Ind. Eng. Chem. Fundam. 1983, 22, 110-116
110
Measurement of Nitrogen and Sulfur Pollutants in an Entrained-Coal Gasifier Tracy D. Prlce,'
L. Douglas Smoot,' and Paul 0. Hedman
Combustion Laboratory, Chemical Engineering Department, Brigham Young University, Provo, Utah 8460 1
Fifteen tests with a highly volatile bituminous coal were conducted in a laboratory gasifier wherein nitrogen (HCN, NH, NO, fueCN) and sulfur pollutants (H,S, SO,, fuel-S) were measured for gas-liquid-solid samples. The samples were removed from within the gasifier by a water-quench probe at various axial and radial locations. Generally, concentrations of H,S, HCN, and NH, diminished and SO, and NO increased as O,/coal ratio increased. Increasing steam/coal ratio suppressed all pollutant levels, partly through reduction in nitrogen and sulfur emissions from the coal. H@ SO, levels formed quickly in the gasifier, possibly from reaction with oxygen in the gas phase. Significant quantities of sulfur and nitrogen were still in the char at the gasifier exit, since carbon conversion was not complete. High SO, and NO levels in aft-regions were attributed to the oxidizer-rich atmosphere resulting from incomplete carbon conversion.
Introduction Important pollutants formed in coal gasifiers include NO, NH,, HCN, H2S, and SOz. It is important to understand what conditions produce these pollutants, how they are formed, and how their production might be minimized. Bissett (1978) summarized entrained gasification work by The U.S. Bureau of Mines and others, including some information on pollution. Recent reviews of coal gasifier pollutant formation and analysis include those of Luthy (1978), Oldham and Wetherold (19771, Walsh (1978), Massey et al. (19781, and Malte and Rees (1980). Hansen et al. (1980) reported detailed measurements of sulfur and nitrogen-containing pollutant effluents from a high-temperature, entrained coal gasifier. Farnsworth et al. (1974) also discussed some aspects of pollutant emissions from a commercial, entrained coal gasification process. However, no work has been reported wherein sulfur and nitrogen pollutant levels were measured locally throughout an entrained-coal gasifier. In this study, sampling and chemical analysis methods were developed to determine local concentrations of SOz, H2S, HCN, NO, NH, fuel-sulfur and fuel-nitrogen in the laboratory gasifier. These methods were then used to determine the effect of O,/coal and steam/coal mass ratio on the sulfur and nitrogen pollutant species concentrations. Measurements of the distribution of pollutants locally in the laboratory gasifier were made, and mass balances were used to evaluate the accuracy of experimental techniques. Test Facility Gasifier. The laboratory gasifier of Figure 1 was described in detail by Skinner et al. (1980). Coal, steam, oxygen, and argon entered the top of the reactor through two concentric, annular pipes. Oxygen from high-pressure gas cylinders flowed through two control valves which divided flow between the primary stream (the stream flowing through the inner concentric pipe) and the secondary stream (the stream flowing through the outer concentric pipe). Coal particles were entrained in argon and then mixed with preheated oxygen before entering the reactor through the inner primary duct. The secondary oxygen was preheated and mixed with saturated steam. This steam-oxygen mixture was then further heated and 'Exxon Co., Baytown, TX. 0196-4313/83/1022-0110$01.50/0
admitted to the reactor through the annular region between the two concentric pipes. A small amount of helium, used as a tracer gas, also entered the reactor in the secondary stream. The reactor consisted of four movable insulated (cast ceramic) sections which were bolted together. One of the sections contained three to five probes for sampling and could be interchanged with any of the other sections. Sampling System. Smoot and Hedman (1979) discussed the design and operation of the 4-mm i.d. sample probes, which were made of 316 stainless steel and were water-jacketed. Water was injected into the sampling area through small injection ports near the tip of the probe, quenching particles and gas from the reactor. Isokinetic sampling was attempted by measuring the pressure inside one of the probes and balancing it with the static pressure inside the reactor. The sampling system included one steel collection cell for each probe. A nylon sample bag, with low permeability to combustion gas products, was fitted inside the cells. The bags and containers were initially evacuated, and the cooling water flow rate was adjusted to approximately 4 mL/s in each individual probe. In these tests, the reduced pressure caused by opening the evacuated sample cylinders disturbed the isokinetic sampling, increasing the collection rate of particles and gases. Thus a carbon elemental mass balance was used to calculate the amount of gas which corresponded to the amount of char sampled. Test Program Tests were conducted with a high-volatile, Utah bituminous coal. The proximate analysis was 2.4-2.7% moisture, 45.0-45.2% volatiles, 43.6-44.270 fixed carbon, and 8.2-8.7'70 ash; the maf ultimate analysis was 77.6-80.2% C, 5.7-6.0% H, 1.4-1.6% N, 0.5-0.6% S, and 11.8-14.2% 0 (by difference). Mass mean diameter of the 70-200 mesh coal was 40-43 ym, with particle diameter varying from 2 ym to over 100 ym. Skinner et al. (1980) reported the measured coal particle size distribution. Coal feed rate in all tests was 24.5 kg/h. In a first series of 11 tests, the effects of steam/O, feed rates on pollutant formation were observed. The Oz/coal mass ratio was varied from 0.67 to 1.00. For a given Oz/coal ratio, the steam/coal mass ratio was increased to the point where the flame became unstable. In the second series of four tests, detailed radial and axial pollutant concentration profiles were obtained from @ 1983 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 22, No. 1, 1983 111
r--Pr imar y St r e a m
Table I. Summary of Analysis Methods and Accuracy component phase analysis method av % diffa HCN
Hydrogen Igniter
liquid gas liquid gas liquid gas liquid gas. solid solid solid solid solid solid
H,S
Preheat M e t h a n e
NH, SO, NO
S S N C
H ash
30.5 c m Sample
I I
2
System
specific ion electrode 8.0 Drager tube specific ion electrode 4.5 Drager tube specific ion electrode 7.4 Drager tube 25.0 Henry's Law chemiluminescent analysisb 5.0 LECO analysis 7.0 Eschka methodC 10.0 Perkin-Elmer analysisb 5.0 Perkin-Elmer analysis 5.0 Perkin-Elmer analysis 5.0 10.0-15.0 Perkin-Elmer analysis
a Average % difference = ~ " i , l ( . M i - S ) / n , where Mi = measured value, n = number of trials run, S = standard value. Although these analysis methods were not subjected to the same rigorous tests as the specific ion electrode methods, they were calibrated against a known Accuracy was checked by analysis of standard daily. raw coal of known sulfur content.
Symbol
IS00
n
Quench Water
SteamlCoal Ratio
0 0
0.24
A
0.39
D
0.47
0
0.54
0.00
Solid point =av
of 2 runs
1400
Figure 1. Schematic of the gasification reactor.
various locations within the gasifier. The Oz/coal and steam/coal mass ratios were 0.83 and 0.24, respectively. These ratios were selected because reasonable carbon conversion, good flame stability, and near-minimum pollutant levels were achieved under these conditions. Axial locations of the probe (distance aft of the entrance plane of the primary feed stream) were 48.3 cm (1test), 63.5 cm (2 tests), and 94 cm (1test). Radial locations of the five fixed probes (distance from the centerline) were 0,1.3,2.8, 5.6, and 8.6 cm.
Methods of Sample Analysis The use of water-quenchedprobes necessitated chemical analysis of pollutants in sample gas, sample quench water, and the solid residue. Gas-phase HzS was analyzed with Drager tubes. Drager tubes are small indicator tubes containing a compound which reacts with the specific gas species to be measured. As the gas sample is pulled through the tube, a length of the tube is discolored in proportion to the pollutant gas concentration in the sample. Drager tubes were also employed in analyzing gas samples for SOz. No method was used for analyzing aqueous sulfur dioxide, but the amount of aqueous SOz was estimated from the gaseous composition with Henry's law, assuming the gas-liquid sample to be in equilibrium. This method has since been replaced by a better technique (Burkinshaw et al., 1981). The sample gas was analyzed for NO with a Thermo Electron Model 10 AR chemiluminescent analyzer. Gas chromatography provided molar percentages of the major gas components in the sample (Skinner et al., 1980). The aqueous cyanide, sulfide, and ammonia determinations were performed with Orion specific ion electrodes, Models 94-06,94-16, and 95-10, respectively. Calibration curves were prepared with known standards for cyanide and sulfide while a standard addition technique was used for ammonia calibration. Once the analysis of the gas and quench-water in the sample bag was complete, the char
-* k4. :1
1200 -
0 1000800600-
"f
, '0.6
0.7
,
,
0.8 0.9 02/Coal Ratio
,
,
1.0
Figure 2. Effect of oxygen/steam/coal ratio on mean NH3 concentration; axial location = 63.5 cm.
particles were filtered, dried, and analyzed for carbon, hydrogen, nitrogen, and ash with a Perkin-Elmer Model 240 elemental analyzer. Sulfur was analyzed with a Leco Model 532-000 sulfur analyzer. Estimates of sample analysis accuracy determined by analyzing known standards are summarized in Table I. Skinner (1980) discussed the accuracy of other parameters such as reactor feed rates, sampling probe effects, gas absorption in quench water, and material balances on both gas and particle flows.
Test Results Mixture Ratio Tests. Skinner et al. (1980) reported that carbon conversion levels for these tests ranged from 42 to 70%,increased with oxygen addition, and decreased with steam addition. Pollutant concentration data were obtained for the first 11 tests at three different radial positions (0, 3.2, 5 cm), all at an axial location of 63.5 cm. This axial position was chosen in order to avoid potential recirculation problems toward the bottom of the reactor and to alleviate probe-capping problems with coal char observed toward the reactor entrance. The data are summarized in Table 11. No SOz measurements were made in these 11 tests. Since the data were not sufficient to permit determination of a mean (mixing-cup) concentration for each pollutant, results are reported as arithmetic
112
Ind. Eng. Chem. Fundam., Vol. 22, No. 1, 1983
Table 11. Pollutant Concentration Data from Mixture-Ratio Tests (ppm)= OJcoal ratio
steam/ coal ratio
Ocm
1.00 1.00 1.00 1.00 1.00 0.83 0.83 0.84 0.84 0.68 0.68
0.00 0.24 0.39 0.47 0.54 0.0 0.24 0.39 0.39 0.00 0.24
856 1037 856 1123 1531 187 1144 2843 848 1084 1695
a
NH, concn
HCN m n c n
H,S concn
3 . 2 c m 5.0 cm 0 cm 955 -.479 713 377 507 341 1115 753 383 _-_ 3 51 894 1102 743 43 822 577 1129 __. 211 568 486 1774 ___ 1077 358 1084 1403 841 427 -1531
3 . 2 c m 5.0 cm __586 97 116 263 149 --. 7 45 73 135 472 .__ 431 437 258 .__ 7 6 3 1077 714 .._ 317
0 cm 1538 1005 2306 2183 1626 508 1560 1570 1429 2254 867
NO concn
0 cm 3 . 2 c m 5.0cm 75 75 ___ -_690 560 563 809 520 510 470 _-- 560 490 _-460 --_ 1050 439 ___ 210 195 1166 -.- 520 780 --657 190 780 770 723 320 ___ 315 506 540 500 410 160 ___ 1471 27 5
3.2 cm
5.0 cm
___
1374 888 693 408 521 1259 1677 641
___
2900 _--
In the gas, dry basis. 2000
\
0
,800[
-
800
c
vi
12
2001
0
k
7
08
09
10
O2 Coal Ratlo
Figure 3. Effect of oxygen/steam/coal ratio on mean HCN concentration; axial location = 63.5 cm (see code in Figure 2).
;::I ;
