Sarma V. Pisupati

Natural weathering and laboratory oxidation of bituminous coals: Organic and inorganic structural changes

Abstract The compositional and structural changes to selected bituminous coals during natural weathering (outcropping) and low-temperature laboratory oxidation were determined. The volatile matter contents of the oxidized coals were higher than those of the corresponding fresh coals. Lower carbon and hydrogen contents, higher oxygen contents and therefore lower heating values were characteristic of the crop and laboratory-oxidized coals. The sulphur contents of the crop coals were significantly lower than those of the laboratory-oxidized and fresh coals, due primarily to the oxidation of pyrite in the crop coals. Changes in the organic structure of the coals upon natural weathering and laboratory oxidation, as shown by diffuse reflectance and 13 C n.m.r. spectroscopy, were similar in nature but different in magnitude. One structural difference between the laboratory-oxidized and naturally weathered coals was the formation of ester groups in the former, whereas acid functional groups were the predominant species formed in the latter. Important structural differences in the inorganic phases between the fresh and crop coals were due to oxidation of pyrite, leaching of calcite, and incorporation of some ion-exchangeable metal cations such as calcium in the crop coals. No such differences were observed in the composition of the ash from the fresh and laboratory-oxidized coals.

Partitioning behavior of trace elements during pilot-scale combustion of pulverized coal and coal-water slurry fuel

Release pathways for inorganic hazardous air pollutants (IHAPs) from a pilot-scale, down-fired combustor (DFC) when firing pulverized coal (PC) and coal-water slurry fuel (CWSF) were identified and quantified to demonstrate the effect of fuel form on IHAP partitioning, enrichment and emissions. The baghouse capturing efficiency for each element was calculated to determine the effectiveness of IHAP emission control. Most of the IHAPs were enriched in the fly ash and depleted in the bottom ash. Mercury was found to be enriched in the flue gas, and preferentially emitted in the vapor phase. When firing CWSF, more IHAPs were partitioned in the bottom ash than when firing PC. Significant reduction of Hg emissions during CWSF combustion was also observed.

Devolatilization behaviour of naturally weathered and laboratory oxidized bituminous coals

Abstract The influence of natural weathering and low temperature laboratory oxidation on the devolatilization behaviour of bituminous coals was investigated. The study was conducted on six sets of coals, each comprising a fresh and an oxidized (crop) coal from the same seam. One of the fresh coals was oxidized in a laboratory at 200 °C for 2 and 72 h to elucidate the differences between natural weathering and low temperature laboratory oxidation and their effect on the devolatilization behaviour of the coal. The compositional analyses revealed that the volatile matter of the oxidized coals were higher than those of the corresponding fresh coals. However, the volatiles release rates, as observed in a thermogravimetric analyser, were slower and the volatiles were released over a wider temperature range for the oxidized coals than for the fresh coals. The volatiles released during flash pyrolysis of the crop coals, as determined by gas chromatography-mass spectrometry, were leaner in aliphatic and aromatic hydrocarbons and other combustible gases and richer in oxygenated species than those from their fresh companions. The differences in the nature and the quality of the volatile matter between the naturally weathered, laboratory oxidized and fresh coals indicate possible differences in the ignition behaviour of these categories of coals.

Sorbent behaviour in circulating fluidized bed combustors: Relevance of thermally induced fractures to particle size dependence

Abstract The particle size dependence of the performance of various limestones and dolostones in capturing SO 2 in fluidized bed combustors was determined and explained in terms of the occurrence of thermally induced fractures (TIFs). Data were obtained in a bench-scale fluidized bed reactor, a pilot-scale down-fired combustor and a 30 MW(e) circulating fluidized bed combustor (CFBC). Finer particle size fractions (100 × 400 mesh, 38–150 μm) had lower Ca/S molar ratios than coarser size fractions (> 100 mesh, > 150 μ m) in the bed ash and recycle ash from the 30 MW(e) CFBC. Upon further sulfation of the ashes in a thermogravimetric analyser, the μ m) fraction captured the most additional sulfur, indicating that these particles did not have sufficient residence time in the CFBC to be fully sulfated. For larger particles, the slow rate of SO 2 diffusion through the product layer limited the extent of sulfation. Hot-stage scanning electron microscopy and microprobe analysis of the sulfur distribution in the particles indicated that some sorbents developed thermally induced fractures, while others with comparable CaCO 3 contents did not. The TIFs promoted SO 2 diffusion into the particle and, as a consequence, the sulfation behaviour of such sorbents was less dependent on particle size than was that for sorbents which did not develop TIFs.

Effects of natural weathering and low-temperature oxidation on some aspects of the combustion behaviour of bituminous coals

Abstract The influence of natural weathering ( in situ ) and low-temperature laboratory oxidation on the combustion behaviour of bituminous coals was investigated. Five sets of coals, each comprising a fresh and crop coal from the same seam, were used. One selected fresh coal sample was oxidized in the laboratory at 200 °C for 2 and 72 h to elucidate the effect of low-temperature oxidation (simulated weathering) on the subsequent combustion behaviour. The combustion behaviour of the coals was evaluated using char reactivity and burning profiles obtained in a thermogravimetric analyser (TGA) and combustion efficiency determined in a drop-tube reactor (DTR). The reactivities of the chars produced from the crop and laboratory-oxidized coals were higher than those for the chars from the corresponding fresh coals. The reactivities of the crop coal chars were higher than those of the laboratory-oxidized coals due to the presence of inorganic species (cations) in the former which could be removed by acid washing. Reactivity changes due to such inorganic species changes did not occur upon laboratory oxidation. The combustion efficiencies of the weathered coals were higher than those of the corresponding fresh coals. A correlation was obtained between the difference in combustion efficiency between a crop coal and its fresh counterpart and oxygen uptake during natural weathering. There was an inverse relationship between the combustion efficiencies determined in the DTR and the ‘initial temperature’ in the TGA burning profiles.

Effect of blending low-grade anthracite products with bituminous coals on combustion characteristics in a bench-scale stoker simulator

Abstract The combustion behavior of blends of two low-grade anthracite byproducts with two bituminous coals was determined in a bench-scale test apparatus that was designed to simulate the combustion conditions (bed density, bed height, air flow rate, and particle residence time) on a travelling-grate stoker. Axial temperature profiles in the bed at various locations, pressure drops across the bed, and flue gas compositions were monitored at regular time intervals during the experimental runs. Residual ash was analyzed for carbon burnout and clinkering potential. The study revealed that the particle size distribution of the coals influenced the bed conditions during combustion and ultimately the carbon burnout. Blending anthracite byproducts, of appropriate size distribution, with bituminous coals decreased the pressure drop across the bed during combustion and reduced the clinkering potential by reducing the caking properties of the blend and improving the air distribution through the bed. Tempering the fuel bed (with water) had a greater effect on combustion performance than the particle size distribution of the feed coals, since tempering was found to alter the size distribution by agglomerating the fines. Tempering lowered the bed density, produced rapid and uniform travel of the ignition plane, and increased the carbon burnout. Several correlations between the condition of the fuel bed during combustion and the carbon conversion efficiency were obtained.

Combustion characteristics of naturally weathered (in situ) bituminous coals

Abstract The relative combustion behavior of five companion outcrop bituminous coals (naturally weathered) and five deep-mined (unoxidized) coals was determined. Chemical and structural differences between the weathered and fresh coals and the extent of oxidation were also determined. The combustion behavior was evaluated in terms of ignition temperature, char reactivity and combustion efficiency (carbon burnout). The relative ignition temperatures of the crop coals were less than those of the corresponding fresh coals in all cases. Devolatilization of the crop coals began at a lower temperature and extended over a wider temperature range than for the fresh coals. The reactivity and the combustion efficiency of the chars produced from the crop coals were higher than those of the corresponding fresh coals. Consequently, the “ease of combustion” was greater for the crop coals than their fresh companions.

Prediction of Sorbent Performance in a Circulating Fluidized Bed Boiler Based on Petrographic Properties

A method for prediction of sorbent consumption is presented here and has been developed based on plant operating data for a boiler in which several limestone and dolostone products were tested under similar firing conditions. The method considers the characteristic partitioning of calcium and sulfur between the flyash and bottom ash stream for the boiler, the feed particle size distribution of the sorbent, and petrographic properties of the sorbents. The predictions of sorbent usage were compared to plant operating data for five sorbents, of two distinct petrographic types. The plant operating data used featured full load operation. The five sorbents tested were all from Pennsylvania, and each contained greater than 40 wt. % CaO. In four of the five cases, the predicted sorbent usage was within 10 wt. % of the average full load sorbent usage by the boiler.

Influence of Petrographic Composition of Sorbents on Attrition Characteristics in a CFB Boiler

The cost of sorbent use can be significant for a CFB power project, notably where the plant fuel includes high sulfur coal or petroleum coke products. Minimizing sorbent costs will improve the economic competitiveness of individual CFB power plants, as well as the technology in general.Copyright

Utilization of coal in IGCC systems

Abstract The chapter deals primarily with the utilization of coal in IGCC systems. Coal gasification for power generation is gaining popularity due to the availability of the raw material (coal) worldwide, reduced environmental-related issues compared to the utilization of coal in combustion technologies, and the ability to capture CO 2 . To improve the overall efficiency of the IGCC process, an in-depth understanding of various coal properties that influence the design and operation of an IGCC plant is required. These coal properties determine the type of gasifier suitable for the process, the feeding system, the kind of coal preparation required, the amount of oxidants required for the gasification process, the flux to be added to keep the slag flowing, and so on. Besides coal, utilization of other solid feedstocks that are commonly used in IGCC plants, such as biomass and petcoke, are also presented in this chapter.