Summary: Four biomass conversion technologies were reviewed and assessed for potential use to convert the forest waste wood (biomass) in the Gisborne area. These conversion technologies include combustion, gasification, pyrolysis and torrefaction. The assessment is based on the maturity and complexity of the technology, products and applications, production costs and the environmental impact.

The biomass combustion process is a mature technology and can produce heat or combined heat and power (CHP). Heat generation requires local users who can be wood processors or district heating. Power generated can be sent to the grid. Although biomass contains low contents of sulphur and nitrogen, emissions from the flue gas still need to be carefully monitored.

Biomass gasification is a proven technology operating at 700 to 1200C, although 700-900C is most common. Both demonstration and commercial biomass gasification plants are available in the world. This technology produces a gas mixture, termed producer gas, with H2, CO, CO2 and CH4 being the major components. The producer gas is expected to contain a significant amount of N2 if air is used as the gasification agent. On the other hand, the producer gas also contains tar and gaseous contaminants, which need to be removed before the producer gas is further used. The cleaned producer gas can be used for power generation using a gas turbine or gas engine. In addition, the producer gas can be further processed for gaseous fuel (synthetic natural gas, hydrogen) or liquid fuel. In this case, the overall process is complex and capital investment is significantly increased.

Biomass pyrolysis is also a proven technology operating at 400-650C with demonstration and commercial plants available around the world. In general, biomass can produce products in liquid (bio-oil), gas (non-condensable) and solid (bio-char). The target product in most cases is the liquid bio-oil, which is further processed for liquid fuel. In this case, sophisticated upgrading is needed due to the complex composition of the bio-oil.

Torrefaction is a relatively simple process in which the wood is heated in the absence of oxygen to remove volatiles; thus, only solid char is produced at a yield of 85% or higher. The solid char can be used to substitute coal in combustion or co-firing. Recently, solid char has been applied in ironmaking to replace coal-derived coke.

Based on the assessment of technology maturity, products and market demand, production costs and environmental impacts for the four conversion technologies, biomass torrefaction stands out as a preferred technology to process the forest waste wood to solid char.