Moving away from fossil fuels, we will need to find alternatives to coal, oil and gas for heating our homes and offices. One obvious solution is biomass – typically in the form of wood.
It is a renewable fuel that truly “grows on trees” and it can be produced locally and sustainably – as long as a new tree is planted for every tree that is cut down. Burning wood biomass emits CO2 through the chimney, but as growing trees absorb CO2 from the atmosphere biomass can be a nearly carbon neutral source of fuel as long as trees are replanted.
Batch log boilers and pellet boilers offer more convenient and efficient ways to heat with wood fuel than traditional stoves.
Can I heat with pellets without using a hopper store?
It is possible to heat with pellets without having a large hopper store connected to a boiler. There are compact pellet boilers – sometimes referred to as pellet stoves with back boilers – which can be filled manually with bags of pellets.
This can be an advantage where there is no space for a hopper store, or where pellets can only be delivered in bags anyway (e.g. because there is no access for a pellet delivery truck).
Can you burn wood biomass without smoke?
Emissions from wood fuel contain virtually no sulphur dioxide and very low levels of nitrous oxides, so won’t cause acid rain. Burning wood cleanly gives off very low amounts of smoke particulates, and many wood-fired appliances are certified for smokeless zones (Defra publish a list of Smoke Control Zones).
It’s important to burn efficiently; use properly seasoned wood (with low moisture content) and make sure that equipment is used properly. Manually fed stoves can produce lots of pollutants if operated badly.
Logs should be burned fiercely with lots of air input until they are almost charcoal, after which the stove can be ‘damped down’. Reducing the air supply too early creates lots of smoke & tar. The key is good ‘secondary combustion’ of the high-energy volatile gases given off by burning wood. Some stoves are fitted with a ‘Lamda’ sensor, to regulate the amount of oxygen added and so optimise efficiency. Avoid burning treated, painted or glued wood, or non-wood waste, as these will give off toxic and polluting gases.
Building regulations require all fuel burners to have a dedicated vent to avoid production of carbon monoxide. The chimney needs an insulated flue to prevent fumes condensing as tar. With complete combustion, wood burns to a small amount of ash, which (unlike coal ash) is an excellent fertiliser.
How do I store wood biomass?
Delivered wood should be stored for at least one year, preferably two, to air-dry the wood to a moisture content below 25%. Bringing logs inside for the last week or so improves them to room dryness. Stoves might be specified to cope with 50% moisture content, but efficiency will suffer. Compressed wood pellet fuel has only about 8% moisture. Manufacturers specify pellets of a certain size, shape and moisture content to ensure reliable operation. Some pellets are produced for power station co-firing, and are not good enough quality for domestic appliances.
The required storage space depends on how big and how well-insulated your home is. A small cottage is likely to need 8 cubic metres (m3) of logs each year, a 3-bedroom house 12m3, and a large detached house 16m3. Pellets have a higher energy content and so take up less than half as much space. Stoves use 0.5 to 1.5kg of pellets per hour, so a 15kg bag should last a few days.
Larger systems can use chipped wood – this allows more automation than logs and is cheaper than pellets. A wood chip boiler heats several buildings at CAT; seasoned wood is delivered, chipped, and stored until it reaches 15% moisture content. For larger schemes, it’s a good idea to have a supply contract to ensure a reliable supply of wood.
How much will a wood biomass system cost?
A log stove is likely to cost between £500 and £1000, with installation costs probably the same again. Pellet Stoves cost £1,500 to £2,500 plus installation. A log boiler will be roughly £4,000. Together with a water storage tank, flue and installation the total will be about £10,000. Automated pellet boilers are more advanced, and so is the price tag: £6,000 to £8,000 for the boiler; the total perhaps £10,000 to £15,000.
Remember to factor in the ongoing purchase of fuel. Bought in bulk, log fuel should be cheaper than gas, oil or coal. A 15kg bag of pellets will cost £3 to £4, but bulk delivery should be similar to or less than fossil fuel. The Log Pile service (below) lists suppliers around the UK. Also, find out about the support structure in place for the equipment you choose, before going for the cheapest. Will it be easy to get the appliance serviced annually? Are there enough plumbers or engineers with the relevant knowledge?
Which wood biomass fired appliance should I choose?
Before switching to any new heating system it is vital to maximise energy efficiency. Measures such as increasing insulation, lagging pipes and draught-proofing will save money on fuel, and also on equipment – as they’ll allow you to specify a smaller boiler. A combination of wood fuel and solar water heating (for hot water in summer), can give renewably-generated heat all year round.
Open fires are a poor choice, financially and environmentally. Most of the heat goes up the chimney and the rate at which the fire draws in oxygen creates draughts across the room that reduce the benefit from the fire. A simple wood stove is a great improvement; it should need only one-third as much fuel, as the efficiency can reach 70%. Automated pellet stoves are more convenient, and can even have an automatic de-ashing function.
Advanced wood heating systems for larger houses have been common for many years in mainland Europe and the USA, and are as efficient as modern gas boilers – converting well over 80% of the fuel into useful heat.
A ‘batch’ log boiler can be fired up once a day (or less often) and the heat stored in a large water cylinder. Automated pellet boilers make wood fuel almost as convenient as gas. They are more costly, but ease of use is a big plus. For those keen on range cooking there are specially designed wood-fired ranges, with a bigger firebox to accommodate logs. Doing everything from one appliance is not ideal, so the efficiency of these is less than dedicated boilers or stoves, but they would give the satisfaction of a wood-fired sunday roast!
Why burn wood biomass?
Biomass fuels include wood, energy crops such as oilseed rape or miscanthus (‘elephant grass’), animal wastes and other agricultural by-products such as straw and grain husks. When burned, these fuels release only the amount of carbon dioxide (CO2) that they absorbed whilst growing – unlike the carbon in coal, oil and gas, which was absorbed over millions of years but is being released in the space of a few decades. The energy used to harvest, process and transport the fuel does need to be factored in as well. Very little energy is needed to harvest wood, so when used locally it is a very low-carbon option.
Planting trees to absorb carbon dioxide may provide temporary mitigation from climate change, but doesn’t address the fundamental problem. To meet our energy needs in a zero-carbon future we must make sustainable use of trees as fuel, and replant them as we harvest them – creating a continuous carbon cycle. Growing our own fuel also creates jobs and is ideal for strong, local economies.
Free, independent and impartial advice on renewable energy and sustainable living provided by the RENEWABLE ENERGY CENTER SOLAR SERDAR
Head of business association