• Energy Research

Air staging is a well-known effective method to control NOx emissions from solid fuel combustion boilers. However, further research is still needed to clarify the effect of air staging at different injection locations on the gaseous emissions of Fluidised Bed Combustion (FBC) boilers that fire 100% biomass fuels, particularly non-woody biomass fuels. The main objective of this work is to investigate the effect of the staging air injection location on the gaseous emissions (NOx and CO) and temperature profiles of a 20 kWth bubbling fluidised bed combustor firing three non-woody (straw, miscanthus and peanuts) and two woody biomass fuels. The experimental results showed that injecting the secondary air at the higher location could lead to a greater NOx reduction due to the fact that the biomass combustion reaction mainly took place in the splash zone and/or beginning of the freeboard. Up to 30% of NOx reduction, compared with no air staging, was achieved for the non-woody fuels when the staging air was injected at the higher position. Air staging also significantly reduced the CO emissions as a result of the higher temperatures in the freeboard and longer residence time in the primary combustion zone.

Abstract Fluid Catalytic Cracking (FCC) units are responsible for roughly 25% of CO2 emissions from oil refineries, which themselves account for 4–6% of total global CO2 emissions. Although post- and oxy-combustion technologies have been proposed for CO2 capture in FCC, Chemical Looping Combustion (CLC) may also be a potential approach that has lower energy consumption. An equilibrium catalyst (ECat) was first modified with oxidised oxygen carriers (CuO, Co3O4, Mn2O3) using wet-impregnation, and their reduced states (Cu, CoO, Mn3O4, MnO) were generated by hydrogen reduction. To demonstrate that the impregnated reduced oxygen carriers had no significant negative effects on cracking, the prepared catalysts were used to crack n-hexadecane using the standard FCC microactivity test (ASTM D3907-13). The CLC behaviour of coke deposited on the reduced oxygen carrier impregnated ECats, was investigated with the stoichiometrically required amount of oxidised oxygen carrier impregnated ECat in lab scale fixed-bed and fluidised-bed reactors equipped with an online mass spectrometer to monitor CO2 release. Although the conversion and liquid to gas ratio were largely unaffected, coke selectivity did increase with the impregnation of reduced oxygen carriers. However, this increase is mostly attributed to solvent extractable coke. It is possible to reach about 90 vol% combustion efficiency of the coke deposited on ECat using mechanically mixed with CuO and Mn2O3, but the regeneration temperature required, 800 °C, is considerably higher than that under typical regenerator conditions of 650–750 °C for 30–60 min. However, relatively high combustion efficiencies of greater than 94 vol% of the coke deposited on reduced Cu and Mn3O4 impregnated ECat were achieved with the stoichiometrically required amount of CuO and Mn2O3 impregnated ECat at 750 °C for 45 min., close to conventional FCC regenerator conditions.

Abstract Although biomass co-firing is now well established in pulverised fuel (PF) combustion under conventional air-fired conditions, there is little information available on how biomass will behave in oxy-fuel firing. Using thermogravimetric analysis (TGA) and a drop tube furnace (DTF), this study examines the impact of co-firing biomass and coal under oxy-fuel conditions compared to normal air firing, with the emphasis on the potential catalytic effect of biomass-contained alkali and alkaline metals on coal char burnout. Individual chars and their blends prepared from sawdust, pinewood and a South African coal in a DTF and normal tube furnace under slow-heating conditions have been used in TGA char burnout tests. The results demonstrate that the coal/biomass char blends burned off significantly faster than predicted under both oxy-fuel and air-firing conditions, and this synergistic catalytic effect was found to be considerably more pronounced in oxy-fuel conditions. In particular, the DTF biomass/coal char blends from devolatilisation in CO 2 burn off approximately two times faster than those prepared in nitrogen. To further examine the catalytic effect, the raw sawdust sample was first extracted with 5 M hydrochloric (HCl) acid to remove its contained alkali and alkaline metals before the char preparation and subsequent char burnout tests. It was found that the removal of the alkali and alkaline metals led to almost complete loss of the catalytic effect as observed with the untreated sawdust derived char samples. The results indicate that biomass having relatively high contents of alkali and alkaline metals can serve as effective combustion catalysts to improve coal combustion efficiency.

Abstract Combustion characteristics and ash fusion behaviours of Qinghai coal (QH) and Fushun oil shale (FS) and their blends were investigated. It was found that ignition index and burnout index of the blends reached maximum for the blend with 10 wt% FS, while its comprehensive combustibility index remained nearly unchanged when compared with the coal sample. With the increase in heating rates, combustion performance of the samples improved significantly. The statistical analysis demonstrated that combustion temperature contributed significantly (about 73% of the impact ratio) to the thermogravimetric mass loss, followed by oil shale blending ratio and heating rate. In addition, there is noticeable deviation between the experimental and theoretical curves of the blends in the temperature range of 410–480 °C, which indicates the existence of synergistic interactions. Moreover, the lowest apparent activation energy, determined using two model-free integral methods, was found to be 64.1 kJ/mol for the blend with 10 wt% FS. In addition, slag formation and mineral transformation of different samples were determined using the thermochemical database package FactSage 6.3. For the blend with 10 wt% of FS, anorthite, hematite, diopside and quartz were found to be the main crystalline phases at high temperatures. It is shown that the addition of FS mitigated the slagging and fouling tendency of the QH coal combustion.

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.

Phase Change Materials (PCMs) has gained considerable interest for storing thermal energy originating from the solar irradiation, industrial waste heat and surplus heat. Here, we present the facile and scalable synthesis of PCM-carbon foam composites by using polyisocyanurate (PIR) foam derived carbon foam as porous support. The unique 3D molecular configuration of the carbon foam materials embedded the composites with high PCM loading capacity, excellent shape stabilization and thermal reliability and chemical stability. The carbon foams prepared by facile chemical activation method with high surface area up to 1968 m2/g exhibit high PCM loading capacity of up to 90.8 wt% and excellent energy storage capacity of up to 105.2 J/g. Advanced characterization demonstrated that the total pore volume of carbon foam governs the PCM loading capacity as well as the energy storage performance of the composites. This work provides a potential pathway to recycle PIR foams, which have been widely used in construction industry, by producing cost-effective PCM composites for thermal energy storage.

Lignite with high moisture content needs to be dried. The variations of internal and external water of Mindong lignite during microwave drying were studied. The effects of coal particle size (<0.2 ...

Abstract Siliceous foams with three-dimensional mesoporous structures were synthesised and used to prepare polyethyleneimine (PEI) and tetraethylenepentamine (TEPA)-functionalised sorbent materials for CO2 capture, with a particular focus on the performance of impregnated amine blends versus single amine sorbent systems. Using thermal gravimetric analysis supported by other characterisations, the obtained results demonstrated that compared to the impregnated mono-component PEI and TEPA sorbent systems, the binary PEI-TEPA blend sorbents all achieved significantly higher CO2 capacities and faster adsorption kinetics, due to the enhanced formation of micro-cavities within the supported amine layers that led to reduced CO2 diffusion resistance and increased accessibility of the amines to CO2. It was found that at 70 °C and 15% CO2 in N2, the CO2 adsorption capacity of the silica-supported PEI–TEPA (3:2) at 70 wt% amine loading increased by 40% compared to the supported PEI at the same level of amine impregnation, whilst the time to achieve 80% and 90% of the equilibrium adsorption capacity was reduced by 70% and 35%, respectively. Extended cyclic adsorption-desorption tests showed that the TEPA-blended PEI sorbents all exhibited considerably higher thermal stability than both the supported PEI and TEPA sorbents, being indicative of the suppressed urea formation even in the pure and dry CO2 gas stream used in the desorption cycles. Calculations indicated that compared to the silica-supported PEI sorbents, the higher adsorption capacities achieved by the binary PEI-TEPA sorbent systems could lead up to 10% reduction in the energy requirement for sorbent regeneration, highlighting the suitability of using amine blending as a facile effective strategy to promote the overall performance of polyamine-based adsorbents for CO2 separation.