Air gasification of woody biomass from short rotation forests : opportunities for small scale biomass-electricity systems : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Agricultural Engineering at Massey University, New Zealand
Downdraft gasification of short rotation forestry (SRF) biomass was investigated to identify opportunities for small scale biomass-electricity systems. Case studies were conducted in Kenya to identify these opportunities by (i) defining the energy demand and supply structure to identify markets; (ii) evaluating the biomass resources available; and (iii) identifying the availability of other facilities required for such a system. At the same time, the yield potential of 12 SRF species, planted in Palmerston North, New Zealand in small plots at 3470 stems/ha and harvested after 2, 3, 4 and 5 years was evaluated. Samples were collected from each species to determine their energy properties. Data on tree growth, yield, and biomass properties were used to develop two multi-objective indices - the relative yield index (RYI) and the fuelwood value index (FVI) for evaluating SRF species. Biomass from the 4 year rotation harvest was used as feedstock to fuel a downdraft air gasifier rated 35 kW (electric). Feedstock gasification processes, gas quantity and quality were correlated with the biomass properties to define the characteristics of a good fuelwood species for gasification purposes. The Kenyan studies highlighted constraints in the energy sector and identified opportunities for new bioenergy technologies. Small scale biomass gasification systems showed potential but suitable sites were restricted to sawmills where processing residues could be used as gasifier feedstock. Field trials of SRF systems were recommended to evaluate tree species over different silvicultural treatments, and to intensify biomass production. A demonstration plant at one of the bigger sawmills was recommended to stimulate interest among investors. Species yields of the trial plantings in New Zealand in the 12 species assessed ranged from 6 ODt/ha/y for Alnus glutinosa to 73 ODt/ha/y for Eucalyptus globulus at 5 year rotations. A stocking density trial of E.saligna showed that 3,500 stems/ha managed on 4-5 year rotations provided the highest yields. Though these yields may not be achieved in field plantings or in Kenya, the study demonstrated the feasibility and methodology that could be applied. Like yield, the bioenergy properties varied between species. Higher heating values ranged from 19.6-20.5 MJ/kg for wood, 17.8-20.6 MJ/kg for bark, and 19.5-24.1 MJ/kg for leaves. Gas yields varied between 1.88-2.89 g/g dry wood due mainly to moisture content variations which also affected the composition of the gas. Gas heating values varied from 4.602 to 6.112 MJ/Nm3, and were considered to be of sufficient quality to fuel internal combustion engines. Both RYI and FVI showed that yield factors outweighed bioenergy properties when identifying a good fuelwood species. The large differences in yields indicated the benefits that could be achieved by selecting appropriate species for a specific region. Although feedstock properties affected the gasification processes and products, their overall influence was not statistically significant. The inclusion of bark in the feedstock did not adversely affect the suitability of the feedstock.