New small-scale CHP plant for biomass fuels
by M Palonen, the CADDET Finnish National Team

International efforts in ecological energy production have led to the increasing use of biomass as a fuel in combined heat and power (CHP) units. In response to this, a Finnish company has developed an innovative and economically feasible steam cycle CHP technique (“BioPower”) suitable for small-scale applications.

The first BioPower plant at a sawmill in Finland

CHP

The objective of CHP production is to replace some, or occasionally all, of the electricity that is purchased from another supply. Thermal energy is the main output of steam-cycle CHP technology, making it best suited to small industrial plants such as sawmills or other industrial processes which can provide wood as fuel from a process, and which use electricity and heat simultaneously. Owners of CHP units can earn extra income by supplying heat to other local consumers, such as other industrial plants or district heating networks, in addition to saving money on their own energy costs.

Other feasible applications are in remote areas where local electricity generation is based on high-cost power plants and where local biomass fuels such as wood chips are available at reasonable cost. By producing electricity locally, the high cost of grid connection can be avoided.

BioPower

Finnish company Sermet Oy has developed the BioPower process using its long experience of boiler and combustion techniques. Since 1975 the company has delivered more than 1,000 boiler plants with outputs of 1–120 MW for district heating and industry from arctic to tropical regions. However, the company’s main marketing focus is on regions where evergreen trees are the prevailing wood type.

A patented combustion technique (“BioGrate”), capable of burning biomass fuels with moisture content from 30% to 65%, is used in the plant.  This allows wood material to be used without pre-handling other than crushing of over-sized particles. The combustion technology is also suitable for burning source-separated combustible waste materials such as packaging waste and household waste alongside bark and sawdust.

Superheated steam, at low pressure (20–30 bar) and low temperature (300–390°C), is produced by a fire-tube boiler. Electricity is generated using a steam engine and generator. A modern steam engine gives a very high turn-down ratio (ie can run at part load) when the plant is running according to the heat load, and is therefore well suited to small electricity capacities. Electricity production can be maximised for short periods by adding a steam-to-air cooling system, which can be integrated easily. If more than 1 MW of electricity is needed, a steam turbine replaces the steam engine.

The overall efficiency of the BioPower CHP process is between 80% and 90% and is competitive with other small-scale CHP processes. The electricity-to-heat ratio can be improved by raising the steam pressure and temperature, and lowering the output steam pressure and temperature, which can be chosen according to the process requirements. 

First application

The first BioPower CHP unit is under construction at a sawmill in Finland and will be completed in the autumn of 1999. Complementing an existing 3.5 MW boiler, the new unit has a thermal capacity of 6 MW and will produce heat for the sawmill dryers and the district heating network. In addition, it will produce 900 kW of electricity for use by the sawmill. Installation of the new unit will allow the sawmill to improve the processing grade of its by-products, wet bark and sawdust, and maximise the use of its raw wood material.

In the town of Kiuruvesi, district heating is already produced from the sawmill’s older 3.5 MW boiler plant. However, the proportion of energy produced from renewable biomass was only 60% of the total. The new plant will increase the renewable energy fraction to 90%. Heavy fuel oil will be used as stand-by, but will be needed only during periods with the highest heat demand. The district heating system is operated by an energy company, Savon Voima Oy.

The steam temperature required by the sawmill’s dryers and the district heating system is almost the same, but the load fluctuations differ, stabilising the heat output from the 3.5 MW boiler.

Schematic diagram of the CHP plant

Economics

BioPower plants from 0.2 MWe to 3 MWe and from 2 MWth to 10 MWth will be available. A 6 MWth plant will cost about e2.7 million (where e is the euro). The payback period will be 7–10 years depending on the prices of fuel and district heating, and the price of replaced purchased electricity. Despite the recent trend towards liberalisation of energy markets, these prices still vary a great deal around Europe.

The economic competitiveness of this CHP solution is based on reliable, simple and safe structures and components. The plants are made in factory packages or combinations of prefabricated modules for on-site assembly. Fully-automatic remote control ensures that the plants run efficiently, with minimal operating costs.

For more information contact the CADDET Finnish National Team in Helsinki.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.