CA1062915A - Production of synthesis gas - Google Patents

Abstract

ABSTRACT:
A process for the fluid-bed gasification of ash-containing carbonaceous solids with steam to produce synthesis gas wherein heat is supplied to the gasification zone by passing therethrough a stream of agglomerated ash particles, formed by heating ash particles derived from the carbonaceous solids in a combustion zone at a temperature sufficient to render the ash particles tacky and to cause them to undergo agglomeration. The agglomerated ash particles are formed in a separate fluid-bed agglomerating combustion zone and the resulting agglomerated ash particles introduced into the main combustion zone where they are maintained below agglomerating temperatures but at sufficient temperatures to sustain the gasification zone.

Description

~ FMC 5526 ~6Z9lS ~
This invention relates to a process for the fluid-bed gasification of ash-containing carbonaceous solids with steam to produce synthesis gas.
The world's supply of recoverable gas and petroleum, at present levels of consumption, cannot last much beyond the year 2000. Coal reserves, on the other hand, are relatively abundant. Coal and lignite, for example, contain 55.9 x 10l5 thermal kilowatt-hours of energy and represent 88.8 percent of the energy obtainable from the world's initial supply of recoverable ~ossil fuels. Even if present coal production of 3 billion metric tons per year should double three times, coal supplies would lask about 300 years. In a modern industrial society such as the U.S., 95.9% of the energy consumed (19693 comes from the burning of fossil fuels; 20.0% from coal and 75.8% from gas and oil. Non-fossil fuel sources include 3.8% hydropower and 0.3% nuclear. Although the use of atomic power is increasing, it is anticipated that fossil fuels will continue to be the principal source 20 of energy for the remainder of this century. ~ -In order to relieve the enormous drain on petroleum resources~ more use of coal is being strongly advocated.
It is the most plentiful of the fossil fuels, and con-stitutes a prodigious reserve of energy. However~ the burning of coal results in severe atmospheric pollution ;~
due to the release of sulfur dioxide and particulate -matter. If it is to contribute significantly as a ~-world energy source, coal must be substantially freed of ; -its pollution causing elements.
One method of upgrading coal into a clean energy ~ 1~629~5 .,..~,,, fuel regards coal as a raw material for the manufacture of synthetic gas and oil. In this approach, finely divided coal is heated at successively higher tempera-tures in a series of fluid-bed reactors to give a medium btu gas (about 500 btu), tar and devolatilized coal particles or char. On hydrotreatment in a catalytic reactor, the tar gives a synthetic crude oil which can `
be refined in the usual manner. A detailed description of the process is disclosed in U. S. Patent 3,375,175 to Eddinger et al. This char constitutes an excellent grade of carbonaceous solid for fluid-bed steam gasifica-tion to form synthesis gas containing hydrogen and carbon monoxide which can be catalytically methanated to give methane commonly referred to as substitute natural gas (SNG). Appropriate measures must be taken to remove atmospheric pollutants such as sulfur dioxide, ammonia and ash residues. Such upgrading of coal, although embodying certain known technQlogy, has yet to be realized on a scale capable of providing oil and gas in quantities sufficient to ease the acute shortage of natural petroleum products.
One of the major difficulties in coal conversion processes, as above described, occurs in the gasification stage wherein a bed of fluidized char is contacted with steam to form synthesis gas. The heart of the problem .
is providing enough heat to sustain the gasification reaction which is highly endothermic.
Desirably, the heat is supplied by burning a portion of the char and various means of achieving this have been proposed and tried. For instance, oxygen has been mixed - ~. ' : ~ . ' ~062915 -~ ~
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with the steam in order to combust some of the char in the gasifier. ~owever, the use of oxygen is economically ob~ectionable. Air can be substituted for pure oxygen but the synthesis gas is then contaminated with nitrogen. In ;
another approach, a recycle stream of char is withdrawn from the gaisfication bed and partially combusted to raise its temperature to such a point that on reintroduction to `~
the gasifier along with fresh char, a substantial portion of the heat of reaction is provided. Such a scheme is disclosed in U. S. Patent 3,440,117 to Patton et al.
One of the drawbaoks to using recycle char streams for heating fluid-beds is the attritional generation of fines which tend to be blown out of the gasifier with the synthesis -~
gas along with the fines formed by breakdown of char -particles due to reaction with steam. The fines either -are lost or must be recovered and returned to the system thereby contributing to increased operation costs. More- ~- -over, the build-up of fines necessitates reduction in gas velocity in the reactor resulting in an over-all lower throughput.
Another proposal for heating a gasification zone is set forth in U. S. Patent 3,171,369 to Stephens et al.
In this scheme a portion of the char is burned in a com- -bustion zone at just below the incipient fusion temperature of the ash formed during combustion. The hot ash particles, being tacky, adhere to one another and form agglomerates which are withdrawn from the combustor and introduced into the gasifier to supply heat to the highly endothermic ~;
steam-carbon reaction. After emerging from the gasifier, the ash agglomerates can be recycled to the combustion ""' ' zone for reheating. The advantages of the Stephens et al method over the prior methods of supplying heat to a steam gasification zone are: (1) it does not contaminate the synthesis gas with nitrogen as occurs when air is admitted to the gasifier to burn a portion of the char; (2) it is less costly than processes that use oxygen in place of air to avoid nitrogen dilution; (3) it is not sub~ect to the high carbon losses which occur when a recycle char stream is heated by contact with hot combustion gases as disclosed in the previously cited Patton et al patent;
(4) it utilizes a by-product particulate heat exchanger, namely fused ash, with concomitant savings in raw material costs; and (5) the circulating fused ash is mechanically superior to recycle char which undergoes excessive ~ -attrition thereby engendering the formation of carbon fines which must be recovered or confined to avoid further `
carbon losses and atmospheric pollution.
Although it constitutes an improved concept in heating fluid-bed carbon gasifiers, the Stephens et al process is not commercially feasible. Its chief drawback resides in failing to provide a practicable means of balancing ^
the heat requirements of the gasifier, on the one hand, and the combustor, on the other. The need and reasons for maintaining such heat balance is discussed below.
In the Stephens et al process of fluidiæed carbon gasification, there are two separable heat requirements for gasification; one for supplying heat to the endothermic carbon-steam reaction and the other for agglomerating char ash to form a recycle heat carrier between the gasifier and the combustor. In order to maintain the gasifier ., . , ~ . . , , ~ .

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1~:9629:~5 temperature at a given level, either the recycle rate of heat carrier must be varied or the temperature of the primary combustor must be varied. Varying the circulation rate of heat carrier is not practical as this would require large complex valves and associated control systems. Varying the temperature of the primary 1;, . .
combustor is practical, provided the combustor is `
operated below the fusion temperature of the char-ash heat carrier. However, to control the agglomeration of char-ash and to produce a desired particle size for use as heat carrier requires a precise control of temperature ;
and other operating variables. Either the gasifier temperature will swing undesirably, causing inefficiencies, or the combustor temperature will vary leading to defluidiza-tion or to inadequate agglomeration. Stephens et al propose control of combustor temperature by injection o~ water but such an expedient is both wasteful and inefficient.
So far as we are aware, the utilization of agglomerated ash for supplying heat to a steam-carbon gasi~ier has not been achieved on a practical scale.
In accordance with the present invention there is provided an improvement in the fluid-bed gasification of ;
ash-containing carbonaceous solids with steam to produce synthesis gas wherein heat is supplied to the gasification zone by passing therethrough a stream of agglomerated ash particles formed by heating ash particles derived from the carbonaceous solids in a combustion zone at a temperature sufficient to cause agglomeration of tacky ash particles; a substantial improvement is reallzed wherein -~
the agglomerated ash particles are formed in a separate ~. ,~ ' '''.

1~2g~5 `' agglomerating combustion zone and the resulting agglomerated ash particles introduced into a main combustion zone where they are maintained below agglomerating temperatures but at sufficient temperatures to sustain the gasification zone.
The present process is an improvement in the known process of producing synthesis gas from solid carbonaceous materia].s in a fluid-bed. In such present process, the carbonaceous raw material, of the size such that lt can be suspended in a gas stream to form a suspended bed of solids surrounded with gas which acts like a fluid, is reacted with steam in accordance with the following equation: C ~ H20 ~ CO + H2. An excellent source of carbonaceous material is the char produced by the pyrolysis of finely divided coal in a series of fluid-bed reactors at successively higher temperatures until all the tar forming components have been removed from the coal as described in the previously cited U. S. Patent 3,375,175.
The temperature at which the steam-carbon reaction takes 20 place is generally from about 1600F (871C) to about 1800F (982C), pre~erably about 1600F (871C) to 1700F
(971C) using the char material of the patent.
The steam-carbon reaction is exceedingly endothermic, requiring about 2700 calories per gram of carbon. Part of this heat can be supplied from the superheat put into the steam used in the process, as reactant and as fluidizing gas for the bed of carbonaceous material, but large amounts of additional heat are needed.
In accordance with this invention, such heat is supplied by introducing into the gasification zone, a .

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circulating stream of agglomerated ash particles which -~
have been heated to about 1900~ (1038C) by contact with ;;
the hot combustion gases produced by burning a separate source of char, preferably the char fines which are -expelled from the gasifier. The agglomerated ash particles, in passing through the gasification zone, transfer their heat to the reacting system, from whence they are conveyed `
back to the combustion zone of burnlng fines to be reheated for another passage through the gasification zone and so on in a continuous stream between the two stations.
The agglomerated ash particles are produced by burning -a portion of the char in a separate agglomerating combustion zone at temperatures sufficient to form partially fused or tacky ash particles which undergo agglomeration.
~ eferring to the single figure drawing, there is pro- -vided a gasifier 10 in which a bed 12 of char is maintained on a grid 14 by a fluidizing stream 16 of superheated steam. The char is fed into bed 12 via entry port 18. `~
In gasifier 10, the char reacts with steam to form 20 a gaseous mixture consisting mostly of synthesis gas (CO -and Hz) but also containing some C02 and H2O. These gases, carrying entrained solids, pass through internal cyclone 11, and then are exhausted through line 20 to external cyclone system 22. In the cyclones, the entrained solids are separated from the synthesis gas stream which exits from the process through line 24. The larger solids are returned to the reactor bed by the cyclone - ;
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system, while the finer solids are withdrawn through line 26 to combustion chamber 28.
The solids fed to combustion chamber 28 comprise .

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11)6Z915 ... .. .
the finest solids coming from the fluid-bed in gasifier 10, since the synthesis gas stream picks up this fines fraction selectively. By burning them, fines are prevented -from building up in the system, thus reducing the load on the cyclone system, permitting a much smaller capital investment and less maintenance in this area. At the same time, selective removal of fines stabilizes the size consist of the bed solids to a larger size consist, per-mitting a high throughput of gas without excessive losses of solids from the bed.
Combustion of the char fines in combustor 28 is effected with air, preferably preheated to about 400F (204C) and supplied from line 30. Steam conveys agglomerated ash pel-lets from the lower portion of gasifier 10 via line 17 to combustor 28 where the particles are m~aintained at a temperature of about 1900~ (1038C) to 2100~ (1149C).
The heated particles emerge from the top of combustor 28 and are propelled upward with combustion gases through lift tube 19 into separator 23 and then drop down through separator exit tube 27 and into gasifier 10. The heated agglomerated ash pellets cascade down through the bed of fluidized char particles imparting heat thereto and exiting from ~:
the bottom of the gasifier and so back to the combustor 28 for a complete cycle.
Combustion gases and attrited ash particles formed ;~
in combustor 28 move up lift tube 19 into separator 23 where they are disengaged from circulating agglomerated ash pellets and exit from the top of separator 23 via line 32 into primary cyclone 34. Refractory fines from cyclone 34 are recycled through line 25 to combustor 28 -8- ;.
..:

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while combustion gases and attrited refractory not removed ~ :
by primary cyclone 34 enter secondary cyclone 38 through .
line 36. Fly ash and combustion gases are discharged from secondary cyclone 38 and the refractory fines are conveyed ~ :
via line 40 to agglomerator combustor 43 which is heated ~ ~ .
to a sufficiently high temperature to effect agglomeration :
of incoming refractory fines to produce agglomerated ash ::
pellets. Heat for agglomerator combustor 43 is provided . :
by burning char fines in cyclone furnace 45. Char fines 10 enter the furnace through conduit 49 which picks up the ~ -fines at ~uncture 52 of char fines supply line 26. I.ine ..
50 is a source of preheated air ( 400F, 204C) which supplies combustion air via take-off line 56 to cyclone furnace 45. Combustion gases from cyclone furnace 45 are conveyed through connecting pipe 59 to neck-portion 61 of agglomerator combustor 43. The heated agglomerated ash pellets pass out of agglomerator combustor 43 through the bottom of neck-portlon 61 and are carried to primary `
combustor 28 by make-up refractory line 64. The hot 20 ~flue gases exiting from cyclone 65, located within : .
agglomerator combustor 43, are led via line 67 into line ~ .
30 which supplies preheated air for primary combustor : :
.: .
28. Line 50 divides at junction 70 to form tempering air .
line 54 and preheated air line 30 to combustor 28. Waste slag from cyclone furnace 45 exits through line 73.
In the description aforesaid cyclone furnace 45 can be dispensed with and the char fines burned directly in fluidized bed agglomerator 43. The agglomerated ash pellets drop to the bottom of the vessel and exit from 30 boot section 61. Although the detailed description herein -'-.,', 9~

~L~629~5 is based on forming and heating the agglomerated ash pellets in a fluid-bed, a transport heater can be substituted for fluid heating.
Example 17 ~ 900 lb/min ( 8119 ~ ~4 Kg/min) of make-up char produced in accordance with U. S. Patent 3 ~ 375 ~ 175 at 1000F (538C)~ containing 15 percent ash or 2710 lb/min (1229~25 Kg/min), (feed stream 18 of the drawing) is fed to the fluidized bed 12 of the gasifier 10. The tempera-ture of the bed of char is 1600F (871s)~ and the pressure is 6 atmospheric (atm.) absolute (normal range is ~ r 4 to 10 atm. absolute). In the gasifier, the char mass i3 fluidized by 22~800 lb/min (10342~08 Kg/min) of steam entering at 1000F (538C)~ The exit gases (stream 24) contain 18~290 lb/min (8296~ 34 Kg/min) of C0 plus hydrogen (mole ratio 0. 627 moles C0:1 mole hydrogen) along with unconverted steam (5986 lb/min, 2715 ~ 25 Kg/min) and an equilibrium amount of C02 (16~420 lb/min, 7448~11 Kg/min) as defined by the chemical reaction H20 + C0 = C02 + H2.
The fluidized-bed gasifier provides an efficient `~
means of mixing the steam and the particulate char to effect the water-gas reaction, and simultaneously to elute the fine portion of the char from the bed. These fines are separated from the product gases in a series of cyclones, the last of which will collect the finest portion of the char which is preferentially used as fuel for the process.
These fines contain about the same concentration of ash as the char in the gasifier; about 32% by weight. Thus, when the char fines are burned completely, all of the ash introduced with the feed (2710 lb/min, -1229~26 Kg/min) is .

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removed from the system either as fly ash with combustion gases from the primary combustor 28 or as molten slag from ~
the cyclone furnace 45. ~ ;
To supply the necessary heat for the water-gas reaction in the gasifler, a refractory stream made up of char-ash agglomerates is recycled from the gasifier 10 to the primary combustor 28 by line 17 at a rate of 666,667 lb/min (302,400.15 Kg/min). The recycle refractory stream is heated to 1900F (1038C) by the combustion of 4950 lb/min 10 (2245.32 Kg/min) of carbon in 7800 lb/min (3538.08 Kg/min) of char fines fed to the combustor 28 from line 26.
: -. .
50,400 lb/min (22861.44 Kg/min) of preheated air from line 30 and 6667 lb/min (3024.15 Kg/min) of combustion products from the agglomerator 43 are combined in line 30 at 400F (204C) and used to fluidize the recycle refractory and to combust the char fines in the primary .-...., .:.. ,.~ ..
combustor. The heated recycle refractory is carried over-head by combustion gases in line 19 to the separator 23, where the refractory is separated and returned to the gasifier ;
by line 27.
During the recycling of the refractory, attrition causes a breakdown of the particles to fines. In addition, combustion of the char fines is incomplete in the primary combustor. A ma~or portion of the refractory fines and char is removed from the combustion gases by cyclone 34 and returned to the primary combustor. Additional refractory fines and char-ash fines are separated in cyclone 389 with a balance of the fine solids exiting as fly ash at a rate of 2542 lb/min (1153.05 Kg/min) with the combustion gases. The amount of fines separated by .

6;~915 cyclone 38 is controlled to equal the amount of makeup refractory required to of~set the attrition losses, 6667 lb/min (3024.15 Kg/min). These fines are carried by line 40 into the agglomerator 43, which operates at a '~
temperature of about 2000F (1093C) where the char is derived from a typical high volatile, bituminous B coal such as that obtained from the Illinois No. 6 seam. The agglomerator is heated by the combustion of char fines, taken as a slipstream from line 26 at ~unction 52, carried to the cyclone furnace 45 by line 49. Products of combustion from the cyclone furnace are tempered with air to control the temperature of the agglomerator.
Burning about 400 lb/min (181.44 Kg/min) of char fines with 4030 lb/min (1828 Kg/min) or air in the cyclone furnace, followed by tempering with 2300 lb/min (1043.28 Kg/min) of air will provide a temperature of 2000F (1093C) in the agglomerator. The agglomerator is operated to produce a ;~
makeup refractory stream of 1/4- to 3/8-inch (.63 cm to .93 cm) diameter ash agglomerates. These are conveyed through line 64 to primary combustor 28 for reheating.
About 174 lb/min (78.93 Kg/min) of ash is removed from the cyclone furnace as molten slag.

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Claims (4)

The embodiments of the invention in which an ex-clusive property or privilege is claimed are defined as follows:

1. The fluid-bed gasification of ash-containing carbonaceous solids with steam to produce synthesis gas wherein heat is supplied to the gasification zone by passing therethrough a stream of agglomerated ash particles formed by heating ash particles derived from the carbona-ceous solids in a combustion zone at a temperature suf-ficient to render the ash particles tacky and to cause them to undergo agglomeration, the improvement wherein the agglomeration is effected in a separate agglomerating combustion zone maintained at agglomerating temperatures and the resulting agglomerated ash particles introduced into the main combustion zone where they are maintained below agglomerating temperatures but at sufficient temperatures to sustain the gasification zone.

2. The process of claim 1 wherein the carbonaceous solid particles are char.

3. The process of claim 1 wherein the temperature in the combustion zone is 1900°F (1038°C) to 2100°F (1149°C) and the temperature in the gasification zone is from 1600°F (871°C) to 1800°F (982°C).

4. The process of claim 1 wherein the heat for the combusion zone and the agglomerator is produced by burning char fines from the gasifier.