Amassing evidence
With the use of biofuels increasing, the particular requirements for flue arrangements need to be carefully considered, writes Richard Parkhouse
they are less dense than the surrounding air and are displaced by cooler air. However, the challenge of effectively removing the by-products of combustion is one of the key considerations to ensure the successful implementation of biomass boilers. Without proper assessment and application of appropriate flue techniques, the safety, efficiency and environmental impact of biomass boilers can be adversely affected.
W
Biomass fuels Biomass is a term that is used broadly to cover a variety of fuel sources. These range from animal dung and grass to wood, processed bio-liquids and biogas. In UK domestic and commercial boilers, biomass fuels are typically wood, and used in the form of chips or pellets. There are also boilers (including dual-fuel boilers) that can burn waste materials, as well as other biomass materials that are used in applications within the built environment. Bioliquids are usually produced from
oilseed crops such as rape. The liquids can be used in conjunction with petrol products, green diesel for vehicles, and fuel oils for burners and combined heat and power (CHP) engine sets. Biogas is created either by thermo-
chemical (pyrolisis) or anaerobic decomposition (decay in an oxygen-free environment) of organic materials. The biodegradable matter comes from living, or once-living, organisms (plants, animals or their wastes). Biogas fuel may be used for heating processes and, if concentrated, for vehicle transportation as compressed natural gas (CNG). Biogas plant includes sewage systems,
landfill gas and site waste-recovery systems. Its constituents can vary greatly from plant
42 CIBSE Journal January 2012
hen considering how a flue works in terms of buoyancy, it is simple: warm air and exhaust gas rise because
to plant and on a daily basis. Most systems generate a methane-based gas, with levels varying between about 40% and 90% (together with proportions of hydrogen, nitrogen and carbon monoxide). For example, a biomass plant would
generally produce about 74% methane, 25% carbon dioxide and 1% nitrogen, and has a distinctive odour. Its gross calorific value would be about 27.8 MJ/cu m, and this compares to typical natural gas gross calorific value (GCV) of 38.7 MJ/cu m. Biogas often has a specific gravity
approaching 0.9 and must be treated, for safety purposes, as a gas that can both rise and fall in air. This means ventilation must be considered for both heavy and light gases at the same time.
Biomass boiler Although most modern wood-chip and wood-pellet boilers are designed to allow modulation of the output down to, perhaps, 30% of maximum output, biomass is not well suited to this mode of operation. And, if continuous, low-level output is routinely required, biomass may not be the optimum choice. This also means that it is very important not to over-specify biomass boilers, and in larger systems they may need to operate in conjunction with a gas- or liquid-fuelled boiler with modulating burners for faster response to load changes. Biomass systems will also require fuel- storage facilities and, in almost all cases
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