ATMOSPHERIC PRESSURE, ENTRAINED FLOW, COAL GASIFICATION RE-VISITED

Transcription

1 Abstract ATMOSPHERIC PRESSURE, ENTRAINED FLOW, COAL GASIFICATION RE-VISITED William L E Davey Ph D, Pr Eng, C Eng AECI Engineering (Pty) Limited, Pinelands, Modderfontein, South Africa Edward L Taylor M Sc (Chem Tech), Pr Eng and Michael D Newton B Sc Kynoch Fertilizer (Pty) Limited, Modderfontein, South Africa Paul S Larsen P.E. and Paul S Weitzel P.E. Babcock & Wilcox, 20 S Van Buren Avenue, Barberton, Ohio, United States of America In order to keep abreast of the trend in modern ammonia technology towards lower energy consumption, Kynoch Fertilizer Limited and Babcock & Wilcox have developed a new atmospheric pressure coal gasifier. The new gasifier called the Advanced Coal Gasification Process (ACGP) is intended to replace an ageing set of Koppers Totzek units at Kynoch s plant at Modderfontein, South Africa. Atmospheric pressure operation was chosen in this instance as the change to commercial pressurised gasification had been found to be uneconomic. This paper compares the energy balance of a gas production island incorporating the ACGP with that of a typical dry feed entrained flow pressurised gasification process. The shaft power required by the ACGP was found to be 17% greater than that for a pressurised gasifier for similar gas delivery conditions. However, the waste heat recovered by the ACGP covers about 106% of its shaft power requirements compared to about 94% for the pressurised process. Thus the gas production island would be a net power importer if operated at elevated pressure, and a net power exporter if operated at atmospheric pressure. This finding, together with other advantages, such as lower corrosion potential, lower pollution potential, and lower cost, it is argued, could bring about renewed interest in atmospheric pressure coal gasification. Introduction As South Africa has not yet found a competitive supply of natural gas it has had to rely on its cheap and plentiful coal resources as a feedstock for its chemical industry. Kynoch Fertilizer (Pty) Limited, a South African fertilizer company, has successfully operated a world scale coal based ammonia plant at Modderfontein, South Africa, for over 24 years. In a world now dominated by gas based ammonia Kynoch has been concerned for the past ten years about the high energy consumption of its coal based plant. A meticulous review of all of the technologies incorporated in this plant has resulted in a revamp strategy by which the energy consumption may be reduced to a level competitive with gas based ammonia. Key to this revamp strategy is the replacement of its ageing Koppers Totzek (KT) atmospheric pressure entrained flow coal gasifiers with atmospheric pressure gasifiers of an advanced design, incorporating greatly improved recovery of waste heat. 1

2 Comparison of its Advanced Coal Gasification Process (ACGP) with currently available dry feed entrained flow pressurised gasification technologies suggests that the disadvantage of the greater power required to compress the raw gas may be outweighed by other advantages of atmospheric pressure operation. The comparison between atmospheric pressure operation and elevated pressure operation forms the subject of this paper. Kynochs history in coal based ammonia Kynoch, a wholly owned subsidiary of the AECI group of companies in South Africa, has been manufacturing ammonia from coal at its Modderfontein site for over 60 years. Its first plant, commissioned in 1936, made 12 t/day ammonia and was later upgraded to 80 t/day. This plant employed vertical retort type gas generators fed with graded metallurgical coke. They were blown in alternate cycles with a mixture of steam and air, and then with pure air. When blown with pure air the flue gas produced was vented. When blown with air and steam a raw gas was produced which, after CO-shift and carbon dioxide removal, provided the 3:1 hydrogen/nitrogen mixture for ammonia synthesis. Similar plants are widely used in China today. The second plant, commissioned in 1956, made 150 t/day ammonia and was later upgraded to 330 t/day. This plant employed the same cyclic gas generators as the previous plant but, owing to the scarcity of metallurgical coke, these generators were fed with semi-coke made from sub-bituminous coal in a Lurgi low temperature carbonisation plant. These gas generators were later converted to continuous operation by blowing with steam and oxygen. The third coal based plant, commissioned in 1974, was designed to produce 940 t/day ammonia and 60 t/day methanol. This plant broke with tradition by employing Koppers Totzek atmospheric pressure entrained flow coal gasifiers which were blown with oxygen. Whereas the first and second plants have now been demolished the third plant is still in operation and today produces t/day ammonia and 70 t/day methanol. This plant is, to the best of our knowledge, one of the largest and most successful coal based ammonia plants in the world to day. Coal is fed to the Coal Preparation Unit where it is pulverized to a particle size of less than 90 microns and dried to a total moisture content of less than 2%. Air is fed to the Air Separation Unit where it is separated into pure oxygen and nitrogen. The oxygen is mixed with the pulverized coal and fed to the Gasification Unit where it is largely converted to carbon monoxide, hydrogen and carbon dioxide, referred to as raw gas. Flyash and soot are washed from the gas with water in the Dust Removal Unit before being compressed from atmospheric pressure to about 3 MPa. These units together constitute the gas production island. Downstream units process the gas into ammonia. When this plant was first built it produced ammonia at low cost. To-day improvements in technology, and the ready availability of cheap natural gas elsewhere in the world, have eroded this advantage. Recognizing this fact, Kynoch began in 1988 with an intensive 2

3 study into how this plant could be restored to full international competitiveness. Kynoch s initial effort was focused on retrofitting the plant with pressurized coal gasifiers. By 1991 it had become clear that the change to a commercially proven pressurised coal gasification process could not be justified economically. Against this background Kynoch turned its attention to the improvements which could be made to its proven atmospheric pressure coal gasification process. Babcock & Wilcox history in coal gasification The Babcock & Wilcox Company (B&W), a 130 year old company, first became involved with coal gasification in the early 1950 s by supplying an atmospheric pressure, entrained flow, oxygen and steam blown slagging gasifier to the U.S. Bureau of Mines at Morgantown, West Virginia, USA. At about the same time B&W was involved in the supply of a semi-commercial sized gasifier to E.I. Du Pont de Nemours in Belle, West Virginia, USA. This gasifier was followed with the supply of a commercial sized gasifier installed at the same location. The Du Pont gasifier went into service in 1955 and had operated for approximately two years when inexpensive natural gas became available at the Belle plant and made producing synthesis gas from coal uneconomic. In the mid-1950 s B&W performed engineering studies and experimental work on air blown, slagging type, entrained flow gasification for combined gas turbine/ steam turbine cycles. This work resulted in a project jointly operated with General Electric Company over a three year period in the 1960 s at B&W s Alliance Research Center. In 1976 B&W constructed a gasifier for the Bi-Gas Pilot Plant at Homer City, Pennsylvania, which was sponsored by the United States Department of Energy (DOE). In 1980 B&W teamed up with Koppers Company Inc. to form a joint venture known as KBW Gasification Systems Inc. The outcome of this joint venture was the KBW gasifier, described in earlier papers. This gasifier was to have been the gasifier of choice for seven projects awaiting loan and price guarantees from the U.S. Synfuels Corporation. However these projects all lapsed when the oil crisis of that era was resolved. The knowledge gained from the development of the KBW gasifier has formed the basis for the development of the ACGP gasifier for Kynoch Limited s coal based ammonia plant at Modderfontein, South Africa. An important feature of the development of the ACGP gasifier has been the extensive use by B&W of computational fluid dynamics (CFD) to model the complex interaction between geometry, reaction kinetics, fluid dynamics and heat transfer in the gasifier furnace. B&W is a world leader in the modeling of boilers using their proprietary code called COMO. This code was successfully adapted to the process of gasification and has been used to find the optimum location for the burners in the furnace wall, to develop the optimum design for the burners, and to investigate temperature profiles and velocity distributions over the height of the furnace. By using this modeling tool insights have, for the first time, been gained into the nature of the gasification process which could not have been gained any other way. 3

4 B&W also has a considerable amount of practical experience and theoretical knowledge with similar equipment in related fields. The combination of this experience and knowledge with Kynoch s experience in operating its Koppers Totzek gasifiers, has resulted in a new design for an atmospheric pressure entrained flow gasifier which in the authors opinion will be both robust and reliable, and will exhibit high carbon conversion. The Koppers Totzek coal gasification process The Koppers Totzek coal gasification process is depicted in figure 1. It comprises an ellipsoidal vessel, refractory lined on the inside and enclosed by a water jacket on the outside. Pulverized coal from the service and feed bins is mixed with oxygen in a mixing head and then blown into the gasifier, at each end, at high velocity, through blowpipes. The oxygen at first oxidizes the coal yielding temperatures, close to the burner mouth, in the vicinity of o C. Once the oxygen, with a stoichiometric ratio of less than 0,5, has been consumed the endothermic gasification reactions, of carbon dioxide and water with carbon, take over bringing the gas temperature down rapidly to between o C and o C. The composition of the resulting gas is determined almost completely by the shift reaction equilibrium, just as in steam reforming of natural gas. The hot raw gas leaves the gasifier through the neck where it is quenched with water to a temperature of less than 900 o C. The gas is cooled first in a radiant boiler, which is simply a jacketed pipe, then in the so called Lentjes boiler which provides the gas turn, and finally through a tubular boiler. The dust load carried by the gas is removed from the gas stream by water washing in a column approximating a void column, followed by Theissen Washers which promote intimate contact between gas and wash water by the use of rotating bars and sprays. Dust removal is completed in a wet electrofilter. Some of the ash in the coal melts at the gasification temperature and finds its way to the refractory lined gasifier walls. The molten ash runs down the gasifier walls and leaves via the slag tap hole, from where it falls into a water seal tank. The slag freezes instantly on entering the water, forming granules which are removed from the tank by a submerged chain conveyor. Of the waste heat available from the gasification process approximately one third is recovered as saturated steam at 250 kpa generated in the gasifier cooling jacket, one third is recovered as saturated steam at 5,5 MPa generated in the radiant, Lentjes and tubular boilers, and one third is quenched away with water in the gasifier neck and subsequently lost to the wash water cooling tower. One Koppers Totzek gasifier will produce about m 3 n/h of CO + H 2, enough for about 200 t ammonia /day, and Kynoch currently run six gasifiers of this design in parallel. The Advanced Coal Gasification Process (ACGP) 4

5 An eighteen month design study between Kynoch, AECI Engineering and Babcock & Wilcox has resulted in the proposed replacement gasifier depicted in cross section in figure 2. It is based on a standard B&W boiler design and resembles a tall slender column of square cross-section, the walls of which comprise membrane panels resulting in a gas tight, fully water cooled, enclosure. (Membrane panels are vertical tubes located adjacent to one another and interspersed with narrow metal strips in an all welded construction). Water is circulated through the gasifier walls by natural circulation from a steam drum mounted above the gasifier, and from which saturated steam is withdrawn for superheating. Pulverized coal and oxygen are injected near the base of the gasifier through eight burners. Whereas the process of coal gasification is identical to that occurring in the KT gasifiers the gasification chamber, in line with modern boiler design, is several times larger. There is no necessity for a water quench and the gas is cooled by radiation to the gasifier walls as it travels up the gasifier column. The lower third of the gasifier walls are refractory lined and the upper two thirds are bare. At a temperature below which the entrained flyash is no longer sticky the gas then passes through several banks of superheaters. At the top of the gasifier the gas is turned through 180 o and flows down through an economizer. The gas leaves the gasifier enclosure at a temperature of about 200 o C. The proposed new gasifier combined with coal preparation equipment and dust removal equipment, as illustrated in figure 3, comprises what is referred to in this paper as the Advanced Coal Gasification Process, or ACGP. The new process exhibits the following features: A CO + H 2 yield of close to m 3 n/tonne dry and ash free coal, with a CO:H 2 ratio approaching 3,0. One such gasifier will produce about m 3 n/h of CO + H 2, enough for about 750 t ammonia /day. No hydrocarbons in the raw gas other than methane owing to the exceedingly high temperatures which occur in the gasification process. Methane itself is present at a concentration of less than 0,1% by volume on a dry basis. No toxic trace metals leached from the flyash in the wash water as water scrubbing takes place at about 50 o C. With no hydrocarbons in the raw gas no organic load is transferred to the wash water. Inherently safe fully water cooled gasifier enclosure, not subject to hot spots or blowouts, nor dependent on a matrix of temperature sensors covering the enclosure surface. About 85% of the available waste heat is recovered as high pressure superheated steam (6 MPa, 490 o C) making it possible to use the steam to drive steam turbines without the need for a separately fired superheater. 5

6 Low corrosion potential by H 2 S owing to the low partial pressure under atmospheric pressure operation, making it possible to use commonly available carbon and stainless steels in the construction of the gasifier. No moving parts in the furnace, and consequently no steam consumed to cool such parts. Relatively low capital investment owing to the incorporation of components and equipment which are standard in the boiler industry. Continuous pneumatic feed of pulverized coal using commercially proven dust pumps, and continuous extraction of slag using commercially proven submerged chain conveyors. No lock hoppers or switching valves required for either duty. Insensitive to raw coal particle size, as all coal is pulverized. Coal with an ash content as high as 21% can be accepted. The analysis of the gas produced by the ACGP will vary according to the coal that is being gasified. For illustrative purposes an analysis of a low grade South African coal is given in table 1, and the gas it will produce, is given in table 2. Table 1 Analysis of a low grade South African coal. COAL ANALYSIS % (mass/mass) Proximate Moisture 2,38 Ash 19,66 Volatile 24,93 Fixed carbon 53,03 Ultimate Carbon 64,10 Hydrogen 3,77 Nitrogen 1,56 Oxygen 7,53 Sulphur 1,0 Higher heating value 25,52 MJ/kg Table 2 Product gas analysis for the coal identified in table 1. GAS ANALYSIS (dry basis) % (vol/vol) Hydrogen 22,3 6

7 Carbon monoxide 67,5 Carbon dioxide 7,6 Hydrogen sulphide 0,4 Carbonyl sulphide 0,03 Ammonia 0,03 Hydrogen cyanide 0,03 Nitrogen 1,2 Argon 0,8 Methane 0,08 Oxides of nitrogen 20 ppm Synthesis gas production at elevated pressure compared with atmospheric pressure gasification Conventional wisdom holds that coal gasification at elevated pressure is an improvement over gasification at atmospheric pressure, and a comparison between the two is instructive as to the level of improvement. For the purpose of this illustration block flow diagrams for each case are given in figures 4 and 5. The basis for this illustration is taken as the production of a raw gas containing m 3 n/h of CO + H 2, at a delivery pressure of 2,0 MPa. Each process has been assumed to consume the same amount of coal, and to produce raw gas of the same analysis, the implication being that both processes could operate with the same carbon conversion efficiency. As the ACGP has not yet been built and tested there is no evidence to refute this assumption. However there is evidence to suggest that the geometry of the gasification chamber plays an important role in determining conversion efficiency. In principle the larger the gasification chamber, the smaller the heat losses and the closer carbon conversion for atmospheric pressure gasification may be expected to approach that for elevated pressure gasification. The pressurised gasifier is assumed to operate at about 2,5 MPa which is considered representative of a typical dry feed entrained flow gasifier operating at elevated pressure. All of the major machines required for each process are depicted in each diagram, and an energy balance comparing the two processes is given in table 3. The data given in this table are the best estimates that could be calculated from published papers and from vendor quotations for similar duties. The entries to this table are explained in the following discussion: Following from the assumption that the coal consumption is the same for both processes, the air separation unit, air compressor and coal pulveriser should likewise be the same size in both cases. Pulverised coal is pressurised, in the case of elevated pressure operation, using batch operated lock hoppers, and is conveyed pneumatically to the gasifier in dense phase. Since pneumatic conveying depends on actual gas volumes, the quantity of nitrogen 7

8 required is significant, as is the volume of nitrogen lost through the cyclic depressurisation of the lock hoppers. In addition the pressure of the nitrogen must be greater than that of the oxygen so as to prevent the ingress of oxygen into the coal conveying lines. A significant nitrogen compressor is required for this duty. In the case of atmospheric pressure operation lock hoppers are unnecessary and feeding of the pulverised coal can be accomplished with continuously operated dust pumps. The pressure, and quantity, of nitrogen required for coal conveying is such that it may readily be supplied by drawing it from the pressure column of the air separation unit without the need for a compressor. In the case of elevated pressure operation oxygen can be compressed either by a turbo compressor, or by a liquid oxygen pump. Although the latter is generally considered safer there is little difference in the power consumed. For the purpose of the illustration a turbo compressor has been assumed. The delivery pressure of the compressor is set 0,5 MPa above the operating pressure of the gasifier to compensate for pressure losses in control valves and burners. In the case of atmospheric pressure operation a liquid oxygen reboiler will deliver the oxygen at a pressure sufficient to operate the gasifier without the need for any pressure raising machinery. In the case of elevated pressure operation the raw gas arising from the gasification zone is quench cooled from o C to about 900 o C, by recycling clean cooled raw gas, so as to ensure the flyash is not sticky before entering a convection waste heat boiler. For this duty a recycle compressor is required to raise the pressure from the final pressure of about 2,0 MPa to about 3,0 MPa. In the case of atmospheric pressure operation the hot raw gas is cooled to below the ash melting point by radiation to the gasifier walls, and no quench, using either water or recycled gas, is required. A compressor is required in the case of atmospheric pressure operation to bring the pressure of the raw gas up to 2,0 MPa. A cooling tower has been provided in each case to condense steam used to drive turbines, and to intercool compressors. The higher power consumed in the case of elevated pressure operation, under the heading boiler feed water and circulation pumps, reflects a requirement for forced circulation. In the case of atmospheric pressure operation water circulation is accomplished by natural circulation. 8

9 The higher power consumed in the case of elevated pressure operation, under the heading scrubber pumps reflects the fact that these pumps must deliver to a higher pressure than in the case of atmospheric pressure operation. The recovery of waste heat as high pressure superheated steam, in the case of elevated pressure operation is constrained by two factors: - the acid dew point, or more correctly, the ammonium chloride sublimation point, which is about 250 o C, limits the allowable outlet gas temperature from the waste heat boiler to about 320 o C. - recycling cooled clean gas, to cool the gas leaving the gasification zone, reduces the recovery of waste heat. In the case of atmospheric pressure operation the acid dew point is about 140 o C, permitting a waste boiler outlet temperature of about 200 o C, and the gasifier is designed to operate without a quench. For its high conductivity the walls of the gasifier in the region of the gasification zone must be of carbon steel. Owing to the risk of corrosion by hydrogen sulphide, which is known to be a function of partial pressure and metal temperature, the permissable pressure at which steam can be raised, is lower in the case of elevated pressure operation than in the case of atmospheric pressure operation. Consequently the achievable steam cycle efficiency is lower in the case of elevated pressure operation. Table 3 Estimated power balance comparison between elevated pressure and atmospheric pressure coal gasification for identical coal consumption and product gas analysis. Elevated pressure Sinks operation Coal pulverisers and conveying system 1,3 MW e 1,3 MW e Air compressor 16,4 MW e 16,4 MW e Dust pumps 0,6 MW e Nitrogen compressor 3,0 MW e Oxygen compressor 6,7 MW e Recycle gas compressor 1,6 MW e Raw gas compressor 16,2 MW e Atmospheric pressure operation Cooling tower and pumps 2,0 MW e 2,5 MW e Boiler feedwater & circulation pumps 0,6 MW e 0,5 MW e Scrubbing water pumps 0,5 MW e 0,2 MW e 9

10 Total shaft power required 32,1 MW e 37,7 MW e Sources Waste heat recovered 108,0 MW th 128,7 MW th Steam quality 4,0 MPa, 440 o C 6,0 MPa, 490 o C Steam cycle efficiency 28,0% 31,0% Equivalent shaft power 30,2 MW e 39,9 MW e Net power exported (imported) (1,9) MW e 2,2 MW e Steam power can most effectively be converted to shaft power by driving the largest machines with steam turbines. The smaller machines would normally be driven with electric motors. In the case of atmospheric pressure operation two machines the air and raw gas compressors present themselves as candidates to be driven with steam turbines. In the case of elevated pressure operation only the air compressor presents itself as a candidate. The surplus steam in both cases may either be exported, or turned into electricity by coupling an alternator to the air compressor machine string. 10

11 Conclusion Kynoch, working with AECI Engineering, and Babcock & Wilcox have developed the design for an advanced coal gasification process to replace their ageing Koppers Totzek gasifiers. These gasifiers supply raw gas to their ammonia and methanol factory at Modderfontein, South Africa. To answer the question as to whether gasification at elevated pressure is better than at atmospheric pressure a comparison between a gas production unit incorporating the ACGP, and a unit incorporating a typical dry feed entrained flow pressurised gasification process, has been drawn up. As expected, the shaft power required for the raw gas compressor in the case of atmospheric pressure operation exceeds the power of the oxygen compressor in the case of elevated pressure operation by some 2,4 times. However when viewed in the context of a complete gas production island the shaft power penalty for atmospheric pressure operation is about 17%. Of concern to chemicals manufacturers is the extent to which shaft power requirements may be balanced by waste heat recovered from the process. From this illustration it emerges that, despite its lower power requirement, process limitations may constrain waste heat recovery, in the case of elevated pressure operation, to the extent that 94% of the required shaft power may be covered. Atmospheric pressure operation, on the other hand, is less constrained by process limitations and some 106% of the required shaft power may be covered by recovered waste heat. Overall, coal gasification at elevated pressure may require the net import of power from an external source, whereas coal gasification at atmospheric pressure may result in the net export of surplus power. On the strength of this comparison, the expectation of a more favourable cost to construct, lower corrosion potential and lower pollution potential, the authors contend there may yet be a place for atmospheric pressure gasification, especially in the field of chemicals manufacture. Acknowledgement Kynoch Fertilizer wish to acknowledge the valuable contribution, as consultants, made by Krupp Uhde (formerly Krupp Koppers) to the development of the Advanced Coal Gasification Process. 11

12 References 1. Davey W L E, Taylor E L, Newton M D, Larsen P S and Weitzel P S : An Advanced Coal Gasification Process and its Influence on the Manufacture of Ammonia, Methanol and CO Based Chemicals. Proceedings of the Asia Nitrogen 98 Conference held in Kuala Lumpur. British Sulphur Publishing, 31 Mount Pleasant, London, WC1X 0AD, England (February, 1998). 2. Dokuzoguz H Z, Kamody J F, Michaels H J, James D E and Probert P B : The KBW Coal Gasification Process. Handbook of Synfuels Technology, edited by R A Meyers, pgs 3-87 to 3-110, McGraw-Hill Book Company (1984). 3. Vogt E V, Weller P J and Vanderburgt M J : The Shell Coal Gasification Process. Handbook of Synfuels Technology, edited by R A Meyers, pgs 3-27 to 3-44, McGraw- Hill Book Company (1984). 4. Simbeck D R et al : Coal Gasification Guidebook: Status, Applications, and Technologies. Electric Power Research Institute Report No. TR (December, 1993). 12

13 Figure 1 : The Koppers Totzek atmospheric pressure entrained flow coal gasifier. Six of these gasifiers have been in continuous operation at Modderfontein, South Africa, since Kynoch s ammonia plant was commissioned in

14 Figure 2 : Cross-sectional elevation of the ACGP atmospheric pressure entrained flow coal gasifier developed jointly by Kynoch, AECI Engineering and B&W as a replacement for Kynoch s ageing Koppers Totzek gasifiers. 14

15 Atmospheric pressure, entrained flow, coal gasification re-visited Figure 3 : Flow diagram of the equipment making up the Advanced Coal Gasification Process, or ACGP. The equipment is as follows: 1 - raw coal bunker, 2 - coal pulverizes, 3 - bag filter, 4 - pulverized coal storage bin, 5 - solids pump, 6 - gasifier furnace, 7 - slag extractor, 8 - steam drum, 9 - steam superheater, 10 - economizer, 11 - target plate scrubber, 12 - wet electrofilter. 15

16 Atmospheric pressure, entrained flow, coal gasification re-visited Coal Prep Gasific ation Dust Rem ASU Demin plant Water Treat. Cool Tower Figure 4 : Block Flow Diagram for a gas production island based on dry feed entrained flow pressurised coal gasification. The numbers appearing on pumps and compressors are approximate delivery pressures in bar. Steam: 6.0 MPa, 490 o C Turb. Coal Coal Prep Gasifi cation Dust Rem. 20 Raw gas 20 Alt. Turb. N 2 O Air 6 ASU Demin Plant Water Treat. Cool Tower Water Figure 5 : Block Flow Diagram for a gas production island based on the Advanced Coal Gasification Process. The numbers appearing on pumps and compressors are approximate delivery pressures in bar.