Paul O. Hedman

Pulverized-coal combustion research at Brigham Young University

Abstract This review paper presents a comprehensive summary and analysis of research work conducted principally at the Brigham Young University Combustion Laboratory over the past decade. An attempt is made to set forth the philosophy and foundations for the laboratory research, which include commitments to both measurement and modeling of complex combustion processes. Recent work has emphasized pulverized-coal processes. The review paper reports past results, together with some recent, previously unpublished results, and provides an integration and summary of key findings. Information from independent investigators is also included or referenced where comparisons are made or where similar work provided clarification. Among the most significant results include the following: (1) measurement of local properties (e.g. velocity, concentration, temperature, mixing rate) from reacting and nonreacting gaseous and particle-laden flows for a variety of test conditions. These data provide a basis for determining controlling processes and for evaluating comprehensive coal reaction models; (2) development and application of comprehensive pulverized-coal combustion models for premixed and diffusion flames; (3) evaluation of combustion models by extensive comparisons with detailed profile data from this and other laboratories. Research needs and concerns in each area of activity are also identified, and future possible laboratory directions are considered.

Effect of Coal Moisture on Burnout and Nitrogen Oxide Formation

Abstract Near-exit measurements of NO, HCN, NH;1 and coal burnout an: reported for a highmoisture,subbituminous pulverized coal fired with air in a laboratory combustor. Test variableswere coal moisture level, stoichiometric ratio and swirl number for the secondary inlet air stream.Data arc also reported for the undried coal wherein gas-char samples were obtained from throughoutthe combustor. Concentrations of major gas species, clcmcn tal composition of the char, gasmixture fraction and N-pollutant concentrations are reported. Effects of coal moisture on burnoutwere negligible and only small decreases in NO levels wereobserved when coal was dried. Resultscan be used for evaluation of predictive models.

Effect of coal type on entrained gasification

The effect of coal type for four coals of varying rank was studied in an entrained flow gasifier at atmospheric pressure. The reactor was modified to increase residence time and gas temperature and to provide for direct measurement of the exit gas flow rate. Space-resolved samples were collected from within the gasifier with a water-quenched probe. Correlation of results shows that the most important factors on carbon conversion are O2coal ratio, coal particle size, coal heating value, mixing of reactant feedstreams, and coal char reactivity. Premixing of steam with coal and oxygen produced higher levels of hydrogen, but a lower COCO2 ratio. Elimination of steam increased the reaction temperature and raised the carbon conversion.

Turbulent velocity and temperature measurements from a gas-fueled technology combustor with a practical fuel injector☆☆☆

Abstract Combustion characteristics of a propane-fueled, practical injector operating in a burner that closely reproduces the flow patterns of a gas turbine combustor have been investigated. The practical injector converges co-swirling airsheets on either side of a coannular fuel sheet into the central air passage. Instantaneous planar-laser-induced fluorescence (PLIF) images of OH radical, laser doppler anemometer (LDA) measurements of mean and rms velocity, and coherent anti-Stokes Raman spectroscopic (CARS) measurements of mean and rms temperatures in the same burner at the same operating conditions have provided improved understanding of the complicated processes in a gas turbine combustor. The PLIF images of the OH radical have confirmed the vortex characteristics of the swirling flames and the highly variable nature of the flame shape as φ and air flow rate was changed. Correlation of these images with air flow through the various nozzle passageways confirms that the local φ must lie between the lean and rich flammability limits for a flame to be locally present. Three recirculation zones were identified from LDA measurements. The highest axial velocity region is about 75 mm downstream for the fuel lean case, but is near the injector for the fuel rich case. The highest tangential velocities are located near the injector for both lean and rich cases. The effects of the injector on velocity were dissipated by one combustor diameter downstream. Large rms velocities occurred in areas where significant velocity gradients exist. The high temperatures changed location as the fuel equivalence ratio was varied from fuel lean (over the injector) to fuel rich (near the outer recirculation zone). The high-temperature regions are consistent with the PLIF images of OH radical, and become relative uniform by about one combustor diameter downstream. Measured temperatures never exceeded the peak theoretical adiabatic flame temperature.

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.

Carbon conversion in an atmospheric-pressure entrained coal gasifier

Abstract Entrained gasification tests with a Utah high-volatile bituminous coal were performed at atmospheric pressure to assess the influence of particle size, coal feed rate, steam-coal ratio and oxygen-coal ratio. Independent argon-carbon balance and ash balance methods were used to evaluate carbon conversion, with good agreement observed between the methods. A higher O 2 -coal ratio and finer particles increased the carbon conversion. Carbon conversion and hydrogen formation showed little dependence on the amount of steam injected in the secondary stream, indicating minimal steam-coal reaction. When the coal feed rate was varied from 23 to 27 kg h −1 , a small increase in carbon conversion was observed with no significant change in the gas composition.

Observations of Flame Behavior in a Laboratory-Scale Pre-Mixed Natural Gas/Air Gas Turbine Combustor From PLIF Measurements of OH

The objective of this study was to obtain instantaneous planar laser induced fluorescence (PLIF) images of OH in a laboratory-scale, gas-turbine combustor (LSGTC) with a pre-mixed, swirl-stabilized, natural gas flame. Instantaneous PLIF images of OH were obtained at each of four operating conditions (high swirl and medium swirl at fuel equivalence ratios of 0.80 and 0.65). Comparison of the instantaneous images illustrates the stochastic nature of the flame structure. Pixel by pixel statistical analysis of each collection of images allowed both mean and standard deviation images to be generated, and analysis at selected locations has allowed probability density functions to be obtained in various regions of the flame structure. PLIF images of OH, along with visual photographs and video recordings, showed a wide variation in flame structure for the different operating conditions. The variations in flame shapes are primarily a result of the effect of the swirl intensity and fuel equivalence ratio. Changes in the airflow rate over an order of magnitude do not seem to affect the visual flame structure in this experiment. Operation at φ = 0.80 produced the most stable flames with both injectors. The flame with the high swirl injector was more coalesced and closer to the injector than with the medium swirl injector. At φ = 0.65, the flame was quite unstable for both swirl injectors. With the medium swirl injector, the flame would oscillate between two different flame structures, one that was more or less attached to the vortex funnel, and one that was lifted well above the vortex funnel. The MS case at φ = 0.65 was at the very edge of the lean flammability limit, and would on occasion extinguish.Copyright

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.

Nitrogen oxide formation inside a laboratory pulverized coal combustor

Two series of measurements of NO, HCN and NH3 are reported for a laboratory pulverized coal combustor with a bituminous coal and a water-quench probe. In the first series, the stoichiometric, ratio (SR) was varied from 0.5 to 1.15 and measurements were made near the combustor exit. The second series provided detailed radial and axial profiles of NO, HCN, and NH3 at SR values of 0.7 and 1.15. Data are also reported for gaseous species (CO, CO2, CH4, H2, O2), and char composition (N, H, C, ash). NO and CO2 levels declined while HCN, NH3, CH4, CO and H2 levels increased with decreasing SR values. A minimum in the sum of HCN, NO and NH3 at an SR value of 0.6–0.7 was observed, suggesting maximum N2 formation at this value. Axial profiles suggest early formation of NH3 in the combustor. Locally high values of H2 and CH4 were also observed. Rapid NO formation was associated with coal devolatilization.