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Last edited 07 Jul 2016

Biomass

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[edit] Introduction

Biomass is a generic term referring to organic materials that can be used as fuels. Biomass differs from fossil fuels because of the timescale required for replacement. Whilst both take carbon out of the environment during their creation, before releasing it when used as a fuel, fossil fuels deplete faster than they are replaced and so are not sustainable whereas biomass can be replaced relatively quickly and so may be considered 'carbon neutral'.

Solid bioenergy options include woodchips and pellets. Using these types of biomass fuel as a heating source is well established across Europe and the UK. The use of biomass as an energy source is traditionally through combustion within a biomass boiler, providing hot water. This technology can be a central boiler supplying heat via district heating or individual biomass stoves or boilers in each property.

Biomass fuel can also be used to generate power through Combined Heat and Power (CHP) technology. Small scale biomass CHP systems are in development, but they are still considered to be an emerging technology. The specific requirements of a biomass CHP system are similar to a biomass boiler, with the notable difference being additional space requirements, particularly height.

[edit] Technical Information

  • The following figures can be used to determine the approximate storage area required (the actual volume will depend on the moisture content of the fuel):
  1. Pellets: 3,000 kWh/m³ fuel
  2. Chips: 800 kWh/m³ fuel
  • Storing more than 2 months supply is not recommended because of potential problems with decay and fungal growth.
  • Five litres of combustion ash is produced by each of the following:
  1. 1,000 litres of wood pellets
  2. 15 litres of wood chips
  • Attention must be paid to chimney heights. Biomass boilers have to be certified as 'clean' for smoke controlled zones.
  • Exhaust gases are cleaner than those from oil-fired boilers.
  • Feeding systems for boilers greater than 2MW becomes more complicated and requires moving floors etc.

[edit] Loading

Generally biomass systems should be sized lower than peak loads as these occur infrequently. The Carbon Trust has developed a sophisticated tool for sizing boilers and storage requirements.

Biomass boilers turndown (the amount by which heat output can be reduced from the maximum without switching off) is usually 20-30% on dry fuel and 40% on wet fuel. This dictates sizing unless a buffer tank is used.

Commonly a gas/oil back up boiler (or burner) will be specified to provide fuel resilience in case of supply chain problems, this boiler can also be used to 'top-up' to a peak load capacity. An exception to this might be small domestic applications where fitting two boilers may be uneconomic. Typically a biomass boiler sized at around 30-40% peak load will supply 60-70% of heating consumption over a year..

[edit] Wood Resources

In 2007, The Woodfuel Strategy for England estimated that there was a resource of 3.5 million tonnes of green wood per annum, with a potential for 5.5 million tonnes by 2020.

However, some members of the AECB (The Association for Environment Conscious Building) are against biomass (see AECB discussion paper on biomass) on the basis that wood used in boilers could otherwise be used in other wood products that don't release CO2 into the atmosphere. This caused a lot of heated debate and many members felt that the AECB had taken a position on the issue without consulting its members.

In July 2013, the UK government appeared to turn away from biomass when it capped subsidies for bespoke biomass burning plants and announced that it would end subsidies for biomass burning in existing stations by 2027. This followed a report from the BBC revealing that millions of tonnes of wood were being shipped from the USA to help meet Britain's renewables targets.

[edit] Air Quality Management

In general, cities must monitor their air quality, and if certain emission are above advisory thresholds they must implement an Air Quality Management Plan (AQMP). It is possible that where there is an AQMP, part of this plan might be to limit biomass installations. However, air quality is predominately a function of diesel powered transport and so it could be argued that restricting biomass boilers is not tackling the root of the problem.

NB Waste wood and other wastes in the UK fall under the Waste Incineration Directive (WID) and require expensive treatment to reduce harmful emissions.


This article was created by --Buro Happold.

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