D. R. Dugwell

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.

Investigation into potential synergy between power generation, cement manufacture and CO2 abatement using the calcium looping cycle

Here, we report the preliminary results of an investigation into the effects on cement chemistry of using as a feed calcium oxide (CaO) which is the spent sorbent from a promising CO2 capture process, the calcium looping cycle.

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.

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.

Heteroatom distribution in pyrolysis products as a function of heating rate and pressure

Abstract Nitrogen and sulphur partitioning between product phases during the pyrolysis of Illinois No. 6 and Tilmanstone (high-rank UK) coals was monitored as a function of heating rate and pressure. An atmospheric-pressure wire-mesh pyrolysis reactor was used at heating rates of 5–5000 K s −1 to 950 °C. High-pressure experiments were performed at 1000 K s −1 to 700 °C in a wire-mesh reactor redesigned to allow tar capture by continuous sweeping of volatile products away from the sample holder into an externally cooled trap. The reactor configurations made it possible to determine yields and recover tars in the relative absence of secondary reactions. Generally, the net transfer into the tar phase of both nitrogen and sulphur was enhanced by increasing heating rate and suppressed by increasing pressure. At 950 °C and 0.1 MPa, depending on coal type and heating rate, between 25 and 50% of the evolved nitrogen was found in the tar. At 700 °C, the split of evolved N between tar and gas as a function of pressure differed sharply between the two coals. Despite comparable organic S contents of the two coals (56% of total S in Illinois No. 6, 51% for Tilmanstone), the fractions of evolved S at 950 °C (0.1 MPa) were different, rising from 60 to 70% for Illinois No. 6 and from 35 to 45% for Tilmanstone with increasing heating rate. With increasing pressure, the distribution of coal-S between product phases differed considerably between the coals: for Illinois No. 6 the volatilized S remained roughly constant over the pressure range, whilst tar-S rapidly shifted (52 to 9%) to the gas phase. For Tilmanstone, volatilized coal-S increased by ~ 10% between 0.1 and 7.0 MPa but the tar-S content also shifted to the gas, suggesting the presence of tar-S within structures thermally more sensitive than those associated with tar-N. Considerably less coal-S in Tilmanstone volatilized. In view of the apparent differences between the combustion chemistries of the different phases formed during devolatilization, the results appear to be of direct relevance to the prediction of NO x and SO x formation during combustion and gasification.

Pyrolysis of coal maceral concentrates under pf-combustion conditions (I): changes in volatile release and char combustibility as a function of rank

Abstract The pyrolytic behaviour of sets of maceral concentrates have been examined under conditions similar to those of pf-combustion. Relative combustion reactivities of chars prepared during the pyrolysis experiments have been compared by standard methods. As expected, for samples of similar elemental C-content, total volatile release decreased as: liptinite > vitrinite > inertinite. The data indicates the absence of synergistic effects between vitrinite and inertinite during pyrolysis. The sensitivity of pyrolysis yields to inertinite content was found to diminish with increasing coal sample rank. In marked contrast to the order established for volatile release, combustion reactivities of maceral chars (of similar carbon content) may be ranked in the order: inertinite > vitrinite > liptinite. A direct comparison with maceral concentrates from the same coal was undertaken. Vitrinite/inertinite graded samples of a South African coal were pyrolysed at 1500°C: these chars exhibited increasing reactivity with increasing inertinite concentration, but observed differences in reactivity between samples from the same coal were not large. Furthermore, for a higher rank coal, char reactivities were found to be essentially independent of original inertinite concentration. It is clearly possible for chars from low volatile coals to be relatively reactive. However, there appears to be no reason to suggest that ‘reactive inertinites’ might release volatile matter in quantities comparable to those of vitrinites from the same coal.

Partitioning of trace elements during the combustion of coal and biomass in a suspension-firing reactor

Abstract A novel, quartz ‘suspension-firing’ reactor is described for monitoring trace element release during solid fuel combustion under conditions relevant to fluidised bed combustors. The new design allows the examination of fuel particle combustion in the absence of bed solids. Experiments have been conducted using two coals, a sample of wood bark and one of straw. Ash from the reactor walls and base have been analysed separately from ash collected on a sintered disc in the path of exit gas. Trace element concentrations in these samples were analysed by Inductively Coupled Plasma (ICP)-mass spectrometry and ICP-atomic emission spectrometry (AES). The fractions of original trace elements retained by the ash have been reported; relative enrichment in the ‘sinter-ash’ was calculated by comparing with ‘bottom ash’. Mercury was almost completely volatilised from all fuels, as was selenium for all except wood-bark. Chromium, manganese and thallium were partially volatilised and nickel mostly retained in all samples. The behaviour of beryllium, lead, molybdenum, vanadium and zinc varied, depending on the fuel sample. Beryllium was released to a greater extent from coal/straw than the other fuels. Vanadium was partially volatilised from wood-bark and coal/straw, while the largest proportion of the zinc released was from the wood-bark. Lead and molybdenum were retained to a greater extent by ‘Colombian coal’ and wood-bark, respectively. Evidence of the enrichment of certain trace elements on the finer ‘sinter-ash’ particles has also been observed, e.g. for As, Cd, Pb and Tl during the combustion of the ‘Colombian-coal’.

Comparison of the pyrolysis and gasification of biomass: effect of reacting gas atmosphere and pressure on Eucalyptus wood

The present study was undertaken to clarify the purpose of using reactive gases during the thermochemical processing of biomass. Conversions observed during pyrolysis (He) have been compared directly with gasification in H2, CO2 and steam-helium mixtures. Experiments have been carried out at pressures between 1 and 20 bar, in a fixed-bed ‘hot-rod’ reactor, operating at a relatively low heating rate (10°C s−1). Conversions in helium were between 85 and 88%, leaving little scope for improvement by the use of reactive gases. Up to 95% has been achieved elsewhere by heating at faster rates. As the use of steam or hydrogen also tends to enhance heat transfer to particles, the use of reactive gases may still be justified when particles are heated rapidly to lower reactor temperatures, and/or when residence times at peak temperature are very short. In pilot or industrial scale fluidised-bed operation, loss of fuel through elutriation of fines is usually suppressed by a combination of increased fuel particle size and the use of reactive gases. The present study shows that comparable or higher conversions can be achieved by operating above 800°C in an inert gas, provided elutriation is suppressed by design options other than increasing particle size, which tends to produce unreactive chars. The use of reactive gases such as steam would appear to partially compensate for possibly non-optimal choice of fluidising parameters.

A comparison of the pyrolysis of coal in wire-mesh and entrained-flow reactors

The properties of chars derived from coals of a wide range of rank produced in wire-mesh (WMR) and entrained-flow (EFR) reactors were compared. The WMR was operated at atmospheric pressure of helium or nitrogen with heating rates of 2-5000 K s- ‘, temperatures up to 1773 K and residence time of 2-30 s. The EFR was operated at 1273 K with a residence time of 1 s in nitrogen at atmospheric pressure. The yield of volatile matter from the EFR was consistently lower than that from the WMR; accordingly, the EFR chars were of significantly higher residual volatile matter than the corresponding WMR chars. Differences in yields and char reactivities were observed between pyrolysis in helium and nitrogen. The extent of secondary reactions and experimental flow arrangements accounted for the remaining difference. The results show that in general there is reasonable agreement between the reactivities of the chars prepared under similar conditions by the two techniques. The differences between the results for the two techniques are discussed in terms of variations in the experimental conditions and their effect on char properties. Novel data for pyrolysis in hydrogen on a wire mesh are included to demonstrate the effect of pressure over the range 0.25-7 MPa.

Internal consistency of coal gasification reactivities determined in bench-scale reactors: effect of pyrolysis conditions on char reactivities under high-pressure CO2

Abstract Relationships between char preparation conditions and CO 2 -gasification conversions have been examined using a wire-mesh, a fluidised-bed and a ‘hot-rod’ (fixed-bed) reactor. Conversions from the direct gasification of untreated coal have been compared with those from the gasification of chars prepared: (i) in-situ under helium in each reactor, before switching over to CO 2 ; and (ii) from gasification in all three reactors of a char sample, pre-prepared in the fixed bed reactor. Conversions from direct gasification were slightly higher than experiments where samples were first pyrolysed in situ and gasified under CO 2 in the same reactor. In some cases, conversions were within experimental error. Under these conditions and within several percentage points, the pyrolysis and gasification steps may be considered as additive. Lower overall conversions observed in the hot-rod reactor have been attributed to the combined effect of: (i) formation of a low reactivity char—owing to tar-solids contact leading to secondary char deposition; and (ii) poor contact between char and the reactive gas. In the wire-mesh and the fluidised-bed reactors, direct CO 2 -gasification conversions were considerably larger than those determined using the common sample of char, pre-prepared in the hot-rod (fixed-bed) reactor. This sharp drop in overall conversion shows that care must be taken in interpreting reactivity data from ‘two-reactor, two-step’ procedures where char preparation conditions are not identical to those of the gasification step. A broadly linear relationship has been observed between conversions from a pilot plant reactor and direct CO 2 -gasification in the bench-scale high-pressure wire-mesh reactor. Despite differences between operating parameters in the two reactors, the results suggest that data from the wire-mesh reactor may be successfully used to compare relative coal reactivities under pilot-plant gasification conditions.