtain a gas containing excess oxygen which is introduced into the fluidized bed gasifier (1) as a gasification gas. Method according to claim 1, characterized in that the solid fraction (16) is separated from the product gas by filtration (15). Process according to claim 1 or 2, characterized in that the temperature in the fluidized bed oxidizer (20) is about 800-900 ° C. Method according to one of the preceding claims, characterized in that a desulphurisation sorbent is supplied to the fluidized bed oxidant (20). A method according to any one of the preceding claims, characterized in that the oxidized (20) is separated from the fine gas (36) from the exhaust gas (34) before •. 20 conducting gas to the fluidized bed gasifier (1). I t Process according to any one of the preceding claims, characterized in that ': the fuel (5) is coal, peat or solid biomass. ;!; Method according to one of the preceding claims, characterized in that the primary gasification gas (6) contains air. f I · A method according to any one of the preceding claims, characterized in that; the fluidized bed gasifier (1) has a bubbling fluidized bed of solid particles; T: (3) · . ' A method according to any one of the preceding claims, characterized in that: - before separating the oxidizable solid fraction (16) from the product gas (10) which has left the gasifier (1), a coarser solid fraction (12) is recovered and recycled. * suttime. • ·> *> 110266