Wood pellets - a fuel for the 21st century
From Building for a Future Autumn 2003
 continues from page 1
Pellet heating appliances
There are two main types of pellet appliance; smaller-scale space heaters and larger scale water heaters suitable for a range of uses from top-up space heating to complete space and water heating.
Small-scale pellet stoves are suitable for top-up space heating in houses or large rooms and provide an elegant feature for any room, typically providing 2 to 12Kw of heating energy. Pellet stoves are far more efficient than open fires or wood burning stoves.
They can take the place of an open fire; wood burning, coal, gas or electric stove and use an attractive genuine flame for heating. They are more efficient, easier to use and require far less maintenance than wood burners or coal stoves. Some stoves come with back burners for water heating.
Pellet stoves can be controlled by thermostats and usually have a variable output, allowing for easy heat output adjustment. They are ideal for providing top-up heating in low energy houses, where the actual amount of fuel required is minimal.
How does a pellet stove work?
A pellet stove consists of a hopper, screw feed, heating element, electronic control, suction & convection fans and a combustion chamber. Heat output is controlled by a thermostat, which regulates how much fuel (how many pellets) are fed into the heating chamber. Unlike a wood burning stove, heat output can be adjusted according to requirement. The hopper typically takes 25Kg of pellets and, depending on heat demand will last 2-5 days. Ash drops down into an ash pan, which, thanks to the high efficiency and combustion temperatures, only requires occasional emptying (typically less than 5 times a year). Ash deposits account for <2% of the total fuel volume of fuel burnt.
Water heating pellet boilers can take the place of an existing gas or oil central heating appliance, many are 90%+ efficient and nearly all are microprocessor controlled. Like oil or gas systems, they are totally automatic but do require some occasional maintenance typically:
• Emptying of ash pan every 3 months
• Cleaning of the burner once a year
The fuel supplier will usually agree to carry out this work as part of a maintenance contract.
These appliances are suitable for variable heat loads and can be timer controlled. They range in sizes from 15kW - 500kW in output and are suitable for both domestic and commercial applications e.g.: houses, schools, offices and factories. There are some appliances available that are capable of running on both wood pellets and wood chips, offering the consumer more choice of fuel source.
Fuel storage
Pellets generally require more storage space than fossil fuels such as heating oil. This is because pellets have a much lower energy density. A house with a total heat (water & space) demand of 26,880 kWh/year, would need 10.6 cubic metres of fuel storage space for pellets, 40.6m2 for wood chips and just 3.3m3 for heating oil. (3)
Capital costs
The capital costs for pellet systems are still higher than fossil fuel systems, however there are grants available to help with the costs.
Oil Fired: entire system fitted: 3-bed house (23 kW);
1. New combi-boiler, new oil tank, Installation & connection to existing hot water system inc. electrical connections approx cost £ 3500 + vat
2. Equivalent wood pellet fired system (23kW): new pellet boiler, pellet storage tank (3m, Insulated flue), installation & connection to existing hot water tank, electrical connections approx cost £6000 + VAT. There is, however, a grant available for biofuel appliances of £50 per kW (up to 30 kW only). So our 23 kW boiler would attract funding of £1150 therefore £6000 - £1150 = £4850 + vat. Only £1350 more expensive to install pellet-heating equipment. Pellet stoves range in price from £1400 to £2500, Some models do not require expensive insulated flues, this can make their capital cost comparable with high quality wood burning stoves.
Case studies
Case study 1. Trinity Special School, Durham:
Pellet heating is saving Trinity school 121.9 tonnes of CO2 per annum compared with mains gas, and 186.7 tonnes of CO2 per annum compared to heating oil.
Two years ago Durham county council took the decision to phase out all of its remaining coal fired boilers in schools. Where schools were not able to receive mains gas (off grid) the existing (coal fired) boilers were converted to take wood pellet fuel instead. There are currently three schools in Durham, which have been converted to pellets, and a further 6 are due for conversion this year.
The project has been so successful that the council is considering phasing out a further 16 oil fired boiler systems in favour of pellet fired systems, saving on fuel costs and potentially saving hundreds of tonnes of CO2 emissions per year.
The heating appliances have been running problem-free for 18 months since the simple conversion work was undertaken.
The school featured in this case study is saving 190 tonnes of carbon dioxide per year.
The conversion process was a very simple. First, the appliance was serviced and the coalbunker was cleaned out, then anti-burn back devices were added to the feed pipes to prevent fire spreading along the stokers and into the fuel bunkers. The total cost of the conversion was approximately £3000.
A note on energy prices. Durham county council are able to bulk-buy their heating fuels, therefore the prices used in this analysis are not representative of a domestic prices but should serve as a guide for larger heating projects eg: businesses or institutions.
Durham County Council supplied us with energy prices for comparison purposes. Pellets are being purchased at £58 per tonne, and Coal at £57 per tonne. Calorific values were supplied by the DTI. Carbon emissions according to different fuel types are taken form SAP 2001. These fuel prices are not typical of residential prices.
The figures are based on a yearly heat load of 600,000 kWh with an average boiler efficiency of 80% for all appliances. Total heating costs include climate change levy. (See Ref 3) In this case wood pellets and mains natural gas are the same price per unit energy. This illustrates that it is possible even at today’s prices for wood pellets to compete with natural gas on a price per kWh basis. However at £58 per tonne, Durham are buying their wood pellets for an exceptionally good price.
Case study 2. Typical and ‘Part L’ UK House.
Annual costs and carbon savings of two types of UK house with a floor area of 120m2:
• typical 3 bed Victorian house with a total annual heat load of 28600 kWh/yr
• new 3 bed house built to the 2002 ‘Part L’ building standard with an annual heat load of 14520 kWh/yr (see ‘Ref 2.’ for further information)
Findings:
• Heating a UK house with biomass leads to a massive 50% reduction in total annual CO2 emissions when compared to mains gas heating.
• Wood pellets are 32% cheaper than delivered LP gas
• Annual space and water heating costs for a 3-bed 120m2 'part L' house are £373 using pellets, £359 using oil and £550 using delivered gas.
• Converting a typical UK house from oil to pellet or chip fired heating would save 8.3 tonnes of CO2 emissions per year
• Mains gas is still the cheapest method of water and space heating
• Converting a typical UK house from mains gas to biomass (pellet) heating would reduce CO2 emissions for heating by 87%
• Wood pellet fuel is considerably cleaner and in some cases cheaper than existing methods of heating.
Author: Andrew Cox xco2
All pictures/illustrations courtesy of Xco2 conisbee
Further Information
www.logpile.org.uk - Promoting the use of wood as a source of renewable energy, contains comprehensive database of UK pellet fuel and equipment suppliers.
www.britishbiogen.co.uk - UK Bioenergy industry association
www.pelletheat.org - Amercian pellet fuels institute
A copy of the full report can be downloaded from the XCo2 website at www.xco2.com
References
1, *expressed as % of total energy produced. source: Biomass Tank THERMIE B Project STR-0940,
2. Heat Loads (from BRE) UK Victorian
- Water heating : 60kWh/m2 per year
- Space heating: 164 kWh/m2 per year
Part L 2002 - Water heating : 50kWh/m2 per year
•Space heating: 71 kWh/m2 per year
3. Assumptions for Trinity School Durham All figures the same as ‘UK house’ except raw fuel costs:
•wood chip £40 per tonne
•wood pellet £58 per tonne
•coal at £57 per tonne
•heating oil at 15.7 pence per litre
Note: prices are based on bulk buy arrangements Durham County Council has with its fuel suppliers.
4. Assumptions for willow coppice heating
•Assumes 12 dry tonnes per hectare of short rotation coppice per year - source: The Scottish agricultural college Alterner Report ‘Energy From Willow’
•Energy at 4670 kWh/tonne (see ref.2)
•Same energy demand as Part L 2002 house. (see ref. 2)
•Total set aside figures for 2002 England and wales
- 723000 hectares source: Environment Agency
|
