Biomass is the most utilised form of renewable energy, especially in developing nations, and is a possible replacement for fossil fuel in power generation. The most commonly used method for recovering energy from biomass is combustion. Many countries are exploring the utilisation of energy crops and indigenous residues to deliver sustainable sources of biomass. For these bioresources, detailed characterisation of the fuel properties is essential in order to optimise the combustion processes. In this study, some potential energy crops and woods from Nigeria, namely Terminalia superba, Gmelina arborea, Lophira alata, Nauclea diderrichii, and also one 2 abundant agricultural residue, palm kernel expellers (PKE) were characterised for their combustion properties. Standard characterisation methods such as proximate and ultimate analyses, metals analysis, and ash fusion test were used for this purpose and the results were compared to some UK biomass. In addition, their thermal conversion was assessed by thermogravimetric analysis and pyrolysis-gas chromatographyÐmass spectrometry. Finally, combustion studies were conducted by suspending single biomass particles in a methane flame to obtain information on reactivities and combustion characteristics. Results indicate that the ash fractions in the Nigerian woods were low in K, Si, and Ca, resulting in low calculated alkali indices, hence these fuels are not predicted to cause severe fouling problems. Furthermore, the analysis of the evolved product during devolatilisation from Py-GC-MS suggests that the content of oil is high in Gmelina. Finally, the results from the single particle combustion experiments revealed longer char burn out rate for Lophira and Nauclea when compared with those of Terminalia and Gmelina.Keywords: biomass, torrefaction, energy, combustion, pyrolysis. Nigeria has large reserves of gas and solid minerals, and the largest oil reserves in Africa.Consequently the country has a very high dependence on crude oil, which contributes seedling nurseries, and plantation management [7]. Woods in the plantations include Gmelina, Terminalia, teak, eucalyptus, and pine. Tropical rain forest is the major source of timber supply and energy crops in Nigeria with high plant diversity of over 4,600 plant species. The forest covers 10% of the countryÕs land area with over 560 tree species at a range of about 30 to 70 species per hectare for trees ≥ 5 cm diameter at breast height (dbh) [8].Terminalia superba is a tree found in the tropical lowland forest in Nigeria. The tree is planted around April at the beginning of the rainy season, and thrives on rich, well-drained alluvial soils, although it can also be cultivated on other soil types namely lateritic sands, gravel and clays, lava, black basaltic clays and crystalline soils. The wood air dries rapidly, degrades slightly and is lightweight to medium-weight, with a density ranging from 370Ð730 kg/m³ at a moisture content of about 12% [9]. Once the wood is dried, it becomes stable. The chemical composition
Many countries are seeking to expand their use of solid biomass for electricity and heat generation. Nigeria, too, is exploring its own potential energy crops and indigenous residues. The use of this biomass for energy production is, however, limited by factors such as high moisture content, low bulk and low energy density. This study examines the torrefaction and combustion properties of four readily available Nigerian woody biomass, Gmelina arborea, Terminalia superba, Nauclea diderrichii, Lophira alata and a residue, palm kernel expeller (PKE). They are considered for their suitability for use in large scale power stations, especially as pulverized fuels.The Fuels were torrefied at 270 and 290 o C for either 30 or 60 min, and assessed for pyrolysis and combustion characteristics in comparison to their untreated counterparts.Energy densities of the woods improved from 19.2-21.2 MJ/kg for the raw fuels to 21.5-24.6 MJ/kg for the torrefied fuels. The milling behaviour of the torrefied fuels improved upon torrefaction, especially for Nauclea; however, torrefaction had very little effect on the grindability of PKE. The apparent first order kinetics for pyrolysis were determined by thermogravimetric analysis (TGA). After torrefaction, the fuels become less reactive; Nauclea and Gmelina were the most reactive fuels, whilst PKE was the least reactive. The combustion behavior of selected fuels was visually examined in a methane air flame. This showed that torrefaction resulted in shorter ignition delay, shorter duration of volatile combustion and longer duration of char burn out.
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