The wood for the trees

Renewable energy is now becoming a ‘must have’ for many new build and major refurbishment projects; driven either by local planning regulations or by the need to compensate for energy-hungry systems such as air conditioning. Of the various renewable options available, biomass boilers using wood pellets are proving particularly popular.

There are several reasons for this; not least the significant advances in the combustion efficiencies of wood burning boilers. Whereas early wood burning boilers could only achieve efficiencies of around 50 percent, this has now increased to 90 percent, with a massive reduction in carbon monoxide emissions

Another reason for this growing popularity is that wood is classified as a carbon-neutral fuel, because the carbon-dioxide released during combustion is equivalent to the carbon dioxide that was fixed by the plants during growth just a few years before. With fossil fuels, the carbon dioxide that was trapped over a period of millions of years is being released into the atmosphere within a relatively short space of time.

Commonly used wood fuels include logs, chips and pellets and, in our experience, pellets currently offer the most practical solution for the majority of projects.

While modern log burning boilers are now very efficient, for practical reasons their use is generally limited to projects where there is a plentiful supply of logs nearby. There is also a considerable amount of manual handling required.
Both woodchips and pellets can be fed to the boiler automatically but chips tend to vary in size and tend to have a higher moisture content than pellets (25-55 percent compared to 10 percent). This means that chips require more sophisticated handling and energy can be wasted in boiling off water, which also makes the combustion characteristics inconsistent, requiring complex and expensive controls to compensate.

Pellets are manufactured from sawdust, shavings and finely reduced wood waste, some of which comes from further processing of wood chips. Wood pellets conform to international standards in terms of size and moisture content, so they flow freely in an automatic feeding system and give a predictable heat output.

As a result, the boilers are as responsive and easy to control as an oil or gas boiler. They can also be delivered by tanker and stored in a relatively small container, such as a silo.

These were important considerations for the Newark Beacon Innovation Centre, where sustainability was a key design criterion. Here, two Hoval BioLyt wood pellet boilers have been installed to supply heating and hot water. Operation, function control and fuel dosage are automated, with pellets from the 20 tonne storage silo being fed to the boiler by an automatic pellet feeding system.

In terms of availability, there is a growing network of pellet providers in the UK, comprising both sawmills and farmers growing short rotation coppice crops. For example, since the Share Holiday Village replaced its existing oil boiler with a Hoval STU 300kW wood pellet boiler it has been able to source its wood pellets from a sawmill just 12 miles away.

In a number of projects it is possible, and even desirable, to combine wood pellet boilers with other renewable sources – such as solar heating or heat pumps – and even with conventional boilers as well. In these projects, it’s important to work with a specialist that has experience of all of these technologies and how to integrate them. Integration of different heating technologies will be discussed further in the next issue of Estates Review.

Hoval Ltd
Northgate
Newark
Notts
NG24 1JN
Tel: 01636 672711
Fax: 01636 673532
E-mail: hoval@hoval.co.uk
Web: www.hoval.co.uk