As fossil fuel prices continue to soar and Gordon Brown commits to a 60% reduction in carbon dioxide emissions by 2050, there is a growing understanding of the benefits biomass can deliver, both by significantly reducing energy costs and cutting CO2 emissions.
With buildings responsible for around half of the CO2 emissions in the UK, legislation such as the Building Regulations, Climate Change Levy, European Emissions Trading scheme and the ‘Merton Rule’ – to reduce the carbon footprint of new and refurbished buildings – is accelerating demand for biomass heating. The proposed Climate Change Bill will cement the reduction of carbon emissions as a key component of the building agenda. Biomass is now viewed as playing a major role in the fight against global warming, climate change and carbon dioxide emissions. It has been estimated that more than 10% of the UK’s heating needs could be supplied by wood (Biomass Taskforce Report 2005) – contributing a significant share of the energy market.
For building developments where there is a significant space heating or domestic hot water demand, wood fired boilers offer considerable reductions in CO2 emissions, generally greater than any other currently available on-site renewable technology. Recent advances in boiler technology also makes using biomass in towns and cities a viable alternative. Wood Energy Ltd is at the forefront of the biomass heating industry in the UK and Ireland and is the sole distributor and installer of Binder boilers. The company’s current installed biomass capacity is 30MW, yielding CO2 savings of approximately 14,500 tonnes a year. Government grants to help with the costs of biomass boiler are currently smoothing entry to the market for new customers.
Traditionally, gas-fired boilers have been the choice for most building services engineers. Biomass installation is very different and a thorough understanding of the issues involved is necessary in order to weigh up if it is appropriate for a project.
Fuel
Biomass used in wood burning boilers is either wood chips or pellets. Wood chips are much cheaper than pellets – about half the price – but do require a local supply to make them financially viable. Fuel quality is crucial to reliable operation including absence of contamination and particularly moisture content. Seasoning (air drying) is essential to reduce the moisture content. Special wood chippers must be used – for fuel chip rather than bulk reduction.
The moisture content of wood fuel adds to its weight but reduces its heat content when burnt. Above 50% moisture content, the wood fuel is difficult to combust. Boilers must be designed to deal with varying moisture content. As a general rule the useful energy content of perfectly dry wood fuel is approximately 5.2kWh/kg but reduces significantly as moisture content increases. Pellets are fairly uniform in size but wood chips can vary dramatically in size, which has an impact on combustion and fuel handling.
Emissions
Wood is a complex organic material and therefore emissions must be considered. Although the current Building Regulations (Part L) give a calculated value of 0.025kg CO2/kWh – about one-eighth that of natural gas, locally produced wood chippings are likely to have significantly lower carbon intensity.
Potentially hazardous emissions from biomass combustion systems such as oxides of nitrogen and oxides of sulphur have been reduced to very low levels. Control strategies such as lambda control and exhaust gas recycling have greatly improved air quality. The same applies to abatement technology such as cyclone particle separators. Unlike many biomass boilers, all Binder boilers below 1.2MW are approved under the 1993 Clear Air Act for Smoke Control Areas.
Fuel storage
Pellets and wood chips have different storage requirements. As a general rule pellets can be pneumatically delivered and therefore stored above ground, which means gravity feed can be used at some point in the fuel supply system between the fuel store and the boiler. Wood chips are generally stored at ground level or below ground. Consideration should be given to access for fuel deliveries, all of which presents challenges for the specifier and architect.
Suppliers
Security of fuel supply chains has been an issue in the past as large organisations were reluctant to rely on small suppliers. The arrival of (RES) Renewable Energy Systems Ltd, which has established a nationwide, quality assured, wood chip and pellet supply service, has gone a long way to help instil confidence in the supply chain and is likely to continue to be a significant factor in encouraging take-up of biomass. Local authorities are taking advantage of converting council wood waste into chips.
Sizing
Unlike most gas-fired boilers, wood boilers have limited ability to rapidly modulate heat output. The designer should therefore consider sizing the wood boiler to meet the base-load with more responsive plant, such as gas-fired boilers, providing top-up to meet peaks in demand. Some manufacturers recommend that a thermal store – or Accumulator Tank – be incorporated, allowing a smaller boiler to be installed and alleviating the problems of slow responses to load changes. A value engineering analysis should be undertaken to determine the optimum duty in terms of capital cost, running cost and carbon dioxide savings. This needs to take into account the size of the fuel store and thermal store required and the value of plant space. The designer should consider a whole heating season or year in order to optimise boiler sizing.
Costs
Wood boilers have a significantly higher capital cost than a gas-fired boiler, however, over time that extra initial outlay will be recovered. Customers already benefit from the lower cost of wood-fuel compared to fossil fuels. Typical costs of wood chip are currently less than 2p/kWh (metered heat) for wood compared with approximately 3-4p/kWh for gas and oil. The payback period will vary according to the size of the system, the local fuel cost and load factor. As a general rule, medium and large-scale projects typically achieve payback periods of between four and 10 years and this is before any grant support is taken into account.
The potential for large-scale savings is always going to be closely linked to price rises in fossil fuels. Long-term it is widely predicted there will be a continuing trend for a rise in prices due to increasing reliance on imported energy supplies as North Sea reserves of gas and oil decline. Political instability could also be a trigger for significant fluctuations in fuel prices; many organisations are already coming to the conclusion that high capital costs are an acceptable trade-off for the long-term fuel price stability offered by wood fuel. By using the fuel cost comparator on the Wood Energy website (http://www.woodenergyltd.co.uk/) you could discover just how much you could be saving.
Installations
Being able to deliver a building with strong green credentials and enabling planning mandates to be met more cost effectively has been a major factor in persuading public and private sector clients to explore biomass. The National Assembly for Wales’ new Debating Chamber – the Senedd building – in Cardiff is powered by a Binder biomass boiler and two new NHS hospitals in Wales – the 2nd Rhondda Valley Hospital (Ysbyty Cwm Rhondda) and Porthmadog Hospital (Ysbyty Alltwen Hospital) – have also embraced the technology and have recently been commissioned. Schools, prisons, plant nurseries, shopping centres and district heating housing projects across the UK and Ireland are just some of the pioneering users of biomass.
