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Abstract: The processing of agricultural crops results in waste, which is a potential energy resource for alleviating commercial energy supply problems to agricultural-led economies like Tanzania. The energy content of the individual agricultural waste is largely dependent on its chemical composition (C, H and O) and it is negatively affected by the inclusion of inorganic elements and moisture. In this work, fifteen tropical agricultural wastes emanating from export crops for Tanzania were analyzed. The methods used to analyze involved performing proximate and ultimate analysis for determining the biomass composition. Thermal degradation characteristic was established to five selected wastes (coffee husks, sisal bole, cashew nut shells, palm stem, and bagasse) using a thermogravimetric analyzer type NETZSCH STA 409 PC Luxx at a heating rate of 10K/min. On the basis of elemental composition, the palm fibre and cashew nut shells exhibited high energy content due to their higher H:C ratio with relatively low O:C ratio. Results of the thermal degradation characteristic study showed that the cashew nut shells were the most reactive feedstocks due to their highest overall mass loss and lowest burnout temperatures of 364°C. Further, kinetic studies done to the five tropical biomass feedstocks under the pseudo single-component overall model established the activation energy for the bagasse, palm stem, and cashew nut shells to be 460kJ/mole, 542kJ/mole, and 293kJ/mole, respectively. The respective activation energies for coffee husks and sisal bole were 370kJ/mole and 239kJ/mole. With the exception of the sisal bole, which exhibited zero order reaction mechanism, the remaining materials´ reaction mechanism was of first order. These experimental findings form a basis for ranking these materials for energy generation and provide necessary input to equipment and process designers. [ABSTRACT FROM AUTHOR]

The promising properties of torrefied biomass provide a valid co-firing option for large percentage biomass utilization in existing coal-fired boilers. Torrefied biomass is expected to have a better combustion stability than raw biomass and similar to that of coal. The present work will characterizes the oxidation properties of torrefied biomass char and compare with that of raw biomass char. The studied two chars are produced from raw and torrefied biomass in an Isothermal Plug Flow Reactor (IPFR) at high temperature and high heating rate, a sufficient residence time is applied for the completion of the high temperature devolatilization. Char oxidation tests are carried out in the IPFR by varying temperature, oxygen concentration and residence time. The reactivity of two studied chars are analyzed and compared with referenced biomass char and coal char, and the impact of torrefaction on char reactivity is also discussed in this paper. Finally, the char oxidation kinetic parameters are determined using a parameter optimization method, and the obtained kinetics are examined by comparing the experimental and predicted mass conversions. [ABSTRACT FROM AUTHOR]

Abstract: Analyses made on the world''s biomass energy potential show that biomass energy is the most abundant sustainable renewable energy. The available technical biomass energy potential surpasses the total world''s consumption levels of petroleum oils, coal and natural gas. In order to achieve a sustainable harnessing of the biomass energy potential and to increase its contribution to the world''s primary energy consumption, there is therefore a need to develop and sustain contemporary technologies that increase the biomass-to-energy conversion. One such technology is the high temperature air/steam gasification (HTAG) of biomass. In this paper we present findings of gasification experimental studies that were conducted using coffee husks under high temperature conditions. The experiments were performed using a batch facility, which was maintained at three different gasification temperatures of 900°C, 800°C, and 700°C. The study findings exhibited the positive influence of high temperature on increasing the gasification process. Chars left while gasifying at 800°C and 700°C were respectively 1.5 and 2.4times that for the case of 900°C. Furthermore, increased gasification temperature led to a linear increment of CO concentration in the syngas for all gasification conditions. The effect was more pronounced for the generally poorly performing gasification conditions of N2 and 2% oxygen concentration. When gasification temperature was increased from 700°C to 900°C the CO yield for the 2% O2 concentration increased by 6times and that of N2 condition by 2.5times. The respective increment for the 3% and 4% O2 conditions were only twofold. This study estimated the kinetic parameters for the coffee husks thermal degradation that exhibited a reaction mechanism of zero order with apparent activation energy of 161kJ/mol and frequency factor of 3.89×104/min. [Copyright &y& Elsevier]