Challenges of biomass 14 December 2013

With so many organisations building biomass plants and/or converting existing units to biomass fuel, Alan Fitzpatrick and Marcus Robson examine some of issues being seen by the insurance industry.

With significant growth in the uptake of biomass boilers for everything from power generation to district heating and manufacturing – due primarily to government emissions legislation, subsidies and tax incentives – insurers are warning the industry of additional challenges from biomass fuels, both to operational staff and to insurers themselves. And this is the case across all four main types of biomass-using plant – large, medium, and small – the latter comprising both CHP (combined heat and power) plants and small biomass auxiliary boilers.

First some definitions. Large generating plants, such as Tilbury (each unit rated at 350MW) and Ironbridge (500MW per unit) power stations, are generally conversions from burning coal to wood pellets. They have traditional water tube boilers and steam turbines, and, due to the volume of wood pellets required, fuel is generally shipped from forests in the USA and Canada.

Meanwhile, medium sized generating plants, such as Stevens Croft power station (rated at 50MW), have generally been purpose built, again using water tube boiler and steam turbine technology. Wood chip fuel is, in this case, delivered from the local area.

Moving on to small plant, on the one hand, CHP units (mainly in Europe) use either wood pellets or wood chips as the fuel, feeding into specially designed auxiliary boilers that typically heat water for large district heating schemes. They may also use biogas and/or natural gas (methane) to fuel on-site gas engines that generate electricity to export to the national grid. On the other hand, small biomass auxiliary boilers also use wood pellets as the fuel, and are seeing significant growth throughout continental Europe, while in the UK, they are being used to heat water for businesses, large schools and colleges, and sports facilities – generally to replace gas-fired boilers. For the sake of completeness, there is also another plant type – using wood as a fuel in a process known as gasification. However, it is not common in the UK.

Fire and explosion risk
So what are the key issues? Taking plants that use wood as a fuel first, problems beyond the usual machinery breakdowns primarily relate to additional fire and explosion risks. Where either the process or the handling equipment involves biomass materials with particle sizes of less than 80 mesh (0.177mm) and moisture content below 30% by volume, there is a potential explosion hazard. As a result, initial or retrofit plant design, as well as good practices in commercial operation, are critical for safety.

Looking at the fuel first, a plant should clearly specify the material quality being brought on site, but then also check regularly to ensure that the supplier is compliant with the terms of the contract. An example of a wood pellet contract specification might be: diameter 6—8mm; length less than 40mm; water content less than 10%; bulk density greater than 600 kg/m3; maximum bulk temperature 60C; ash content less than 1% by weight; melting temperature greater than 1,200C; dust less than 1%. Dust is key and if tests show excessive amounts, a delivery should be rejected.

Moving on to wood fuel storage, this requires careful consideration. Any storage facility needs to be well away from the power plant, properly ventilated and with limited capacity. The wood itself must be regularly checked for moisture content and temperature, and moved in sequence so that it is always fresh. It should also be equipped with a sprinkler system. That said, there remain two primary concerns. First, if the wood becomes too wet it biodegrades rapidly, leading to a loss of energy content, but also potentially to the formation of moulds that, when inhaled, may be hazardous to health. But second, spontaneous heating may occur, caused by oxidation of unsaturated fatty acids, releasing carbon monoxide, carbon dioxide and methane.

Fuel storage issues
For power generation, wood pellet storage tends to be either in silos or large storage areas. The primary concern with this type of fuel is that it can degrade if moved excessively, creating dust that, with an ignition source, can lead to an explosion. So silos must have smooth sides and, if the chips are blown in, ductwork should be designed with smooth, gentle bends and padding to absorb pellet impacts. Silo will also be fitted with vents for filling, as well as explosion and over-pressure release devices. As for construction, any silo should be sited to avoid direct sunlight and/or colour used to minimise solar heat penetrating and heating the wood.

Additionally, and importantly, insurers require that wood dust be minimised, using vacuum techniques to keep levels below 3.2mm over a maximum 5% area; a fire detection system must be fitted; and spark ignition hazards, including overhead cranes (unless compliant with ATEX zoning), must be absent.

Considering next fuel transport to the boiler, while several methods are used, the most common is conveyor systems. That being the case, it's worth reviewing overall plant design. For example, as already mentioned, to reduce the risk of explosion, it is critical that surface areas on which dust might settle are minimised and that good housekeeping is standard plant practice. It is also important that the plant complies with the ATEX directive. This zones a plant into explosion risk areas, and only correctly rated electrical equipment should then be used per zone.

Clearly, conveyors raise concerns in this context with electric motors, switchgear and lighting systems, all of which need to be suitable under the ATEX directive. Additionally, though, the system should use non-combustible belts and avoid sudden changes in direction to reduce damage to the pellets and hence also dust formation. Also, since roller bearings might overheat, due to excessive dust or inadequate cooling and/or lubrication, conveyors need regular maintenance and cleaning.

Good practice includes the use of thermography to detect hot spots (guidance is to keep all dust exposure from machinery to below 260C) and the provision of a protective sprinkler system along with a 'firefly' system, which uses infrared for spark and flame detection. A linear temperature cabling system may also be installed so that, in the event of a fire, the conveyor stops and the sprinkler has a chance to operate and contain the blaze.

Advice and guidance
The insurance industry is very keen on compliance with guidance provided by legislation but also other recognised and readily accessible fire protection bodies.

For example, the National Fire Protection Association (NFPA) publishes guidance including two documents used widely in the power generation business: NFPA 664 Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities; and NFPA 850 Recommended Practice for Fire Protection for Electric Generating Plants and High Voltage Direct Current. Converter Stations.

Other recommended reading includes: insurance company guidance, which is available to all; Factory Mutual Insurance Company FM 8-27 Storage of Wood; and Factory Mutual Insurance Company FM 7-11 Belt Conveyors.

Advantages and disadvantages of biomass

i Reduces the need for fossil fuels
ii Biomass crops produce oxygen and absorb carbon dioxide
iii Waste used to produce steam instead of going to landfill
iv Local electricity generation, such as with district heating plants, reduces transmission losses from centralised power stations
v Biomass fuels provide an economic incentive to manage woodland, which improves diversity
vi Availability of biomass plants is above 90% compared onshore wind generation at 30%
vii A large generation biomass plant can base load, respond to frequency changes and load demand

i Biomass uses land that might be better harnessed for food crops
ii Neighbours may raise issues over the number of trucks bringing feedstock (a 25MW plant uses 180,000 tonnes of fuel/year, requiring 33 trucks per day delivering 20 tonnes of wood each)
iii The consequences of poor control and/or maintenance can include not only plant failures but also explosions and serious pollution events.

Alan Fitzpatrick and Marcus Robson are with CNA Insurance Company

Alan Fitzpatrick and Marcus Robson

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