Plants offer an almost limitless renewable source of energy. Graham Blandford, marketing director of Viessmann UK, looks at some of the technical points to be considered with bio-energy systems
Bio-FUELS, in the form of wood and oil derived from plants, promise carbon-neutral heating. Both deliver their heat through boilers, which makes them ideal for replacement systems where solar or heat pumps may not be a viable option. But, with both, there are technical points to be considered.
Changing an existing gas- or oil-fired boiler for a biomass (and in our market that means wood from managed sources, mostly in pellet form) offers a clean, carbon-neutral form of heat. Wood used in bio-mass is often rejected from other processes and would otherwise be waste. So it has great all-round sustainability.
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Pellets can be difficult to store, and maintaining a consistent quality is also essential if boilers are to burn properly. Providing pellets storage and quality can be secured, boiler installation is relatively straightforward. Pellet boilers, such as the Viessmann Vitoligno range, can be hand-fuelled through their 150l hopper. They then burn with modulating output, under automatic control, with a high efficiency and a low slumber rate.
Typically a hopper would hold two days of fuel - a minimum for winter use. But, in all but small domestic installations, a feed system from a remote pellet store is essential. De-ashing is straightforward, and on smaller boilers this needs only be done two to four times a year. Their ease of use makes pellet boilers a replacement option for small oil boilers up to 26kW (with up to 50kW available early in 2008).
For larger projects, pellet, wood shaving or log burners up to 1250kW are available in the KOB range. KOB joined the Viessmann Group in 2007. Importantly, the KOB range includes fuel-handling and storage systems. Even larger boilers up to small CHP station size are available.
The other main bio-fuel, and one that is now attracting increased interest, is oil derived from plants. In the UK, that means rapeseed oil as it is UK grown and refined. There is a refinery in Kent, which means, for example, that the greater London area can be served with low-CO2 mileage deliveries.
Rapeseed oil can be stored and handled in similar ways to that of gas oil. It is non corrosive, has a high flash point and even if spilt is bio-degradable. It is proving therefore a practical option where a renewable energy source is required in, for example, urban buildings where fuel storage for wood is out of the question.
Rapeseed oil does, however, have different combustion characteristics to gas oil, and these have to be allowed for in the design of the boiler/burner and oil handling. The oil must be heated to ensure efficient combustion.
The calorific value of rapeseed is lower than that of gas oil but once its higher burning temperatures are taken into account, the kilowatt per unit volume is similar. Servicing is similar to oil and emissions are lower as the fuel does not contain sulphur.
The burning characteristics mean that the boiler combustion and flueways have to be wider than for gas-burning boilers. This requirement is met by the larger flueways of the Viessmann Vitoplex range. The Vitoplex is a steel three-pass design giving up to 86% efficiency in standard form and further heat transfer from the Viessmann Vitotrans heat exchanger.
Specially designed burners are required for rapeseed. Dunphy Combustion, for example, can provide a dual rapeseed/natural gas version. This gives a standby source of energy should oil supplies are interrupted.
What about costs? Both wood and rapeseed oil are subject to market pressures and reflect both demand and the cost of transport.
But, in the long term, both are sustainable forms of energy and are CO2 neutral. Both are relatively straightforward technologies, well within the technical scope of the designer and contractor.
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