Biomass for heating and electricity generation

• ‘Biomass’ refers to a fuel that is produced by organic means.

• It is very nearly ‘carbon neutral’, limited by the need to transport the fuel.

• Biomass is produced in two formats:
          - As a fuel directly grown as an ‘energy crop’ eg wood; high yield
          - As a waste bi product eg agricultural residue; industrial waste and co-products

• Energy from biomass is generated at different scales:
          - Large / grid scale, from dedicated and co-firing power stations
          -  Medium / district scale power plants that provide energy to eg schools and housing  schemes
          -  Small / Domestic scale biomass boilers
 

The 'Carbon Cycle'

   
The central principle of biomass is that it takes carbon out of the atmosphere during growth and returns it when burnt.

 
A ‘closed carbon cycle’ is maintained where the biomass is harvested as part of a constantly replenished crop. Harvesting is followed by replanting, with the new growth taking up the C02 from the atmosphere released by the combustion of the previous harvest.
 

Large / grid scale biomass power plants

   
At the grid scale the Government is using the Renewable Obligation scheme to promote the development of large-scale integrated biomass generation. Because of the ease and low-risk of retrofitting fossil-fuel power plants to enable ‘co-firing’ (where both fossil and biomass fuels are used together), most of the initial expansion was in the area of existing power stations; But as the production and delivery of biomass at an industrial scale has become more refined, we are starting to witness a growth of the dedicated biomass power plant.
 

Examples

 
• The Eccenshall plant near Stafford generates 2.65 MW of electricity, enough to power some 2,000 households. The fuel is Miscanthus (an SRC, see below) grown within a 25 mile radius. The ash residue will be used as fertiliser on local farms.
 
• The world’s largest biomass power plant is being planned at Port Talbot. The 350MW station will be wood-burning with fuel being imported from sustainable sources in North America.
 

Fuels being used in large scale power plants

 
The Renewables Obligation defines a number of eligible renewable energy sources which include combustion of biomass in dedicated plant; co-firing of biomass and energy crops ; and pyrolysis, gasification & anaerobic digestion of biomass and biomass/waste blends.

• Short-rotation coppice (SRC): Fast growing trees that provide the bulk of fuel being currently used. SRC can be used alongside coal in co-firing power plants.

• Wood waste: The waste from forest maintenance, other timber industries and other crop residues is commonly transformed into compact fuel pellets.

• Municipal solid waste: An increasing amount of waste previously destined to landfill is being used to generate both heat and power on a municipal scale.
 

Medium / district scale power plants (15 – 1000 kW)

 
Biomass district energy systems are those that feed energy from a centralised location to a number of units eg flats, houses, offices, industrial units. Heat generated centrally is piped to each building where it feeds a heat exchanger. Likewise, electricity generated in a CHP process is fed to each building as required.
 

 
District heating and power systems are common in Europe. The UK is only beginning to progress from a long-term base of around 1% of heating provided by district heating systems, compared with, say, Denmark (60%) or even Germany (12%). But with the drive towards CO₂ reduction and the development of boiler technologies, along with more secure fuel planning, district heating systems are becoming increasingly viable and popular. CHP systems on the other-hand are still in the early-days of development – though they offer much promise, experience of early systems, such as that at BedZED have been disappointing.
   

  District heating and power – pros & cons

Small / Domestic scale biomass boilers (typically 15 kW)

Biomass is beginning to compete in the domestic market alongside gas and oil as a fuel. Most boilers are imported from Austria where currently 80% of all boilers installed are wood pellet boilers. More costly than wood chip fuel boilers, wood pellet boilers are more like conventional boilers including self-igniting and self-regulating. They are also more efficient, typically burning at up to 94% efficiency. As the technology develops, domestic-scale CHP boilers are also likely to feature on the market in the future.

Small scale heating -  pros & cons

Wood fuel types

Wood chip – 10kW – 10MW

 
• More suitable for medium / large scale operations
• A local supplier is needed
• Quality tends to be variable
• Fuel reception design is key and can be expensive
• Fuel does not flow and is difficult to handle.
• Care should be taken when using recycled wood because of the risk of contaminates such as formica or melamine, plastics and paint.
 

Wood pellet – 10 kw – 1000kW

 
• More suitable for small / medium scale operations
• The fuel is usually made from compressed sawdust and wood shavings but can be made from most biomass material (eg straw, forestry residues and specially grown energy crops)
• Pellets are usually produced at an industrial scale, though developing technology is enabling smaller-scale production.
• There isn't' t yet a consistent quality of pellet in the UK. Pellets vary according to source. Care should be taken when using recycled wood because of the risk of contaminates.
• Pellets are more expensive than wood chip / logs.
• Sources are unlikely to be local – necessitating transportation over long distances – so adding to cost and carbon emissions.
• Fuel is delivered by blower – fuel storage is generally simpler and cheaper than for other wood fuels.
• Unlike other wood fuels, pellets ‘flow’ – resulting in simple delivery to combustion.
• Pellet quality is crucial – boilers can be ‘intolerant’ to certain sources.
  

Logs – 15kW – 70kW

 
• More suitable for small operations
• Logs are easy to handle.
• Logs are more often sourced from a known and existing supply chain.
• They can be produced from small scale woodland
• They need to be loaded manually at least once a day

Design considerations

   
• Capacity: Calculate peak heat loss (which in a district system will include distribution losses).
• Network design in district systems
• Wood fuel selection:
   - Either chip (including moisture and particle size), pellet or log
   - Availability and price
   - Wood fuel specification and boiler type
   - Wood fuel delivery and fuel store design
• Think carefully about where to locate plant in relation to the thermal / airtightness envelope - putting a wood-fired system within the envelope can lead to significant heat loss owing to its contribution to air leakage and thermal bridging.
• Space and access for biomass boiler plant
   - Fuel delivery, receipt, storage options and delivery vehicle needs
• Planning issues
   - Visual
   - Air quality: In smokeless zones, wood can only be burnt in exempted appliances.
   - Traffic
   - Noise
• User controls

Movies

Biomass: The basics

A walk around a 95kW wood-fuel boiler at the RJ Mitchell Primary School
   

 
Retro installing a domestic biomass boiler and radiator system
   

Further information

• The BioMass Energy Centre - ( www.biomassenergycentre.org.uk )
• The Log Pile Web site– an online database of wood fuel suppliers - (  www.nef.org.uk/logpile )
• Wood Energy Business Scheme - ( www.forestry.gov.uk/woodenergywales )

 

 Biomass boiler products on GreenSpec

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