Heating technology

Since the revisions to Part L earlier this year, it’s become clear that many modern buildings will need to incorporate renewable energy technologies to achieve compliance. And, while newer technologies such as photovoltaics and solar energy have received the most publicity, long-established fuels such as wood also have a significant role to play.

There are several reasons for this; not least significant advances in the combustion efficiencies of wood burning boilers. Whereas early models only achieved efficiencies of around 50%, this has increased to 90% with a massive reduction in carbon monoxide emissions - from 20,000 mg/m³ to 100 mg/m³.

In addition, the development of wood-based fuels such as wood pellets, which burn consistently and can be fed automatically to the boiler, has made wood burning more viable. Some farmers have started growing short rotation coppice crops like willow and poplar and we have recently seen this wood supplied on pallets; so fuel supplies can be local and secure.

In exploiting the benefits of wood, there are essentially two approaches, depending on the project – installing new wood-burning boilers in conjunction with other technologies, or converting existing coal-fired boilers to wood.

In new installations, the optimum arrangement will often be to integrate renewable heat supply technologies, such as wood and solar thermal energy.

In many projects, it also makes sense to use wood-fired boilers in conjunction with gas-fired condensing boilers, using the strengths of each to achieve optimum performance. In this scenario, the wood boilers provide heat for domestic hot water where the return water temperatures are too high for efficient condensing. In parallel, the gas boilers supply space heating requirements and exploit the lower return water temperatures to achieve high efficiencies through condensing.

Converting from coal

Where a building is already using coal-fired boilers up to 750 kW, it is relatively straightforward to convert them to wood pellets. Usually this requires improving the safety and position of the screw feed and increasing its speed, while decreasing the air mix to give combustion efficiencies of up to 86%.

This type of conversion has now been carried out on many school boilers – some of them 25 years old – achieving very high efficiencies with low emissions.

Further benefits come from the fact that as wood pellets produce less than 1% ash and contain no sulphur, they are a much cleaner fuel than coal. Burning wood in a converted coal-fired boiler will burn away existing soot from the boiler and chimneys, so that maintenance requirements are considerably reduced. The ash also makes a good fertiliser (potash) for local use.

Wood fuel types

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

Modern log-burning boilers use a two-stage process, with gasification followed by high temperature combustion. This is very efficient (>90%) with properly seasoned wood, but requires manual handling of the logs and considerable storage space.

Both woodchips and pellets can be conveyed with a screw worm or augur for automatic feeding, but chips require more storage space and more sophisticated handling. Most chips, however, vary in size and moisture content and require complex controls to handle this variation. Chips also have a higher moisture content than pellets so more heat is wasted in boiling off water, producing water vapour.

Pellets are manufactured from sawdust, shavings and finely reduced wood waste, some of which comes from further processing of wood chips. Compressing these materials into pellets releases lignin from the wood, binding the pellets firmly with no additional binding agents.

A major advantage of pellets is that they are a consistent size, so they burn predictably, provide a consistent heat output and flow freely in a screw feeder. They have around 10% moisture content, considerably less than the 25-55% typical of chips.

Pellets can also be delivered by tanker and stored in a relatively small container, such as a silo. For example, in a building with a 50 kW heating capacity using oil-fired boilers, the fuel consumption would be around 8500 litres of oil per year. To achieve the same heating using wood pellets would require 17 tonnes of pellets, occupying just 26 m³.

Carbon neutral

Because the CO₂ produced by wood burning is equivalent to that absorbed in its growth, it is virtually carbon-neutral, other than the carbon used in transporting it.

In terms of price, wood fuels cost less than oil and slightly more than gas, but this differential is changing rapidly in line with the fluctuation in world energy prices.

Wood remains the primary fuel for many developing countries and, because of its environmental, economic and social advantages, it is rapidly making a comeback in developed nations. It is now a serious contender for consideration in future projects.