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A comparison between the properties of chars produced by pyrolysis of rice husk and eucalyptus at different temperatures and heating rates has been performed. Low heating rate (LHR) devolatilization experiments were conducted in a fixed bed reactor at temperatures ranging from 600 to 900 °C, while a fluidized bed reactor was used for preparing chars at high heating rate (HHR) and temperatures of 800 and 900 °C. The morphological changes in carbonaceous solids produced in the different thermal treatments were observed by scanning electron microscopy (SEM). X-ray diffraction (XRD) and Raman spectra were obtained to evaluate the degree of char structural order. The chars were characterized by their ultimate analysis, oxygen functional group content, and CO 2 adsorption at 0 °C using the Dubinin-Radushkevich method. The results obtained from the different techniques were contrasted to give an overview of the chemical and physical properties of the biomass char samples studied. The influence of the parent material and char properties on char reactivity toward O 2 and NO reduction was further investigated. It was found that, though rice husk chars have a greater reactivity toward oxygen, the NO reduction ability was significantly higher for the eucalyptus chars.

The ability of non-hydrocarbon fuels such as CO and H2 to reduce nitric oxide under conditions relevant for the reburning process is investigated experimentally and theoretically. Flow reactor experiments on reduction of NO by CO and H2 are conducted under fuel-rich conditions, covering temperatures of 1200−1800 K and a range of stoichiometries and reactant levels. Bench and pilot scale results from literature on reburning with CO, H2, and low calorific value gases are also considered. The experimental data are interpreted in terms of a detailed reaction mechanism, and the reactions responsible for removal of NO are identified. The experimental results indicate that under typical reburn process conditions these non-hydrocarbon fuels may remove 20−30% of the nitric oxide entering the reburn zone. However, results indicate that the process potential increases with temperature and reburn fuel fraction, and at high temperatures and reburn fuel fractions of about 30%, the reduction efficiency approaches that of hydrocarbon gases. If dilution effects and the lowering of the primary zone NO (maintaining the overall load) are accounted for, the reduction potential is further increased. Modeling results indicate that the mixing process may affect the NO reduction in the reducing zone. The modeling predictions are in qualitative agreement with the experimental results but tend to underestimate the reduction of NO. Conversion of NO to N2 in the reburn zone proceeds primarily through the following sequence:  H + NO + M ⇌ HNO + M, HNO + H ⇌ NH + OH, NH + NO → N2 + ... The implications of the results for reburning with fuels with a low hydrocarbon content are discussed, with special emphasis on gasified fuels.

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A model for simulating reburning in semi-industrial scale has been developed. It consists of a recently developed reaction mechanism for reburning with C1 and C2 hydrocarbons, in combination with ideal reactor modeling and a simplified mixing approach. The reaction mechanism as well as the mixing model has been validated separately against experimental data from laboratory and pilot scale tests. Modeling predictions have been compared with experimental data from a number of pilot scale studies of gas reburning with good results. The large differences in reburn efficiency reported in different low-temperature pilot scale experiments are reconciled in terms of the different operating conditions used. The model has been used to assess the potential of the reburn process at low temperatures, and recommendations for process optimization are provided. Results show that the low-temperature gas reburn process has a significant potential for NO reduction and that both the reburn and burnout regions are important in process optimization.