Bio-energy potential: why small is beautiful

When it comes to the combustion of biomass, few countries can claim a greater level of expertise than Finland. For example, based on their experience of working with the environmentally conscious Finnish forest industry, Kvaerner and Foster Wheeler have established themselves as global leaders at the larger end of the biomass market.

Finnish reciprocating-engine maker and distributed power generation system provider, Wärtsilä, a relative newcomer to the biomass business, is taking a different tack. With its recent acquisition of biopower boiler specialist Sermet Oy and the establishment of Wärtsilä BioPower, the company is planning to focus on smaller-scale bio-energy plants.

This strategy was adopted because of the exponentially greater availability of suitable fuel sources when you go down to this size range.

Wärtsilä BioPower is initially offering modular boiler plants in the range of 2-25 MWt and power plants in the range of 1-5 MWe, burning wood-based fuels (although the company says it might expand both the fuel spectrum and power range in the future).

Smaller bio-energy plants have typically been supplied for industrial applications such as sawmills, where the heat is used for drying timber, and for general heating purposes. Large plants are commonly built when there is access to a large bio fuel supply, as is the case at pulp and paper mills or sugar mills. Large projects are typically CHP plants while the smaller ones have tended to be heat-only boilers.

The future market is being driven by general concern about the environment and global warming and the global biomass-fired boiler and power plant market is projected to grow significantly in many regions of the world. The European Parliament is expected to issue a directive in the near future setting targets for bio-electricity production for individual EU nations. Incentives such as tax benefits and subsidies for biomass-produced energy are driving a variety of bio energy developments in Europe, which appears to be one of the strongest markets for small-scale bio-power in the short term.

Technology for the small scale

One obstacle to the advance of bio-power at the smaller end of the market has traditionally been that existing technologies have problems with combusting soaking wet and icy fuels in periods of cold weather, such as those experienced in the Nordic countries during winter. Sermet Oy has been working on this problem since the early 1990s, with the first 4 MWt installation using its advanced BioGrate technology starting operation at a Finnish sawmill in 1994.

Currently, the two main customer groups for BioGrate technology are Nordic sawmills and municipal energy (district heating) companies. To date, over 60 BioGrate installations have been delivered, to Scandinavia, Baltic countries, Russia, Canada and France.

Wärtsilä's planned future developments of the BioGrate technology will build on this success to address operation in other environments.

The BioGrate solution is based on a rotating conical grate. In the primary combustion chamber, fuel is fed upwards on to the rotating grate bars through a channel in the centre of the chamber. Combustion is stable because the fresh fuel entering at the centre of the rotating conical grate does not disturb the surrounding burning fuel bed and the rotating grate bars control the thickness of the fuel layer.

As the fresh fuel flows in a controlled fashion down to the combustion grates it is dried by heat radiation from burning gas and the refractory surfaces lining the primary combustion chamber. The further the fuel flows from the centre, the more combustion takes place; when the fuel reaches the outer edge of the grate it has been completely combusted and only ash is left.

This elegant technology - which requires neither fuel pre-drying nor support fuel - provides a very stable constant combustion process for fuels with a moisture content of up to 65 per cent. Furthermore, after-burning time and temperature can be very accurately adjusted.

In addition, the technology allows a very wide variety of bio fuels to be utilised.

The combustion gases from the primary combustion chamber flow into the secondary combustion chamber to ensure the highest possible temperature and thorough mixing in order to completely burn out gas and particles. Hot gases flow from the secondary combustion chamber into the fire tube steam boiler and into a steam superheating device, when superheated steam is needed.

The system produces very low emissions of NOx and particulates. Where a thermal output of more than 12 MWt is needed, a water tube boiler can be assembled above the primary combustion chamber. The necessary insulation linings partly cover the water tube walls - depending on fuel properties - in order to maintain the high combustion temperature. In all applications the connecting channel between the combustor and the boiler creates high level of turbulence, encouraging more efficient combustion of gas.

The BioGrate concept therefore provides operational reliability, high thermal efficiency, fuel flexibility and a long equipment lifetime (as evidenced by operating experience to date).

Future development work will focus on increasing power generation capabilities, reducing emissions even further and extending the fuel range, to include, for example, RDF, fast growing tropical woods and agricultural biomass such as bagasse and rice husks.