R. Kandiyoti

Combustion reactivity and morphological change in coal chars: Effect of pyrolysis temperature, heating rate and pressure

Abstract Changes in the combustion reactivity of sets of chars from five coals have been determined as a function of increasing devolatilisation temperature, heating rate and pressure. Chars were prepared in wire-mesh pyrolysis reactors, where evolving volatiles are rapidly removed from the vicinity of the heating coal. Samples were heated at rates between 1 and 5000 K s −1 to temperatures up to 1500°C and H 2 -pressures up to 150 bar. Combustion reactivities of the chars have been determined by iso-thermal thermogravimetric methods. For rapidly heated chars, reactivity was found related to temperature by: In (R max = a − b ∗ T . Chat reactivities were found to increase with increasing heating rates up to 1000 K s −1 , but to level off between 1000 and 5000 K s −1 . At 700°C, reactivities of hydropyrolysis chars were found to go through a minimum at around 40 bar, whilst at 850°C the upturn of char reactivity took place at lower pyrolysis pressure. Taken together, our results suggest that char reactivity is related to tar release and that an upper limit exists to the enhancement of char porosity and reactivity, determined by patterns— and limits—of tar release from coal particles.

Variable‐heating‐rate wire‐mesh pyrolysis apparatus

An electrically heated wire‐mesh apparatus for pyrolysis studies has been developed which uses computer‐driven feedback control for the heating system and thus can apply virtually any time‐temperature history to the sample. Internal components are water cooled to prevent heat buildup during long runs. Using this system, coal pyrolysis has been studied at heating rates from 0.1 to about 5000 K/s and temperatures up to 1000 °C. Alternating current is used for heating; this allows the thermocouples to be attached directly to the sample holder and also makes power regulation relatively simple. For atmospheric‐pressure experiments, a gas sweep can be forced through the sample holder to remove products from the heated zone and also to concentrate them in a trap which can be removed from the apparatus and weighed to establish tar yields directly. Although the design is optimized for atmospheric‐pressure operation, relatively simple modifications allow operation under vacuum or at pressures of up to 160 bars in i...

Secondary reactions of flash pyrolysis tars measured in a fluidized bed pyrolysis reactor with some novel design features

Abstract A fluidized bed pyrolysis reactor with variable freeboard height was constructed to investigate the effect of time-temperature history on freshly formed pyrolysis tars. The apparatus allowed direct measurement of tar loss through gas phase reactions in the freeboard, and calculation of the quantities of tar cracked or coked within the fluidized bed itself. At 600 °C about 30 wt% more tar (daf cellulose) appears to crack in the freeboard for 3.5 s residence time compared with 0.25 s. At this temperature, 19 wt% (daf cellulose) tar is destroyed within the fluidized bed before reaching the freeboard. The amount of tar destroyed within the bed was found to increase sharply with temperature, reaching nearly 50 wt% (daf cellulose) at 700 °C. Four other substrates were studied: silver birch, pulverized municipal solid waste, a Turkish lignite (Can), and a low rank British bituminous coal (Linby). Results indicate that the temperatures corresponding to maximum tar yields and thermal stabilities of the tars broadly correlate with substrate rank. At higher reactor temperatures, biomass tars lose their predominantly carbohydrate character and heteroatom content, and become more aromatic in character, with increasing contents of condensed aromatic ring systems.

Vacuum pyrolysis of maceral concentrates in a wire-mesh reactor

Abstract The pyrolytic behaviour of maceral concentrates from three bituminous coals was investigated in a wire-mesh reactor under vacuum and at atmospheric pressure. A new set of traps, designed to overcome difficulties encountered in tar capture and recovery during vacuum pyrolysis, has been developed. Experiments reported were carried out at 1000 K s −1 to 700 °C with a 5 s hold; tar characterization was carried out by size exclusion chromatography, u.v. absorption and u.v. fluorescence spectroscopy. During pyrolysis experiments with whole coals, vitrinite, liptinite and inertinite concentrates, it was systematically found that about 5 wt% more gas was formed at atmospheric pressure than under vacuum. No major synergistic effects were observed between the petrographie components of coals during pyrolysis. Liptinite concentrate tars were found to have larger molecular mass distributions than tars from other maceral concentrates, and were found to undergo greater change from vacuum to atmospheric pressure. U.v. absorption and u.v. fluorescence spectra suggested that tars produced at atmospheric pressure were richer in aromatic content compared to vacuum tars. Liptinite concentrate tars appear to be more aliphatic/hydroaromatic in character and to have shown greater sensitivity to intraparticle secondary reactions. Spectra of tars from all three maceral concentrates show similar structural features suggesting that aromatic cluster size distributions of tars from different macerals may not be significantly different even though their aromaticities may differ.

Structural characterization of biomass pyrolysis tars/oils from eucalyptus wood waste: effect of H2 pressure and sample configuration

Changes in yields and structural characteristics of tars/oils produced in the first stage of a two-stage fixed-bed (‘hot-rod’) reactor were determined as a function of sample configuration and pressure. The work included a critical evaluation of analytical methods for monitoring structural changes in biomass-derived liquids. Total volatile and tar/oil yields decreased and significant structural changes were observed with increasing sample bed height and pressure, leading to ‘lighter’ tars/oils. Products were observed to become more aromatic and less oxygenated. Less intense degradation reactions were observed under hydrogen than under helium. 1-Methyl-2-pyrrolidinone (NMP) was used as eluent in size-exclusion chromatography; solubility limitations were found with tetrahydrofuran as eluent. Good agreement was obtained between structural changes inferred from size-exclusion chromatography and u.v. fluorescence and u.v. absorption spectroscopies. The characterization of liquid products by 1H n.m.r. was found to be less informative than expected, due to the multiplicity of structural assignments in relatively narrow bands. The work suggests that mild hydropyrolysis in deeper beds may be considered as a process step more likely to produce lighter, less oxygenated and more stable tars/oils than liquids produced at atmospheric pressure. However, the reforming process achieved by extraparticle reactions and by the effect of pressure gave rise to significant loss of liquid product.

A two-stage fixed-bed reactor for direct hydrotreatment of volatiles from the hydropyrolysis of biomass: effect of catalyst temperature, pressure and catalyst ageing time on product characteristics

This investigation involved the hydropyrolysis of biomass (eucalyptus globulus) and the immediate catalytic hydrocracking of pyrolytic oils in the second stage of the reactor. The effects of temperature, pressure and the catalyst ageing time on the final product tar have been studied using the catalyst Zeolite H-ZSM5. The catalytically hydrocracked tar/oil products were characterised and compared with the hydropyrolysis product from the first stage of the reactor to determine the effect of catalytic hydrocracking. The carbon deposition on the catalyst has been examined using thermogravimetric analysis. The tar yields after catalytic hydrocracking decreased with increasing pressure and temperature of the cracking stage. The tar yields at 10 bar pressure were greater than those at 40 bar pressure. The fresh zeolite catalyst trapped more than 40% of the product from the hydropyrolysis stage and TGA evidence indicated that this was not as carbon deposition but as volatiles trapped in the zeolite matrix. Reuse of the catalyst resulted in little more uptake of volatiles; however, extended use of the catalyst did not result in increased yields of liquid products but in increased production of light volatiles or gas. The H-ZSM5 catalyst appeared to act as a more active cracking catalyst rather than to promote hydrogenation or deoxygenation of the liquids produced in the hydropyrolysis stage. Characterisation of the liquids by SEC and UV fluoresence indicated that structural changes were relatively minor despite the significant changes in yields of liquids with process conditions. Available reaction routes do not appear to allow specific deoxygenation pathways to predominate without disintegration of parent molecules to lighter volatiles, under the conditions used here.

The effect of variations in time-temperature history on product distribution from coal pyrolysis

An electrically-heated wire-mesh pyrolysis reactor is described. The apparatus is capable of heating rates ranging from virtually zero to 5000 K s−1 as well as multi-step heating with variable heating rates and holding times at intermediate temperatures and at the peak temperature. An alternating current power supply is used, allowing thermocouples to be attached directly to the wire mesh to give close tracking of temperature changes. The heating circuit is driven by a micro-computer for real-time logging and control. A continuous stream of carrier gas is forced through the sample holder, giving mean volatiles residence times of around 2ms in the heated zone. The gas sweep gives sample cooling rates of 500–1000 K s−1; slower cooling rates may be selected by pre-programming the heating circuit. Pyrolysis yields of tar and total volatiles from a low rank British bituminous coal (Linby) obtained with single-step heating, show clear changes in the product distribution with increasing heating rate. The effect of holding times, relatively unimportant during slow heating (1 K s−1) runs, is quite pronounced for fast heating rates (l000 K s−1) at peak temperatures below 650–700 °C. Because of this, unless pyrolysis reactions are allowed to run to completion, the effect of the heating rate on product distributions may be masked.

Determination of 17 trace elements in coal and ash reference materials by ICP-MS applied to milligram sample sizes

This paper describes the evaluation of two digestion methods used to extract 17 elements (Be, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, As, Se, Mo, Cd, Sn, Sb, Ba and Pb) from coal and coal ash, obtained as standard reference materials. An acid digestion method in open vessels using sulfuric, hydrofluoric, perchloric and nitric acid was compared with a sealed microwave digestion method using nitric acid only. The microwave method cannot break down silicates, which harbour many trace elements, but can extract As and Se quantitatively. Volatile elements such as As and Se might be lost during the open vessel digestion; therefore, the closed vessel is the method of choice. The effect of reducing the sample size from several hundred milligrams to amounts as small as 5 mg on the accuracy of determinations was investigated. No significant differences were observed so long as the total dissolved solids and the dilution factors of the final solutions remained constant.

Characterization of biomass pyrolysis tars produced in the relative absence of extraparticle secondary reactions

Product distributions from the atmospheric pressure pyrolysis of sugar-cane bagasse and silver birch were determined in a wire-mesh reactor as a function of temperature (300–900 °C) and heating rate (1–1000 K s−1), over a range of holding times between 0 and 100 s. Major groups of components present in the tars were identified by g.c.-m.s.. Above 500 °C when increasing the heating rate from 1 to 1000 K s−1 in experiments using holding times longer than several seconds, tar yields were observed to increase by about 10 wt% to nearly 54% w/w daf bagasse; total volatile yields increased by similar amounts. Number average molecular masses (MMn) of tars determined by vapour pressure osmometry (v.p.o.) were in the range 150–250 Da, those of silver birch tars being systematically larger than those of sugar-cane bagasse by 15–30 Da. For both sets of tars, MMn were observed to decrease with increasing temperature between 400 °C and 600 °C, but remained unchanged between 600 °C and 900 °C. The molecular masses were also observed to decline with increasing heating rate. Lignocellulosic substrates appear to cleave readily to give relatively small, compact and relatively stable molecules able to evaporate completely at temperatures around 500 °C. Extensive chemical modification of primary products appears to result from intraparticle secondary reactions, these reactions being intensified during pyrolysis at higher heating rates. Whilst the wire-mesh reactor configuration appears not to alter tar yields or structures subsequent to release from parent substrate particles, comparison with tars obtained at 600 °C in a ‘hot-rod’ (fixed-bed) reactor suggests that whilst overall yields may be adversely affected and the relative abundance of some major components altered, extraparticle secondary reactions do not substantially change the character of the tar at this temperature.

CO2 and steam-gasification in a high-pressure wire-mesh reactor: the reactivity of Daw Mill coal and combustion reactivity of its chars

Interrelationships between extents of coal gasification, char gasification and combustion reactivities have been examined as a function of CO2 and steam pressure and holding time. Experiments have been carried out in a high-pressure wire-mesh reactor equipped with a steam injection facility. Evidence has been presented linking minima in weight loss vs. reactive gas pressure curves with deactivation as a result of secondary char deposition. At longer times, extents of gasification were about 2–3 times higher in steam compared to CO2. With increasing reactivity of the ambient gas, the minimum in the weight loss vs. pressure curve appears only at the shortest reaction times. At higher pressures, reactivities between zero and 10 s were lower than those between 10 and 20 s. These data are novel and support the suggestion that a relatively unreactive layer of re-polymerised tar tends to slow down the gasification of the main body of the char in the initial stages – until it is itself consumed. Combustion reactivities of pyrolysis and gasification chars decrease with increasing pressure. When exposed to a temperature of 1000°C, combustion reactivities of chars were found drop rapidly within about 10 s to relatively low and stable values. The end values were independent of pressure and composition of the reactive gas. The results are relevant to the design of pilot and commercial scale reactors. On the basis of limited available evidence, it appears that secondary char deposition caused by tar re-polymerisation plays an increasingly significant effect with increasing particle size.