L. D. Smoot

Entrained flow gasification of coal: 1. Evaluation of mixing and reaction processes from local measurements

Abstract Local mixing and reaction processes were studied within a laboratory-scale, entrained coal gasifier at atmospheric pressure, using a Utah high-volatile, low-sulphur bituminous coal at a design flow rate of 24.5 kg h −1 . The coal-oxygen-steam feed mass ratio was 1.00:0.91:0.27. A water-quenched sample probe was used to collect radial gas and char samples at seven different axial positions in the 124 cm long reactor for the measurement of gasification products and residual char composition. The observed carbon conversion was 79 ± 3%. Coal hydrogen and oxygen were converted more rapidly and more completely than carbon. Devolatilization, which occurred very rapidly near the inlet, led to most of this carbon conversion; heterogeneous char reactions with CO 2 and steam apparently accounted for the balance. Oxygen was consumed through reaction with volatiles very quickly in the upper gasifier region. These data were used to evaluate mixing and reaction characteristics within the reactor. Agreement of measurements with predictions from a generalized two-dimensional entrained coal gasification model was good.

Release and reaction of fuel-nitrogen in a high-pressure entrained-coal gasifier

Abstract Effects of pressure, flame type and coal feed rate on fuel-nitrogen release and nitrogen pollutant formation were examined in a laboratory scale, entrained-coal gasifier. A Utah, high-volatile bituminous coal was used. With a water-quenched probe, gas-particulate samples were collected for oxygen-coal mass ratios from 0.6 to 1.1, pressures of 1, 4.9 and 10.4 atm and coal feed rates of 25 and 35 kg h −1 . Two injector types were utilized; one produced a diffusion flame, the other a premixed flame. Fuel-nitrogen release from the coal showed little dependence on oxygen-coal ratio, pressure or coal feed rate. Values at the gasifier exit averaged 83% for the diffusion flame and 92% for the premixed flame. Fuel-nitrogen release, mostly during devolatilization, exceeded fuel-carbon release by ≈ 10% for the premixed flame and ≈ 30% for the diffusion flame, depending on oxygen-coal mass ratio. Over 50% of the released fuel-nitrogen formed N 2 , with significant amounts of NH 3 and HCN, and smaller amounts of NO. Increased pressure at constant mass feed rates caused sharp decreases in effluent NO concentrations (to near zero) for both flame types which was explained by a combination of increased residence time and increased homogeneous NO decay rate. Elevated pressure also increased the effluent NH 3 and decreased HCN concentrations for the diffusion flame whereas the more complete mixing of the premixed flame resulted in lower NH 3 and HCN levels, and higher N 2 levels. In general, nitrogen species concentrations were not largely affected by coal feed rate, though increased coal feed rate decreased NH 3 levels somewhat.

Entrained flow gasification of coal: 2. Fate of nitrogen and sulphur pollutants as assessed from local measurements

A laboratory-scale, entrained flow gasifier was used to investigate the local details of the coal gasification process. Results were obtained from a series of mapping tests at an oxygen-coal ratio of 0.91 and a steam-coal ratio of 0.27, using a Utah high-volatile, low-sulphur bituminous coal. Sulphur pollutant (H2S, SO2, COS and CS2) and nitrogen pollutant (HCN, NH3 and NO) concentrations were determined by detailed radial measurements. As was revealed by measurements of the main gasification products reported in Part 1, three separate flame zones were also found for pollutants: an intense oxygen-rich flame region where very rapid reaction took place, a recirculation region and a downstream region where slow heterogeneous reactions were observed. Oxygenated pollutant species (SO2 and NO) were found to form in significant amounts (up to 3168 and 2767 ppm respectively) in the intense, oxygen-rich flame region. These oxygenated species were converted to varying amounts of H2S, COS, CS2, HCN and NH3 towards the reactor exit, more consistent with expected values based on the overall stoichiometry. Values of local carbon, sulphur and nitrogen conversion from the coal to the gas phase were determined from ash-carbon, ash-sulphur and ash-nitrogen material balances, respectively. Elemental conversion values near the reactor exit were ≈ 79% for carbon, 78% for sulphur and 85% for nitrogen. These data allow the important reaction processes to be postulated and also provide insight into the chemical mechanisms of pollutant formation. The local data are also useful for comparison and validation of generalized entrained coal gasification models.

Sulphur pollutant formation during coal combustion

A laboratory-scale pulverized coal combustor was used to determine the effects of secondary air swirl, stoichiometric ratio (O2fuel), and coal type on the formation and reaction of sulphur pollutants (SO2, H2S, COS and CS2). Detailed local measurements within the reactor were obtained by analysing solid-liquid-gas samples collected with a water-quenched probe. Increasing the stoichiometric ratio increased sulphur conversion and SO2 levels, and decreased H2S, COS, and CS2 levels. Swirl of secondary combustion air had a pronounced effect on the distribution of sulphur species formed at an O2-coal stoichiometric ratio of 0.8, but had very little effect at stoichiometric ratios of 0.57 and 1.17. Combustion of a bituminous coal produced more SO2 and less H2S, COS, and CS2 compared with a subbituminous coal.

The behaviour of chlorine in Kentucky and Illinois coals during combustion and its effects on ash deposits

Abstract Ash deposition tests were performed in a laboratory-scale, pulverized coal combustor with four different coals. These four coals, Kentucky No. 9, Kentucky No. 11, Illinois No. 5 and Illinois No. 6 had chlorine contents of 0.18, 0.10, 0.42 and 0.36 wt%, respectively. Five repetitive, 1 h firings were performed for each coal at a coal feed rate of 11 kg h −1 . Ash samples were obtained from simulated waterwall and superheater probes, from an exhaust cyclone, and from a water-quenched char sample probe. Chlorine was found to release quickly from the coal to the gas phase. The amount of gas phase chlorine that concentrated on the ash collection surfaces was inversely dependent upon the temperature of the collection surface. The chlorine conversion rate from the char was equal to the carbon conversion rate for levels above 65%. Ash fusion temperature, ash sintering temperature, emittance, thermal conductivity, shear strength and compressive strength measurements, which were performed on samples from the waterwall and superheater probes showed no observable differences among the four coals tested. Obvious corrosion of the stainless steel test surfaces was observed during the combustion tests with the Illinois coals. The l h firings were too short for many of the physical properties of the ash deposits to reflect the influence of metal corrosion. Emittance, ash sintering temperature, compressive strength and shear strength of ash deposits were dependent on sample location.

Effects of coal quality on utility furnace performance

Abstract Use of lower than design grade coals can cause problems in furnaces and boilers due to increased ash deposits. A single-zone model has been developed to relate coal quality to thermal performance of pulverized, coal-fired power-generating boilers. The model, based on algebraic mass and energy balances and necessary auxiliary equations, estimates some of the required chemical and physical coal/ash properties. Three wall ash deposit parameters, thermal conductivity, emittance and thickness, have been determined by a sensitivity analysis to be critical to furnace performance and have been obtained by experimental procedures described herein. Data for ash properties are reported for Utah, Illinois and Western Kentucky bituminous coals. With these measured properties, the model has been used to predict effects of coal quality on furnace performance and to interpret changes in ash property data from a small scale combustor to a large scale utility boiler.