Torrefaction is a mild thermal pretreatment (T<300°C) that improves biomass milling and storage properties. The impact of torrefaction on the gasification and oxidation reactivity of chars from torrefied and raw biomass was investigated. Thermogravimetric analysis was used to study the differences in O(2) and steam reactivity, between chars prepared from torrefied and raw willow, under both high- and low-heating-rate conditions. High-heating-rate chars were prepared at 900°C with a residence time of 2s. Low-heating-rate chars were prepared with a heating rate of 33°C/min, a maximum temperature of 850 or 1000°C, and a residence time of 30 min or 1h, respectively, at the maximum temperature. Pretreatment by torrefaction consistently reduced char reactivity. Torrefaction's impact was greatest for high-heating-rate chars, reducing reactivity by a factor of two to three. The effect of torrefaction on a residence time requirements for char burnout and gasification was estimated.

This paper is concerned with the kinetics of the thermal decomposition of a woody biomass, willow. It addresses two questions. First, what method of data analysis is appropriate for extracting reliable kinetic data from thermogravimetric analysis (TGA) experiments? Second, what kinetics are most suitable for high heating rate situations such as those present in pulverized fuel power stations? It contains kinetic analysis of willow TGA data using a variety of approaches. A review of previously published work on biomass and its polymeric components helps ascertain the variation in kinetics, reasons for differences, and extrapolation to flame temperatures. The data falls into two main categories: (1) very high E and A values (>100 and up to 270 kJ/mol, and up to 10 17 s -1 ) derived when model biomass components are studied, for example, cellulose; or the data is interpreted as the sum of a number of individual first-order reactions, for example, FG-BioMass; (2) intermediate and low E and A values (50-100 kJ/mol and <∼10 5 s -1 ) derived using a number of global approaches. For low heating rate experiments (10 1 K/min), the global first-order reaction kinetic models that tend to yield low E values, such as the reaction rate constant method, work well. High E kinetics can also work well at low heating rate, but only if the reaction is assumed to be due to the sum of a number of individual steps, such as the biochemical components degrading independently, or the functional group approach. For higher heating rates (>10 3 K/s) high E kinetics predict conversion well, and this can be rationalized since primary cracking reactions will dominate under these conditions. However, at heating rates of 10 5 K/s and temperatures of 1500 °C (i.e., flame conditions), a compensation on the rates is seen and the choice of rate parameters is less critical. Two sets of kinetic data, E = 178.7 kJ/ mol, A = 2.2 Â 10 13 s -1 and E = 48.7 kJ/mol, A = 6.84 Â 10 3 s -1 , both predict conversions in keeping with the available experimental data.

Torrefaction of biomass is rapidly gaining popularity as a viable pretreatment for use with co-combustion with coal or with other thermochemical conversion processes. This work explores the effects of combining pretreatment washing techniques using water, ammonium acetate, and hydrochloric acid. Four biomasses were studied, short-rotation coppiced willow, eucalyptus, Miscanthus, and wheat straw, all in chipped or chopped form. The resultant fuels, after the pretreatments, were characterized for ultimate analysis, proximate analysis, heating value, and pyrolysis behavior (via thermogravimetric analysis), and mass and energy yields in a fixed-bed torrefier were measured. The ease of removal of certain metals, Na, K, Mg, and Ca, as well as PO4 3–, SO4 2–, and Cl– was assessed by ion chromatography on the leachates from the water washing, and influences on fouling behavior were predicted. Fuel ashes (both prior to and after torrefaction) were also assessed in the ash fusion test, a probe for slagging behavior. Water washing resulted in a high removal of alkali metal ions and chloride, particularly for the herbaceous biomass, where up to 92% of sodium and 62% of potassium were removed, together with up to 100% of the chloride. There was a general trend of higher concentrations of water-soluble species for the herbaceous biomass compared to the woody biomass, although there were a few exceptions, such as phosphate. As a consequence of water washing, the alkali indices (an index for fouling) decreased markedly. Because the ash composition changes as a result of the different washing procedures, the ash melting behavior also changes, and hemisphere temperatures (oxidizing conditions) were seen to increase substantially, by approximately 400 °C for wheat straw to 1500 °C and 290 °C for Miscanthus to 1490 °C. Different pretreatment methods also influenced the progress of torrefaction. After all washings, the fuels became less reactive to thermal degradation, and therefore, mass (and energy yields) increased during a fixed torrefaction operation. This could be explained through measurement of the pyrolysis kinetics; removal of key catalytic metal species (such as Na and K, in particular) by washing results in slower reaction rates. Water washing was seen as the most beneficial pretreatment, because it produced the most marked improvement in the torrefied fuel in terms of its ash fusion test behavior.

No abstract

Biomass is an especially reactive fuel. There have been large increases in the transportation and utilization of biomass fuels over the past 10 years and this has raised concerns over its safe handling and utilization. Fires, and sometimes explosions, are a risk during all stages of fuel production as well as during the handling and utilization of the product. This paper presents a method for assessing ignition risk and provides a ranking of relative risk of ignition of biomass fuels. Tests involved single particle measurements, thermal analysis, dust layer and basket ignition tests. In all cases, smouldering combustion was observed, whereby the fuels pyrolyse to produce a black char, which then subsequently ignites. Low temperature pyrolysis kinetics have been utilised to predict ignition delay times at low temperatures. A method for evaluating risk was explored based on the activation energy for pyrolysis and a characteristic temperature from {TGA} analysis. Here, olive cake, sunflower husk and Miscanthus fall into the high risk category, while the woods, plane, pine, mesquite and red berry juniper, fall into the medium risk category. This method is able to capture the impact of low activation energy for pyrolysis on the increased risk of ignition