Over the long term, rising fossil fuel prices, renewable energy target legislation and potential carbon emission costs will lead power consumers to increase the percentage of renewable energy in their fuel portfolio. The two methods of turning biomass into electricity are combustion and gasification, which differ in terms of efficiency and risk.

Biomass combustion Combustion is a well-known method of converting biomass into electricity, with hundreds of biomass combustion plants operating successfully worldwide. The two main firing systems in these plants are grates and fluidised beds. Grates are generally suitable for a heterogeneous biomass, while fluidised bed systems prefer homogeneous fuels. Both firing systems accept clean and contaminated biomass; are very low risk; and for a typical 5 megawatt electric (MWe) plant, cycle efficiencies of 20–24 per cent can be achieved.

Biomass gasification Gasification systems have existed for decades but they are less commonly used, as clients are more familiar with combustion and fewer suppliers offer gasification systems. Biomass gasification can be separated into two types:

1. Gasification and the use of syngas in a boiler 2. Gasification and the use of syngas in an engine/turbine.

Boilers – easier to use with problematic fuels Several industrial biomass gasification systems which burn syngas in a boiler are in operation worldwide and their capacities reach up to 70 megawatts thermal (MWth). The advantage of this approach is that some problematic fuel components, for example chlorine compounds, end up significantly reduced in the syngas. Burning a reasonably clean syngas in a boiler allows steam temperatures of 525°Celcius with significantly reduced high-temperature corrosion and ash fusion issues. Most gasification references (of efficiency and temperature) exist for clean biomass as contaminated materials cause complex chemical reactions.

This gasification approach is low risk and a 5 MWe plant can reach a cycle efficiency of up to 27 per cent.

Engine/turbine – more efficient, more complex The most efficient way to turn biomass into electricity is gasification and the use of syngas in an engine/turbine in a process called biomass-integrated gasification combined cycle (IGCC).

The use of highly efficient and standardised gas engine/turbines allows simple cycle efficiencies of 35 per cent. However, this is the least mature and most technologically complex approach.

Some plants generating up to 1.5 MWe are in operation but the technology has not yet reached a proper industrial level. The next step to implement this technology is the construction of an approximately 5 MWe reference plant. Most of the components are available already – for example the gasifier and the gas engine – but the crucial syngas cleaning mechanism has to be adapted.

Cleaning fuel for cleaner energy The type and extent of syngas cleaning depends on the raw biomass material as well as the final application of the syngas. A clean forestry waste source will require significantly less processing of the syngas than if the syngas was derived from metropolitan solid waste. Y

For application in a gas engine, the minimum quality requirement would be particle-free and non-condensing syngas at the required fuel pressure. A gas turbine would require a similar syngas feed quality with respect to particulates and moisture but at a higher pressure compared to a gas engine.

The next stage of syngas treatment involves chemical removal of one or more of the problematic components prior to combustion, for example sulphur compounds and carbon dioxide.

To reduce technical risk it is recommended to start with a clean fuel source. A 5MWe plant would achieve a simple cycle efficiency of 35 per cent. Larger plants using the engine/turbine exhaust have the potential to reach up to 50 per cent.

Biogas and syngas – the efficient answer Several technologies are available for electricity production from biomass plants. The highly standardised and very low risk approach is a combustion-type boiler, making it the recommended technology for fuel mixtures and contaminated biomass.

Higher fuel costs of forestry waste and plantation wood make gasification with syngas use in a boiler an interesting option due to better efficiencies.

However, a range of gas cleaning technologies exist that can be easily adapted to biomass gasification applications. From small-scale gas engine generator set-ups to large-scale IGCC power plants, gas cleaning technology can be utilised to remove impurities and reduce harmful exhaust emissions.

All renewable technologies have to further increase their efficiencies and biomass is no exception. However, to significantly raise energy efficiency the development of industrial-scale biomass IGCC plants is necessary.