• Energy Research

Abstract In this study, pyrolysis of sawdust and pinewood (120–250 μm) was conducted in a drop-tube furnace (DTF) at temperatures of 900, 1100, 1300 °C and residence times of 50–600 ms in both CO2 and N2 atmospheres. The samples are fed at a rate of 5–10 g/h in a gentle flow of nitrogen (1 L/min) to ensure laminar flow. A silica tracer method has been developed to accurately determine the high temperature volatile matter yields. The elemental analysis of chars collected allowed the study of the release of nitrogen. BET surface area, scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses were also carried out to study the chars produced. Burnout tests were conducted at 1100 °C, an O2 concentration of 5% v/v in N2 and CO2 respectively, using the chars produced at the same temperature and a residence time of 200 ms. In nitrogen, the maximum volatile yield achieved was 97 wt% while in CO2, the maximum volatile yield was over 99 wt% for residence times above 200 ms, indicating virtually complete gasification of the char. These are the highest reported volatile matter yields for biomass obtained using a DTF. The release of nitrogen into the volatile phase is proportional to the yield of volatiles both for air and oxy-fuel conditions. SEM images revealed higher porosities of the DTF CO2 chars than those of N2, being consistent with their higher BET surface areas. Faster char burnout was obtained for oxy-fuel firing attributable to the CO2/char gasification reactions. The results will be useful for modeling dedicated oxy-biomass firing and co-firing systems.

Activated carbons represent one of the important categories of the adsorbent materials for CO2 capture currently under development. However, the low adsorption capacity and selectivity at low CO2 partial pressure or relatively high flue gas temperatures is the main barrier for carbons to be applied in post-combustion CO2 capture under practical conditions. Here, we report the successful preparation of hierarchical ultra-micro/mesoporous bio-carbons from using a facile one-step method with a low-grade biomass waste as the feedstock. The bio-carbons exhibit high adsorption capacities (1.90 mmol/g) and record-high Henry’s law CO2/N2 selectivities up to 212 at ambient temperature and low CO2 partial pressure. Unlike conventional chemical activation process for manufacturing carbon materials, the integrated compaction-carbonization-activation method proposed endows the biowaste-derived carbons with unique hierarchical bio-modal pore structures, which is highly characterised by their high mesoporosity and high ultra-microporosity with narrow pore size distributions. The results demonstrated that the unique surface textural properties along with the enhanced surface chemistry due to the simultaneously achieved potassium intercalation created favourable conditions for CO2 adsorption with high CO2/N2 selectivity at low CO2 partial pressures, whilst the presence of mesoporosity greatly increased the CO2 adsorption kinetics. Measurements of CO2 adsorption heat confirmed the strong surface affinity of the prepared bio-carbons to CO2 molecules.

AbstractAmine solvents have long been used by industry as absorbents for acid gas (CO2, H2S) removal and are the current technology of choice for post-combustion carbon capture from fossil fuel power plants. However, these technologies are energy intensive and have a number of short comings. In an effort to reduce the capital cost and energy penalty of carbon capture alternative technologies are being explored, of which solid sorbents have shown good potential. Adsorbents containing basic nitrogen functional groups to increase adsorbent / adsorbate interaction have been demonstrated to be effective for post-combustion capture. These materials are usually synthesised by the impregnation of basic amine polymers or bonding of amine groups to the surface of inorganic porous substrates. In this work the development of a range of novel high nitrogen content activated carbon adsorbents will be described. The aim of this research is to introduce basic nitrogen directly into the matrix of activated carbon to yield high capacity adsorbents with high thermal stability in terms of volatile and thermal loss of nitrogen.Novel nitrogen enriched activated carbons have been synthesised by a templating or nanocasting technique by which a removable inorganic template is used to generate porous polymers with high surface area. A range of adsorbents have been synthesised by templating of melamine–formaldehyde resin with silica followed by activation over a range of temperatures from 400–700 ∘C. The resultant adsorbents have been characterised in terms of their textural properties, elemental composition and surface chemistry, with materials containing up to 42 wt.% nitrogen and 880 m2 g−1 surface area generated. Adsorption capacities up to 2.25 mmol g−1 of CO2 at 25 ∘C were measured using thermogravimetric analysis and will be discussed in terms of the textural and surface chemical properties of the carbons as determined by X-ray Photoelectron Spectroscopy (XPS). Both texture and surface chemistry influence the CO2 capture performance of the adsorbents. The activation temperature used during the synthesis step controls the nitrogen functional groups present, as determined by XPS, with the loss of triazine nitrogen with increasing activation temperature proposed to account for the decreased CO2 affinity. Finally the stability and regeneration of the carbons over numerous thermal swing adsorption cycles will be described.

AbstractComplementary analytical methods have been used to study the effect of potassium on the pyrolysis mechanisms of cellulose and lignocellulosic biomasses. Thermogravimetry, calorimetry, high‐temperature 1H NMR spectroscopy (in situ and real‐time analysis of the fluid phase formed during pyrolysis), and water extraction of quenched char followed by size‐exclusion chromatography coupled with mass spectrometry have been combined. Potassium impregnated in cellulose suppresses the formation of anhydrosugars, reduces the formation of mobile protons, and gives rise to a mainly exothermic signal. The evolution of mobile protons formed from K‐impregnated cellulose has a very similar pattern to the evolution of the mass loss rate. This methodology has been also applied to analyze miscanthus, demineralized miscanthus, miscanthus re‐impregnated with potassium after demineralization, raw oak, and Douglas fir. Hydrogen mobility and transfer are of high importance in the mechanisms of biomass pyrolysis.

Abstract The activity of potassium-containing char pellets prepared from different low-cost carbon precursors towards NOx reduction in an oxygen-rich environment has been investigated by isothermal reactions at 325 °C. From the overall data, it can be asserted that high potassium content is a key factor in allowing carbon-based pellets to achieve better NOx reduction capacities whilst exhibiting higher selectivity to NOx reduction and inhibiting carbon burn-off, but the intrinsic characteristics of the carbon precursor must also be taken into account (mineral matter content, carbon nature…). Having chosen the most appropriate formulation for pellet preparation (high potassium content and suitable carbon precursor, a metallurgical coke), the 3HT-C pellets demonstrated the most promise for consideration for NOx removal. This sample, when tested as a packed bed of pellets at 325 °C, gave more moderate and very constant NOx reduction rates in combination with very high selectivity towards NOx. On the other hand, when tested at 400–450 °C, very high and constant NOx reduction rates were achieved, with decreased but still acceptable selectivity. Reaction data from a lifetime test were not significantly worse than those from a 2 h reaction, which is very encouraging. Neither was sample efficiency compromised in the lifetime test, in which only 18% (maximum) of oxygen was consumed compared with 84.1% (maximum) NOx converted.

During carbonisation coal undergoes both physical and chemical changes that result in the generation of gas and tar and the formation of an intermediate plastic state. This transformation is known to generate high internal gas pressures for some coals during carbonisation that translate to high pressures at the oven wall. In this study, three low volatile coals A, B and C with oven wall pressures of 100 kPa, 60 kPa and 20 kPa respectively were investigated using high-temperature rheometry, H-1 NMR, thermogravimetric analysis and SEM, with the primary aim to better understand the mechanisms behind the coking pressure phenomenon. Rheometer plate displacement measurements (Delta L) have shown differences in the expansion and contraction behaviour of the three coals, which seem to correlate with changes in rheological properties; while SEM images have shown that the expansion process coincides with development of pore structure. It is considered that the point of maximum plate height (Delta L-max) prior to contraction may be indicative of a cell opening or pore network forming process, based on analogies with other foam systems. Such a process may be considered important for coking pressure since it provides a potential mechanism for volatile escape, relieving internal gas pressure and inducing charge contraction. For coal C, which has the highest fluidity Delta L-max occurs quite early in the softening process and consequently a large degree of contraction is observed; while for the lower fluidity coal B, the process is delayed since pore development and consequently wall thinning progress at a slower rate. When Delta L-max is attained, a lower degree of contraction is observed because the event occurs closer to resolidification where the increasing viscosity/elasticity can stabilise the expanded pore structure. For coal A which is relatively high fluidity, but also high coking pressure, a greater degree of swelling is observed prior to cell rupture, which may be due to greater fluid elasticity during the expansion process. This excessive expansion is considered to be a potential reason for its high coking pressure.

Hydrothermal carbonisation (HTC) is an attractive biomass pre-treatment as it produces a coal-like fuel, can easily process wet biomass and wastes, and lowers the risk of slagging and fouling in pulverised fuel (PF) combustion boilers. One of the major factors in determining the suitability of a fuel as a coal replacement for PF combustion is matching the char reactivity and volatile matter content to that of coals, as these significantly affect heat release and flame stability. The char reactivity of wood and olive cake biocoals and their respective drop tube furnace chars have been studied using thermogravimetric analysis in comparison to other biomass fuels and high-volatile bituminous coal. It was found that HTC reduces the reactivity of biomass, and in the case of HTC of wood pellets the resulting biocoal has a char reactivity similar to that of high-volatile bituminous coal. Proximate analysis, X-ray fluorescence analysis, and textural characterisation were used to show that this effect is caused primarily by removal of catalytic alkali and alkaline earth metals. Subsequent torrefaction of the wood biocoals was performed to tailor their volatile matter content to match that of sub-bituminous and high volatile bituminous coals without major impact on char reactivity.

Proceedings of the 9th International Conference on Greenhouse Gas Control Technologies (GHGT-9), 16–20 November 2008, Washington DC, USA This paper describes the development of carbon-based adsorbents for CO2 separation in integrated gasification combined cycle (IGCC) processes for energy generation and hydrogen production. The research presented forms part of a Research Fund for Coal and Steel funded project “Hydrogen separation in advanced gasification processes” (HYDROSEP) with the ultimate aim of developing technologies to reduce the costs for the capture of CO2 when compared to existing absorption processes. A range of carbon adsorbents were developed by MAST Carbon. They present significant microporosity and in some cases also meso or macroporosity. CO2 adsorption isotherms have been determined using a dual limb differential pressure apparatus under realistic operating conditions. CO2 and H2 high pressure adsorption isotherms at room temperature have also been evaluated in a high pressure adsorption balance. Maxima CO2 uptakes of 58 wt.% at 3 MPa and H2 uptakes of 0.3 wt.% at 4 MPa were obtained. The significant differences observed in CO2 and H2 adsorption at high pressures showed the high selectivity for CO2 of the tested MAST Carbon adsorbents. This work was carried out with financial support from the Research Fund for Coal and Steel (Project RFCR-CT- 2006-00003).MGP acknowledges support from the CSIC I3P Program co-financed by the European Social Fund. TD would like to thank EPSRC for funding (Adcanced Research Fellowship, EP/C543203/1). Peer reviewed