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. It is expected that 75% of electricity generation in Nigeria will come from renewable energy by the end of 2025 [1], and this includes hydro, solar, wind and biomass. The use of this biomass for energy production is, however, limited by some critical factors such as high moisture content, low bulk density and low energy density, all of which negatively affect the viability of biomass energy. Torrefaction is a mild pyrolysis method which shows promise for improving the energy density and some of the other undesirable properties of raw biomass.
This study examines the torrefaction and combustion properties of four Nigerian woody biomass, Gmelina arborea, Terminalia superba, Nauclea diderrichii, Lophira alata and a residue; palm kernel expeller (PKE). Two of these woods (Gmelina and Terminalia) are energy crops under cultivation trials in Nigeria, and two (Nauclea and Lophira) are woods used in the timber industry which results in large quantities of sawmill residues. Fuels were torrefied at 270 and 290oC for either 30 or 60 min, and assessed for pyrolysis and combustion characteristics in comparison to their untreated (raw) counterparts. Torrefied fuels were analysed for proximate, ultimate and higher heating value, and mass and energy yields were calculated. Results show that the Nigerian fuels have low N, S, Cl and high carbon contents. The high carbon content in the Nigerian fuels resulted in a relatively higher calorific value compared to typical European biomass such as willow. These two together makes the biomass very attractive for energy production. For the Nigerian fuels mass yields were in the range 70-93% and energy yields in the range 79-93%. 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 torrefaction process also results in loss of nitrogen from the fuel, mainly in the tar (condensables). Generally, while the wt% N increases in the torrefied fuel compared to the original material, the N content on an energy basis, generally decrease. This is very significant since amount of NOx that is formed in combustion is related, in part, to fuel nitrogen (fuel dependent N/GJ).
A modified Hardgove Index test was used to study the milling behaviour of the fuels, which demonstrated improved grindability of the woods upon torrefaction, especially for Nauclea. This is important for pulverized fuel combustion applications where energy in milling and mill throughput are expected to decrease for the torrefied fuels. 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 as evidenced by both the slightly higher temperature of maximum pyrolysis rate and by the lower reactivity rate (as calculated at 300oC). The A and Ea values for the fuels ranged from 10 – 24.9 s-1 and 78.8 -156 kJ/mol respectively. Overall, Nauclea and Gmelina were the most reactive fuels, whilst PKE was the least reactive.
The combustion behaviour of the selected raw and some of the torrefied fuels was examined by suspending particles in a methane air flame and interrogating using a high-speed camera. The observations showed that torrefaction changes the combustion properties of biomass resulting in shorter ignition delay, shorter duration of volatile combustion and longer duration of char burn out. TGA combustion analysis of chars and apparent first order kinetic parameters for char combustion revealed that torrefaction of biomass results in an increase in activation energy, Ea, resulting in a slower apparent reactivity in combustion i.e. torrefied chars are less reactive than their raw counterparts and the more severe the torrefaction conditions, the less reactive the fuels become. The fuels are becoming more coal-like in their combustion behaviour.
In summary, the Nigerian biomass and torrefied biomass show high potential for large scale electricity production. Their high calorific value, low nitrogen, low sulphur and high melting ash, together with their combustion properties make them an attractive resource. Future studies should examine the sustainability of supply chains for these fuels to ensure good carbon reduction. Torrefaction shows promise for improving many characteristics of the fuels for use in both large scale power stations and more widely for domestic purposes
