Angus U. Blackham

Char oxidation at elevated pressures

Abstract Approximately 100 char oxidation experiments were performed at atmospheric and elevated pressures, with two sizes (70 and 40 μm) of Utah and Pittsburgh bituminous coal chars at 1, 5, 10, and 15 atm total pressure. Reactor temperatures were varied between 1000 and 1500 K with 5% to 21% oxygen in the bulk gas, resulting in average particle temperatures ranging from 1400 to 2100 K and burnoff from 15% to 96% (daf). Independently determined particle temperature and overall reaction rate allowed an internal check on the data consistency and provided insight into the products of combustion. The chars burned in a reducing density and reducing diameter mode in an intermediate regime between the kinetic and pore diffusion zones, irrespective of total pressure. Significant surface CO 2 formation occurred at particle temperatures below about 1800 K over the entire pressure range. Particle temperatures were strongly dependent on the oxygen and total pressures; increasing oxygen pressure at constant total pressure resulted in substantial increases in particle temperature, while increasing the total pressure at constant oxygen pressure led to substantial decreases in particle temperature. Increasing total pressure from 1 to 5 atm in an environment of constant gas composition led to modest increases in the reaction rate; the rate decreased with further increases in pressure. Results indicate that ambient pressure global model kinetic parameters cannot be accurately extrapolated to elevated pressures. The apparent reaction rate coefficients (based on the partial pressure form of the n th-order rate equation) showed significant pressure dependence, since both the activation energy and frequency factor decreased with increasing pressure. This suggests that the empirical n th-order rate equation is not valid over the range of pressures encountered in the experiments. However, simulations indicate that the global model can be used to model elevated pressure char oxidation provided pressure-dependent kinetic parameters are used.

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.

Effect of pressure on oxidation rate of millimetre-sized char particles

Abstract Mass losses and burnout times of large (0.1, 0.2 g) char particles at pressures of 101–760 kPa were measured in a newly designed high-pressure reactor. A cantilever balance measured instantaneous particle mass and an optical pyrometer measured particle temperature continuously. The process was also videotaped. Sixty-two combustion experiments were conducted with a bituminous coal and a lignite. The reactor air temperature was ∼ 900 or 1200 K and the air flow Reynolds number was varied by a factor of two. Coal particles were placed in a platinum-wire basket inside the reactor at the end of the balance beam. An ash layer accumulated around the particles and receded as the char was consumed. In all tests a linear decrease in cube root of char mass with time was observed during oxidation until near the end of burnout. Changes in air velocity had little effect on oxidation time; increasing gas temperature or increasing pressure from 101 to 507 kPa reduced oxidation times by about one-quarter. Further increase in pressure caused no further reduction in burnout time. Pairs of nearly equally sized particles of coal had oxidation times similar to those of single particles with the same total mass.

Reactivity and combustion of coal chars

Seven industrial process chars were tested to determine their reactivities and/or combustion characteristics. Char densities, total surface areas, and elemental and proximate compositions were measured. Reactivity tests were conducted over three temperature ranges: thermogravimetric analysis (TGA) with air (550–750 K), with CO 2 in a fixed-bed (1070–1270 K) and with air in a drop-tube furnace (1300–2100 K). In TGA tests, reactivities among chars differed by almost two orders of magnitude at the same temperature. Fixed-bed CO 2 tests gave mass reactivities that differed, by about one order of magnitude, while mass reactivities at high temperature in the drop-tube furnace differed by only a factor of two. Parent coal type and char formation conditions are major factors influencing reactivity in low and intermediate temperature ranges. N 2 -BET surface area does not adequately explain observed reactivity in any temperature range while variation in external surface area does correlate reactivity at high temperature. High temperature reactivities compared well with predictions from a pore tree/char oxidation model, after adding char volatiles percentage. Only high temperature reactivity data characterized the char consumption rate in a laboratory combustor, but a significant residual char volatiles content appeared to be essential for flame stability.

Fate of coal-sulphur in a laboratory-scale coal gasifier

Abstract This work summarizes several observations concerning the effects of pressure and oxygen-to-coal mass ratio on the fate of coal-sulphur during entrained gasification. A high-volatile bituminous coal was pulverized to a mass mean of near 50μm. The coal was gasified with oxygen in a laboratory-scale entrained-flow gasifier. No steam was used. Test pressures were 1, 5 and 10atm (10, 500, 100 kPa). Oxygen-to-coal mass ratios between 0.60 (SR = 0.30) and 1.10 (SR = 0.56) were investigated. Gas-particulate samples were collected with a water-quenched probe from the gasifier chamber effluent stream. Measurements were made of the sulphur in the char particles and of the levels of H2S, SO2, COS, and CS2 in the product gas. Increasing pressure yielded a significant decrease (about 25%) in net conversion of coal sulphur. This decrease was shown by an increase in the sulphur content of the char samples, and by a decrease in total sulphur concentration in the gas phase. Gas species measurements showed that increased pressure yielded lower SO2 and CS2 concentration, but did not significantly affect H2S and COS concentrations. It was concluded that increased pressure shifted the distribution of sulphur among the gas phase species, yielding higher H2S and lower SO2 and CS2 concentrations. The increase in H2S was not seen in the measurements, because increased pressure also tended to increase the capture of H2S by char. Increasing oxygen-to-coal mass ratio increased sulphur conversion and SO2 and COS levels, while it decreased H2S and CS2 levels.

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.

Reduction of fuel-NO by increased operating pressure in a laboratory-scale coal gasifier

Abstract Measurements of NO during laboratory-scale gasification of a Utah bituminous coal verified that small increases in pressure (from 1 to 2 atm) at constant residence time resulted in dramatic decreases in effluent NO levels. Tests were conducted at 3 target levels of pressure (1, 2 and 4 atm) and 2 target levels of residence time (450 and 900 ms). Oxygen-to-coal ratio for all tests was 0.90 (stoichiometric ratio SR = 0.45). The dominant factor in causing lower effluent NO levels was the increased kinetic rate of NO decay. Increased residence time in the fuel-rich gasifier contributed to lower effluent NO levels, but was of minor importance when compared to the effect of pressure on the decay rate. Concentrations of N 2 appeared to be slightly increased and concentrations of total fuel nitrogen (TFN) decreased as pressure was increased. Also, concentrations of N 2 increased and concentrations of TFN decreased as residence time was increased at 1 atm pressure. For all tests, nitrogen conversion exceeded carbon conversion by about 10%. Neither nitrogen conversion nor carbon conversion was found to increase with increasing pressure. Both increased slightly (4–5%) with increasing residence time, evidence that most of the coal nitrogen and carbon was released during devolatilization.

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.

Rates of oxidation of millimetre-sized char particles : simple experiments

Abstract Rates of oxidation of 5–10 mm particles of chars from six coals at various temperatures were measured in air at ambient pressure in simple devices: a muffle furnace, a Meker burner, and a heated ceramic tube. The chars were first prepared from the coals in the Meker burner at comparable temperatures. As well as coal type and oxidation temperature, initial char particle mass and number of particles were varied. The char particles were oxidized in incremental steps of several minutes for periods up to 1 h. The cube root of particle mass decreased linearly with increasing time in all tests. Ash layers formed and usually remained in place around the particle. Average mass reactivities increased with decreasing initial particle mass. With decreasing furnace temperature, char reactivity decreased at the lower temperatures. Two or four closely spaced char particles burned much more slowly than single particles of the same size. Correlative equations are consistent with the data, elucidating the roles of kinetic reaction and oxygen diffusion.

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.