Wood fuel goes back to the future
by M Thomas, CADDET UK National Team and A Talbott, Talbott’s Ltd, UK

Introduction

Wood was a traditional fuel for heating back in the 11th century when Elvendon Priory in Oxfordshire was built. In more recent times, the Priory has used gas- and oil-fired boilers to meet its heating needs. However, wood (a carbon-neutral fuel) is now once again being used to heat the Priory. Fuel for the system is supplied principally from the country estate’s own sustainable forestry as wood-waste and farm maintenance residues.

A layout diagram of a Talbotts C range system.

Theoretically, the current and future markets for wood fuel in the 21st century are extremely large. In the UK alone, there are thousands of rural businesses, schools and other services that have the potential to install wood-burning heating systems. The system installed at Elvendon Priory is an example of modern computer design and engineering, using a traditional sustainable fuel.

Background

Elvendon Priory is a listed building (ie building preserved for historical interest) that has recently undergone extensive refurbishment and modernisation. As part of this process, the owners wanted to replace the inefficient conventional heating boilers with a state-of-the-art wood fuel heating system. Owing to the listed status of the Priory and its outbuildings, no external alterations were permitted during refurbishment. Consequently, innovative design and installation solutions were needed. Talbott’s Ltd, a leading UK biomass and waste-to-energy specialist, tailored a standard, high-efficiency combustion unit coupled to a programmable computer controller (PCC) to meet the specific requirements of this unusual and specialised installation.

Inside the combustion zone of a wood-fired boiler.

The project

The project involved installing a 300 kW wood-fired boiler and auxiliary plant to meet the heating and hot water needs of a suite of facilities in the Priory complex, including offices and other business premises, the estate manager’s cottage and a large outdoor swimming pool.

Chipped wood waste, mainly from the estate’s own sustainable resource, is stored in a 2m diameter, top-fed silo which has a capacity of 7m3. This provides a continuous supply for between three and five days, depending on the heat demand. One tonne of wood chips occupies, on average, 2–6 m3 (depending on chip size, moisture content, loading procedures and settling).

Wood chips from the storage silo are fed to the combustion module by a screw auger via an agitator and an air-locked rotary valve. The raw fuel is passed slowly beneath a ceramic extension to the combustion zone, ensuring that even the wettest fuel is dried and pre-heated before reaching the grate. Motor-driven fan blowers supply pre-heated primary and secondary air to the combustion zone, while an induced draught fan with integral grit arrestor is mounted downstream between the exhaust port of the combustion unit and the stack.

Combustion air is also circulated outside the ceramic firewalls, lowering the temperature differential across the outer casing of the combustion module, thus minimising thermal losses.

The complete fuel handling and combustion system is controlled through the PCC. This has been specially designed to allow the whole system to be easily operated and controlled by non-specialist, untrained estate staff. Information from temperature sensors in the combustion zone, heat exchanger and flue gas exhaust are fed to the PCC, which controls the induced-draught fan, combustion-air blowers and fuel-handling motors. The drive speeds are monitored continuously and controlled to maintain the hot water temperature set-point, while ensuring that complete combustion takes place at the optimum temperature using the minimum volume of fuel. The unit produces less than 2% ash by volume; it is de-ashed routinely on a two-week cycle using a manually-controlled vacuum extraction system. The computer-based monitoring and control equipment includes data acquisition and storage together with an on-line remote communications facility. Up to 300 hours of continuous operating data can be monitored either on-site or remotely from Talbott’s headquarters. This enables engineers to reset, adjust or reprogramme the complete heating system on-line from virtually any remote terminal or lap-top computer, without the need to visit the installation.

Elevendon Priory.

Economics

The total cost of the system was £30,000 excluding VAT (where £ is the UK pound and Value Added Tax is 17.5%). The plant saves about £560/week during normal operation, when compared to the operational costs of the old heating system. The basic payback period for the new plant is about 13 months.

Conclusion

Elvendon Priory uses some of the most advanced control technologies available for this type of heating installation. The scheme provides an excellent example of how modern technology and a traditional fuel can combine to produce a high-efficiency, sustainable solution for meeting heating needs.

Note:

Under the UK government’s Climate Change Levy, a tax on energy used by businesses which comes into effect in April 2001, electricity generated from renewable energy (with the exception of large-scale hydro) is to be exempt. Renewables used as energy sources in their own right, eg wood fuel for heat, are also exempt from the levy. More information is available from the UK Department of Trade and Industry’s publication “New & Renewable Energy Prospects for the 21st Century”, which can be downloaded from the Web site: www.dti.gov.uk/renew/condoc/policy.pdf

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.