Mass spectroscopic characterization of tars from the gasification of low-rank coals

Mass spectroscopic methods were developed for characterization of gasifier tars. The tars studied were produced during pilot plant operation of the GFETC slagging fixed-bed gasifier. Nine samples were selected, representing four low-rank coals and a range of gasifier conditions. The tars were analysed by gas chromatography-mass spectrometry, high-resolution mass spectrometry, and low-voltage mass spectrometry. Whole samples and fractions separated by column chromatography were analysed by conventional g.c.-m.s. techniques for identification of major compounds. Carbon number analysis was accomplished using h.r.m.s. for mass identification and l.v.m.s. for quantitative analysis. The chemical nature of the coal gasified was shown to be important in determining the composition of the tar. Approximately twenty constituents varied markedly in tars produced from different coals gasified under identical conditions. The important operating parameters affecting tar composition are the amount of hydrogen in the gas phase and the coal residence time. Neither operating pressure nor oxygen rate appear to be related to the composition of the tar.     

The kinetic study of catalytic low-rank coal gasification under CO2 atmosphere using MVRM

In this study, we carried out a kinetic investigation and analysis of the syngas produced by low-rank coal gasification. We conducted a proximate and ultimate analysis of six types of coals in order to measure the amount of sulfur and ash. The coal was then analyzed using a thermo gravimetric analyzer (TGA) to select a suitable sample. The selected Samhwa coal sample was mixed with catalysts. Samples mixed with catalysts were used to determine the activation energy under CO₂ atmosphere using the modified volume reaction model (MVRM). An analysis of the resulting syngas was performed using gas chromatography (GC).     

Determination of carboxyl groups in low-rank coals

The determination of carboxyl groups in low-rank coals by exchange with barium chloride-triethanolamine reagent is re-examined. The difference in carboxyl contents calculated from the barium content of the exchanged coal, and the amount of acid used to extract the barium, is discussed. Reliable carboxyl contents are obtained providing the latter procedure is used.     

Model for drying during fluidized-bed combustion of wet low-rank coals

An unsteady state heat conduction model with a convective boundary condition is proposed for the drying of low-rank, high-porosity coals, such as lignites, during fluidized-bed combustion. The drying front is assumed to be the receding surface of a wet core. The solution technique for this moving boundary problem is based on the heat balance integral approach with immobilization of the moving boundary by a change in space variable. The governing cubic equation describing the drying curve in dimensionless form may be solved easily by the Newton—Raphson method. The model predictions are compared with experimental data for Mississippi lignite with excellent agreement. A correlation for estimation of total drying time is proposed. The temperature profiles obtained may be used for the study of the coupled drying and devolatilization in fluidized-bed combustors. The profiles could also be of importance in the study of formation of fissures/cracks in lignites subjected to intense heating conditions encountered during fluidized-bed combustion.     

Laboratory gasification of five Canadian coals

Results of a gasification study of five Canadian coals in a 100g laboratory fixed-bed gasifier are reported. The coals were three bituminous coals (Byron Creek, Prince and Devco) and two lignites (Coronach and Bienfait). When gasified in an equal mixture of nitrogen and oxygen at 850 °C, the lignites reacted more rapidly and produced more liquid and gaseous products than the bituminous coals, whereas the bituminous coals produced more methane but less carbon monoxide. Overall rates for both the gasification and combustion processes were 3.20, 2.80, 1.84, 1.70 and 1.25 wt% carbon conversion per min for Coronach, Bienfait, Prince, Byron Creek and Devco coals, respectively.     

Transformations of sodium during gasification of low-rank coal

Specially prepared forms of a high-sulphur South Australian lignite containing sodium in two forms; as a carboxylate, forming a part of the coal organic matter or as soluble salt (NaCl) were investigated. The influence of gasification environment (gas type, temperature and time) on the transformations of sodium and the extent of vaporisation were studied. Contrary to theoretical equilibrium calculations for the high levels of sulphur present in the coal, no sodium sulphide formation was found upon the transformation of carboxylate sodium. Experiments confirmed rather the formation of sodium carbonate during gasification or pyrolysis of the sulphur-rich coal. Sodium was found to evaporate, with sodium release higher during gasification in steam or carbon dioxide than for pyrolysis in nitrogen. The release of sodium and chlorine from coal containing sodium chloride was disproportionate. Almost all of the chlorine was measured to have been released during gasification and pyrolysis by 850 °C. The release of sodium was nearly half of that of chlorine and inferred to be in the form of sodium chloride vapour. Identification of sodium carbonate in the residual pyrolysed char confirms the reaction of sodium chloride with hydrogen from coal carboxylic acid groups. Neither sodium sulphide nor sodium hydroxide was identified in char for coal containing sodium chloride.     

Comments on the reactivity of low-rank coals in liquefaction

This communication discusses the subject of the relative reactivities of low-rank and bituminous coals in liquefaction. Accounts given by several researchers have provided seemingly contradictory results. An explanation is presented to reconcile these opposing views.     

Simulation of the steam gasification of coals

Based on three chemical reactions, equations were set up in accordance with the developed model to adequately describe experiments on the steam gasification of solid fuel carbon.     

The catalytic steam gasification of one Australian and three Japanese coals using potassium and sodium carbonates

The catalytic steam gasification of four different coals using potassium and sodium carbonates as catalysts was carried out in a semi-flow type fixed-bed reactor. The coal was gasified with or without the catalyst under a steam—argon atmosphere at a heating rate of 50°C/s at 700–800°C. The catalytic activity of carbonates for gasification was remarkable for Japanese high-volatile coals (Miike and Takashima coals), and moderate for Australian medium-volatile coal (New Lithgow coal); however, the carbonates had little effect on gasification of Japanese lignite (Taiheiyo coal). It is assumed that Miike and Takashima coals soften and melt during the heating process to make the contact between char and catalyst better. New Lithgow and Taiheiyo coals do not have this property. Gasification was promoted significantly at lower temperatures when the catalyst was used. In both catalyzed and uncatalyzed runs the main products were hydrogen and carbon dioxide; the reaction temperature did not affect the composition of the gases much. A water—gas shift reaction occurred during gasification resulting in a large amount of carbon dioxide under a large excess of steam flow.     

Reactivities of 34 coals under steam gasification

The reactivities of 34 coal chars of varying rank with H₂O have been determined to examine the effect of coal rank on the gasification rate of coal char. The reactivities of chars derived from caking coals and anthracites (carbon content > 78 wt%, daf) were very small compared with those from non-caking (lower-rank) coals. The reactivities of low-rank chars do not correlate with the carbon content of the parent coals. To clarify which factor is more important in determining the reactivity, the evolution of CO and CO₂ from char, the moisture content of char and the amount of exchangeable cations were determined for these low-rank coals or their chars. These values were considered to represent the amount of active carbon sties, the porosity and the catalysis by inherent mineral matters, respectively. It was concluded that the amount of surface active sites and/or the amount of exchangeable Ca and Na control the reactivity of low-rank chars in H₂O.     

Fluidized-bed gasification of some Western Canadian coals

Three Western Canadian coals were gasified with air and steam in a fluidized bed of 0.73 mm sand and coal, at atmospheric pressure and temperatures of 1023–1175K to produce a low-calorific-value gas. One non-caking and two caking coals were tested. The effects of temperature, coal feed rate, $\text{air}\text{coal}$ ratio, $\text{steam}\text{coal}$ ratio, coal quality, coal particle size and bed depth on gas composition, gas calorific value and operating stability of the gasifier were established. Results are compared with those previously obtained for the same three coals when gasified in essentially the same equipment, but operated as a spouted bed.     

Studies on catalytic gasification of coal chars. Part 1. Development of pores during gasification

Chars prepared from three coals were impregnated with nickel catalyst and gasified with hydrogen at 850°C and 10 atm. The surface area and porosity of gasified residues at several conversions were measured. Blair Athol char, which had a relatively large surface area, showed a unique gasification pattern, i.e., the rate increased with time until a maximum was attained. Catalytic gasification resulted in an increase in the volume of macropores of a given diameter range, which was characteristic of the catalyst system used. In non-catalytic gasification and in cases when activity of the catalyst was low, mainly pores smaller than 6 nm were enlarged. Possible explanations of the acceleration of the rate are discussed and some consequences of the porosity measurements for this explanation are indicated. As the macropores are enlarged during catalytic gasification, the kinetic pattern could be modified when intraparticle diffusion comes into play.     

Pore entry effects in the solvent-induced swelling of low-rank coals

The swelling of a low-rank coal under the influence of various solvents is examined and results compared with those for a similar coal in briquetted from. Results are interpreted in terms of the effects of briquetting on pore structure.     

Mild oxidation of two low-rank mature coals by alkaline nitrobenzene

Two low-rank mature coals have been oxidized with alkaline nitrobenzene. Soluble phenolic aldehydes and ketones were obtained, similar to those formed from lignites and separable by the same scheme of extraction.     

Flow properties of low-rank coal ash slags: Implications for slagging gasification

For predicting satisfactory operation in slagging fixed-bed gasification, a slag should have a viscosity of < 10 Pa s at 1300 °C when measured in a reducing atmosphere. Various phenomena related to slag flow observed qualitatively in pilot plant testing can be explained or predicted from results obtained in the laboratory. Hysteresis between cooling cycle and heating cycle viscosities requires that slag must be reheated well above its initial temperature to resume slag tapping that was interrupted by a temperature fluctuation. A transition from a reducing to an oxidizing atmosphere in the gasifier hearth, such as may be caused by fuel bed bridging, will raise slag viscosity enough to impair or stop flow even in the absence of hearth cooling. Formation of metallic iron by partial reduction of the slag will raise viscosity, but for low-silica slags which give good slagging operation in the gasifier this effect is not usually serious.     

A novel catalytic process for cellulose gasification to synthesis gas

The catalytic gasification of cellulose to synthesis gas has been studied under mild conditions in a laboratory scale batch-feeding and fluidized bed reactor using air as a gasifying agent and Ar as a cellulose carrier from the feeder to catalyst bed under atmospheric pressure. Various types of support materials and supported metal catalysts have been investigated in this process. The flow conditions were 60 and 50 cm³ (STP)/min of air and Ar, 2 cm height of the fluidized bed and 0.7 s residence time of volatiles. From this investigation CeO₂ has been found as the best support and Rh/CeO₂ has been found as an excellent catalyst in the cellulose gasification at 823 K, which resulted in 100% C-conversion to gas. The use of Rh/CeO₂ catalyst in the secondary bed resulted in lower yield of CO and H₂.     

Oxygen-steam gasification of coals in a spouted bed

A bituminous and a sub-bituminous coal from Western Canada have been gasified in oxygen-steam and air-steam mixtures in a 0.30-m diameter, 50 kg coal/h continuous spouted bed reactor. Results are presented to show the effects of the blast composition and reactor temperature on gas heating value and carbon conversion. Operation in the ash agglomeration mode is illustrated, and the role of K₂CO₃ as catalyst explored. Results from a wide range of experimental gasification conditions are compared with predictions of an equilibrium model.