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There has been an increase in the use of biomass for power generation by means of co-firing with coal as well as by the combustion of 100% biomass. Despite the advantages of biomass in reducing carbon emissions from the electricity sector, the co-firing of high percentages of biomass can potentially aggravate ash related problems in the boiler. In order to develop mitigation strategies for the formation of deposits, an understanding of the ash behaviour during the combustion of high percentages of biomass is required. In this work, ash samples from El Cerrejon coal and pine biomass were characterised for their inorganic composition by X-ray fluorescence and wet chemical methods. Relationships between these two methods were found. Furthermore, the melting behaviour of ashes from pure coal, pine, and their blends were studied through ash fusion tests (AFT) and via a method using a simultaneous thermal analyser coupled to mass spectrometer (STA-MS) for the evolved gas analysis. Pine ash has lower slagging potential than El Cerrejon coal ash and results show that for 20:80 and 80:20 pine:coal ash belends the characteristic ash fusion temperatures increase with increasing pine ash content. There is unusually higher slagging potential (lower ash fusion temperatures) at a 50:50 blend ratio. Viscosity models produced sensible results for coal and coal/pine blends, but further refinement is required for modelling the viscosity of pure biomass ash. Thermodynamic modelling of slag formation was undertaken using the FactSage model. This model was successful in predicting the changes of gas, solid and liquid phases during pure pine, coal and co-combustion. [ABSTRACT FROM AUTHOR]

Torrefaction of biomass is of great interest at the present time, because of its potential to upgrade biomass into a fuel with improved properties. This study considers the fundamentals of combustion of two biomass woods: short rotation willow coppice and eucalyptus and their torrefied counterparts. Chars were prepared from the untreated and torrefied woods in a drop tube furnace at 1100 °C. Fuels and chars were characterised for proximate, ultimate and surface areas. Thermogravimetric analysis was used to derive pyrolysis and char combustion kinetics for the untreated and treated fuels and their chars. It was found that the untreated fuels devolatilise faster than their torrefied counterparts. Similarly, the chars from the untreated biomass were also found to be more reactive than chars from torrefied fuels, when comparing reaction rates. However, the activation energy value (Ea) for combustion of the untreated eucalyptus char was higher than that for the torrefied eucalyptus chars. Moreover, the eucalyptus chars were more reactive than the willow char analogues, although they had seen a lower extent of burn off, which is also a parameter indicative of reactivity. Similar trends in were also observed from their intrinsic reactivities; i.e. chars from the untreated fuel were more reactive than chars from the torrefied fuel and eucalyptus chars were more reactive than willow chars. Chars were also studied using scanning electron microscopy with energy-dispersive X-ray analysis. This latter method enabled a semi-quantitative analysis of char potassium contents, which led to an estimation of potassium partitioning during char formation and burnout. Results show a good correlation between potassium release and percent burnout. With respect to the effect of torrefaction on fuel-N, findings suggest that torrefaction would be beneficial for pf combustion in terms of nitrogen emissions, as it resulted in lower fuel-N contents and ∼72–92% of the fuel-nitrogen was released with the volatile fraction upon devolatilisation at 1100 °C. [ABSTRACT FROM AUTHOR]

Combustion of solid biomass in large scale power generation has been recognized as a key technology for the transition to a decarbonized electricity sector in the UK by 2050. Much of the near-term forecast capacity is likely to be by the conversion of existing coal-fired pulverized fuel plant (DECC, 2012). In such applications, it will be necessary to ensure that the combustion behaviour of the solid biomass fuels is engineered to match, as far as practical, that of the original plant design. While biomass feedstock characteristics vary considerably, one controllable variable for pulverized fuel is the size of the particles. Useful modelling for adaptation and design of boiler plant can be improved with more detailed measurement of the real behaviour of individual particles of the varying fuels. Typical power plant biomass fuels including pine, eucalyptus and willow with particle sizes ranging from up to 3 mm (Van Loo and Koppejan, 2008) and with differing moisture content and aspect ratios were selected for study. Single particles were supported in a water-cooled cover and then exposed above a flame, simulating biomass combustion in a furnace. Measurements of ignition delay, volatile burning time and char burn-out time were undertaken using high speed image capture. Temperatures of the surrounding environment and near to the particle surface were measured with thermocouples and thermometric imaging. Thermo-gravimetric measurements on separate samples complement the single particle measurements as a means of verifying the demarcation between the different stages of combustion and providing kinetic data. Analysis of the data identified correlations between the biomass fundamental characteristics, particle size, and the observed combustion profiles. Empirical expressions for the duration of each combustion stage have been derived. These have been validated with basic modelling including the predicted devolatilisation stage calculated by the FG-Biomass model (Chen et al.,1998). [ABSTRACT FROM AUTHOR]

Highlights: [•] We present findings from a two-year field trial into Miscanthus agronomy. [•] We examine changes in fuel composition and characteristics. [•] Agronomy influences fuel characteristics especially N, Cl, and ash. [•] Agronomy indirectly influences combustion characteristics. [Copyright &y& Elsevier]

Abstract: For many years, the sole use of Dried Distillers Grains and Solubles (DDGS) has been as a feed for livestock, due to its high protein and fiber content. Recently there has been interest in DDGS as a power station fuel. In this study, DDGS produced from wheat has been studied for its fuel properties. Pyrolysis studies were conducted via Thermogravimetric Analysis (TGA) and pyrolysis gas chromatography–mass spectrometry (py GC–MS): the former was used to calculate the apparent first order kinetics while the latter helped to characterise pyrolysis products such as methoxyphenols and long chain fatty acids and esters. Fast heating rate chars were made, and nitrogen and sulphur partitioning between volatiles and chars are reported. In addition the chars were studied for their combustion behavior via Simultaneous Thermogravimetric Analysis with Mass Spectrometry detection (STA–MS) to study the conversion of char nitrogen and sulphur to different nitrogen and sulphur-containing species. In addition DDGS was combusted in a 50kW fluid bed furnace, and the nitrogen species also monitored. DDGS is predicted to have a high alkali index that predicts high fouling tendency due to the high potassium and sodium contents in the ash. The phosphorus content in DDGS ash is also very high. Pyrolysis studies showed some oil evaporation/decomposition occurring at lower temperatures compared to decomposition of the lignocellulosic material. Nitrogen and sulphur partitioning showed that approximately 91% N and 94% S were given off as volatiles. The char was enriched with nitrogen but depleted in sulphur. Char combustion took place in two distinct stages indicating the presence of two types of chars, which yield selective evolution of S and N species during their combustion. Fluid bed combustion studies showed release of both HCN and ammonia during initial stages, when the bed is heating and the fuel undergoes devolatilisation. Steady-state combustion gave NO and N2O emissions of 290 and 30ppm respectively, with HCN and NH3 emissions dropping to below 1ppm. Agglomeration of the bed was avoided by keeping the combustion temperature below 900°C [Copyright &y& Elsevier]

Abstract: In this study, the combustion properties of three of the UK’s commonly imported biomass fuels for co-firing, which are palm kernel expellers, shea residue, and waste from olive oil production are examined. The fuels were characterised and their thermal decomposition properties were studied by thermogravimetric analysis (TGA). Additionally the products from their devolatilisation were identified by gas chromatography–mass spectrometry (py–GC–MS) analysis of the evolved vapours and tars from high heating rate pyrolysis tests. Finally, chars from the fuels were prepared, analysed, and combustion studies were conducted by TGA–MS to determine the conversion of char-nitrogen to different nitrogen-containing species. In general, the main constituents of their ash fractions were K, Si, Ca and Mg, resulting in high alkali indices, which predict a large tendency to fouling. The pyrolysis and combustion kinetic parameters, estimated from TGA studies of these fuels and their chars, are much lower than those reported in the literature for lignocellulosic biomass. It is suspected that there is oil/fat evaporation processes overlapping with the decomposition of their lignocellulosic fractions, which significantly affects the apparent kinetics. The pyrolysis conditions used promoted depletion of nitrogen in the char, resulting in approximately 79–91% of the fuel-N being released with the volatiles. In combustion of the char, NO x and N2 are the major nitrogen compounds detected. Another primary product, HCN, was detected from the combustion of some of the fuel chars, as well as C2N2. [Copyright &y& Elsevier]

Abstract: Potassium, a key nutrient in biomass growth, contributes to problematic ash chemistry and corrosion in combustion. This study seeks to examine the behaviour and fate of potassium in biomass combustion under high temperature flame conditions. A model to predict potassium release is presented. Short rotation willow coppice was treated to reduce metals, by water-washing, and remove them, by demineralisation, and then potassium was doped into the demineralised sample. The resultant fuels have been studied for their combustion behaviours in methane–air flames, both as suspended, moving particles, and as stationary, supported particles, using high speed digital video. In the latter case, potassium release was measured simultaneously by emission spectroscopy. In both experiments, potassium was seen to catalyse devolatilisation, and for the stationary particles it was possible to detect potassium catalysis in the char burn-out rates. Demineralised willow was seen to melt in the flame and combustion resembled heavy oil combustion, rather than solid fuel combustion. The residual char was extremely slow to burn-out. In the potassium-doped particles, potassium was seen to evolve over three regimes, devolatilisation, char burn-out and, less significantly, during ash cooking. The first two evolution processes have been modelled using an apparent first order devolatilisation rate for the first stage, and a KOH evaporation model for the second stage. [Copyright &y& Elsevier]

Abstract: Thermal pre-treatment, or torrefaction, is a process that can improve the handling and grinding properties of biomass for combustion and co-firing. This paper compares the combustion properties of raw and torrefied Short Rotation Coppice Willow (SRC), with those of typical bituminous power station coals. The fuels were analysed using a number of standard fuel characterisation tests. Willow SRC was torrefied at 290°C for two reaction times (10min (short) and 60min (long)). During torrefaction, longer reaction time promotes loss of nitrogen from the solid product. Chars were produced from the fuels under high heating rates up to 1000°C and the char yields and nitrogen partitioning were determined. Results show that the nitrogen partitioning for both raw and torrefied biomass favours release of nitrogen into the volatiles during rapid pyrolysis. In contrast, rapid pyrolysis of the coals favours nitrogen retention in the char. The chars were subjected to thermogravimetric analysis-mass spectrometry (TGA-MS) combustion tests, which were used to calculate the char reactivities and fate of char nitrogen. The char reactivities follow the order coal char