528
Ind. Eng. Chem. Process Des. Dev. 1982,21, 528-530
Oxygen-Enriched Anthracite Gasifkatton Raymond 1.Mllne, John C. Tao, and Timothy W. Thew’ Air Products and Chemicals, Incorporated, Allentown. Pennsylvania 18 105
A renewed interest in producer gas from air-blown gasification of anthracite coal is emerslng k the northeastern U.S. As a result, Air Products and Chemicals, Inc. (APCI), has developed a two-zbne model to Simulate a fixed.bed reactor using an oxy@n&nrich& airhteam blast as the gasification medium. Model-generatedcpredictions of the increase in fuel gas productin with oxygen enrichment agree well with publish& data from tests in the U.S. The results show that for a 10% enrichment of the air feed with oxygen, a 100% increase in energy production is realized, reflecting both increased heat content of the product gas as well as increased throughput.
Introduction Before the advent of the natural gas pipeline network, producer gas provided a major source of fuel for manufacturing, heating, and lighting. In the US. mid-Atlantic states, gasification of anthracite resulted frob the need for a reliable gaseous fuel supply. Typically, producer gas was manufactured in a fixed or gravitating bed by blowing a steam/air mixture through the carbonaceous material to produce carbon monoxide and hydrogen as a combustible fuel gas. This low-Btu gas, containing about 50% nitrogen from the air supply, possessed a heating value of approximately 140 Btu/scf when anthracite feedstock was used. However, with the emergedce of a natural gas transmission network, further development of producer gas technology waned under the influence of competitive economics. Today only a handful of the thousand-plus plants originally constructed are in operation in the U.S. The situation in South Africa is somewhat different in that, although that country has an abundance of mineral wealth, no signs of significant oil or natural gas deposits have been found despite extensive exploration. At present, almost 80% of the South African energy requirement is generated from indigenous sources, mainly coal, and 20% is imported in the form of oil. South Africa is keenly aware of the political and economic vulnerability attached to these oil supplies and, consequently, every effort is made to encourage the utilization of coal resources. The best known examples of South Africa’s thrust toward development o€ coal utilization processes are the Sasol plants, in which low-grade bituminous coal is converted to liquid fuels by means of Lurgi gasification and Fischer-Tropsch synthesis. however, in addition, maJ examples of small, fixed-bed gasifiers, producing low-B: + fuel gas for numerous industrial processes, are also fourit In general, South African producer gas plants opera;‘. on bituminous coal, yielding a product gas with a heating value of 160-170 Btu/scf. Several South African companies have investigated the use of oxygeb enrichment as a means of improving the gas quality and energy output of the standard plant operdting on bituminous feedstock. The process incentive for application of enrichment to existing gas producer facilities is often related to a requirement for additional production capacity (i.e., shorter cycle times) from fufnaces operating on the low-Btu fuel. In the US.,interest in the gasification of anthracite has been revived in recent years as a result of the push toward 7
*International Coal Refining Company, P.O. Box 2752, Allentown. PA 18001. 0196-4305/82/1121-0528$01.25/0
Table I. Analysis of Pea Coke and Anthracite pea cokea anthraciteb Proximate Analysis, wt % moisture 2.8 4.5 volatiles fixed C 86.8 85.8 ash
10.4
9.7
Ultimate Analysis, wt % C H 0
N S
ash a
87.12 0.65 0.59 0.80 0.60 10.24
From Batchelder et al. (1950).
85.48 2.22 1.2 0.9 0.6 9.6
From Jennings
(1970).
developing alternative energy sources. Because it is low in sulfur, ash, and volatiles, anthracite is an attractive feed for this application. Serving primarily industrial fuel mafkets, this technology appears to possess significant potential for small-scale applications that will be economically competitive as natural gas updergoes deregulation. Air Products and Chemicals, Inc, (APCI) has investigated the production enhancement gained by using an oxygen-enriched blast instead of air for anthracite gasification. In this report, oxygen enrichment is defined as an increase in the percentage of oxygen over the 21% level in air; (for example, 25% total pxygen is defined as 4% enrichment). Production improvement occurs in two ways. (1)With higher oxygen concentrations, the heating value of the gas is increased, since there is less diluent nitrogen in the product gas stream. (2) The greater amount of coal processed through a given producer unit enhances the potential for reduced capital for a given Btu requirement; that is, for a given capital investment, greater gasifier capacity is available. APCI’s approach to evaluating the enrichment gasification process has been to develop a fundamental understanding of anthracite gasification through computer simulation techniques. This review of APCI work is based upon the application of fundamentail principles of fixedbed gasification technology to data from published sources. Discussion of Methods Development The simulation of anthracite gasification is based upon the concept that the gasifier bed can be subdivided into distinct zones, identified by the type of process assigned to each zone. Figure 1 provides a schematic diagram of the producer bed divided into four zones, namely, drying 0 1982 Pw. 4can Chemical Society
In& Eng. Chem. Process Des. Dev., Vol. 21, No. 3, 1982
r'
GAS
-7 DRYING 8 DEVOLATILIZATION
REDUCTION OR GAS IF1 CAT1 ON
I
529
Table 11. Comparison of Predicted and Measured Gas Compositions gas composition comcase % 0, ponent pred measd I 21 co 29.8 28.8 5.7 14.6 49.9 36.4 14.7 32.9 16.0
CO,
SIMPLIFIED
HZ I1
50.7
N,
co COZ H,
N;
5.9 14.2 50.3 40.3 12.2 29.9 17.6
210
200
I90
1
i
I80
STEAM 8 AIR I O2
ASH
Figure 1. Fixed-bed gasifier model.
170
BTWSCF 160
0
CARBON I
I50
REDUCTION ZONE
130
.
C O + d Z O ~ C 0 2 + H 2
A " ' & 7 ' " '
c+co2--2co
"
q
'
'
*
'
'
"
Figure 3. Predicted heating values.
c+l120Z-c0
.f
f
Figure 2. Simplified two-zone gasifier model. and devolatilization, reduction (or gasification), comldustion, and ash. With respect to anthracite gasificatioy, simplifying assumptions concerning the relative sigrlificance of these zones have reduced the simulation to consideration of a two-zone gasifier. I t is believed that devolatilization will not contribute substantially to the ovetall gas make since anthracite is viewed as a highly condensed carbon structure with very little associated volatile matter. Table I compares the proximate and ultimate analyses of a typical Pennsylvania anthracite to that of pea coke, which provided the basis for the simulation development. As a result of this assumption and the fact that commercial-sized anthracite has a low moisture content, the top zone shown in Figure 1has been deleted from the model. Since ash behaves essentially as an inert material, no factor accounting for it presence has been included in the model. zffecta on the overall heat balance are minimal, since calculations have shown that the sensible heat of the ash gravitating through the bed accounts for a very small percedtage of the total heating value of the anthracite
charge. With these assumptions, the simplified two-zone model consisting of combustion and reduction zones is shown in Figure 2. Figure 2 depicts six streams used in the model. The input streams are carbon (l),steam (2), and oxygen + nitrogen (3). A chemical equilibrium computer program is used to simulate each zone. The program considers the flow of rectants entering each zope, the thermodynamic molecular properties of the reactants, and allowable reaction products as defined by the model, and then adjusts the composition of products in each zone to minimize the free energy of the zone as a whole. The corresponding adiabatic temperature of each zone is also calculated. In the combustion zone, the blast stream is allowed to react with carbon until all oxygen is consumed. It is assumed that steam and nitrogen are chemically inert in this zone. The combustion zone products (stream 5 ) are then sent to the gasification zone where they are allowed to react endothermicallywith carbon to form a product gas (stream 6). A certain proportion of feed carbon is unreacted and is recycled to the combustion zone in stream 4. A convergence loop in the program campares the carbon flows in streams 4 and 5 and adjusts the carbon flow in stream 1 until convergence is achieved.
Results The data used in simulating anthracite gasification are those of Batchelder et al. (1950). Gasification tests were conducted on pea coke in a 7-ft diameter Kerpely producer at various levels of oxygen enrichment from air (21% 0,) to essentially pure O2 (98.6%). Data on the composition of the pea coke indicated a clwe similarity with anthracite; for simulation purposes, both kedstocks were assumed to
Ind. Eng. Chem. Process Des. Dev. 1982, 21, 530-532
530
I6O
140 120
I 1
COhlBiNEU
/
6
l\ICFIEASE Clr'ER EASE CAS6
o2
N E ~ . R C-IED A
a
BLAS-
Figure 4. Effect o f oxygen enrichment on gasifier output.
be composed of elemental carbon. Table I1 compares the predicted product gas compositions with those reported by Batchelder et al. for two cases involving gasification with air (21% 0,) and gasification with a 50.7% oxygen blast. These results established the basis for the oxygen-enriched cases, which were examined in increments of 5% oxygen. The incremental steam required for additional cooling at higher O2levels was predicted on the basis of volume-for-volume substitution of nitrogen, as suggested by Batchelder et al. Figure 3 shows the resulting heating values, HHV and LHV, for the enrichment cairn as predicted by the model. For example, a 10% enrichment level yielded a 16% predicted improvement in heating value. The production incentive for an oxygen-enriched blast in anthracite gasification is based not only on the incremental improvement in heating value, but also on additional capacity, which results in a significant increase in total Btu production. These effects are shown in Figure 4 (lime B) as a function of oxygen enrichment over the base
case using air (21% OJ. Capacity improvements were calculated from the oxygen-carbon ratio extrapolated from the Kerpely producer data. When the effects of higher Btu content and increased capacity were compounded, the result was a combined production factor, also shown in Figure 4 (line C). The data indicated that at a 10% enrichment level, the total production improvement would be approximately 100%. It is interesting to compare the two-zone model advanced here with a similar "kinetics-free" (KF) model described by Denn et al. (1979). In the KF model, the CO:C02 ratio in the gas leaving the combustion zohe is arbitrarily fixed, and the corresponding combustion zone temperature is computed from the adiabatic temperature rise in the reactor. The "combustion zone" temperature for fixed-bed anthracite gasification predicted by both models lies with 2750-2950 OF. These temperatures are somewhat higher than expected when ash fusion limitations are considered. Denn et al. have calculated that variation from 0 to 100% of the CO fractional conversion used in the KF model causes a variation of only 300 OF in the computed maximum temperature. Conversely, investigations with the two-zone model developed here indicate that the "combustion zone" temperature is relatively sensitive to the fractional heat leaks (typically 3% of the feed HHV) normally ascribed to a commercial gas producer operation. Conclusion A model has been developed to simulate fixed-bed gasification of anthracite and similar feedstocks. Results compare favorably with published data which show the effects of oxygen-enrichment of the air blast. The model predicts improved heating value of the product gas as the oxygen concentration of the air/steam blast is increased. Additionally, potential capacity increases are identified. Literature Cited Batchelder, H. R.; Dressier, R. G.; Tenny, R. F.; Kruger, R. E ; Segur, R. D Am. Gas Assoc. R o c . 1950, 340. Denn, M.; Yu, W,; Wei, J. I d . Eng. Chem. Fundam. 1979, 18, 286. Jennings, E. H. "Environmental Engineering"; International Textbook Co : Scranton, PA, 1970
Received for review May 30, 1980 Revised manuscript received February 1, 1982 Accepted M a r c h 8, 1982
COMMUNICATIONS Sorption of Sulfur Dloxide by Waste Calcareous Materials The conversions of calcareous muds from a sugar mill and acetylene production plant ?ere studied in terms of exposure time and temperature. There is an essential difference in the course of the sulfation reaction of these materials and that of common limestones. While conversions of common limestones are, in general, low, the calcareous muds retain a considerable reactivii until the complete conversion of calcium oxide to sulfate is attained.
Introduction It is a well-established fact that particles of limestone exposed at high temperature to the flue gas containing sulfur dioxide react only partially. This poses a major 0196-4305/82/1121-0530$01.25/0
obstacle to the practical feasibility of dry limestone processes for removal of sulfur dioxide from combustion gas (Krijger et al., 1981). In our previous work we investigated structural changes 0 1982 American Chemical Society
