BIOMASS DESIGN BEST PRACTICE
Safe storage of pellets
We are used to hazards from liquid and gaseous fuels and to the precautions these make necessary. The seemingly benign, environmentally friendly wood pellet fuel is, surprisingly, no less hazardous and we must take appropriate measures to guard against its unexpected hazards.
The normal decomposition of wood includes oxidation of the fatty acids, such as linoleic acid, into methane, carbon monoxide (CO) and hexanal (an inflammable liquid). This proceeds so slowly, however, that no harm is done. Making wood pellets involves crushing the wood down to about the consistency of sawdust in a hammer mill followed by compression to ²/7 of the normal volume and extrusion. This alters the rate at which the above oxidative reaction occurs. The extreme case occurs during sea transport when thousands of tons of pellets can
be confined in bulk carrier chambers for around two to three weeks. The headspace in these chambers has been found to contain as little as 1% oxygen and as much as 1% CO, plus the other gases.
There have been stevedore deaths in Rotterdam and Helsinborg, Sweden. But there have been deaths in domestic, pellet-fired installations in Ireland and Switzerland and in large-scale plant in Germany as well. This atmosphere will asphyxiate and is potentially explosive in contact with the outside air. The risk of dust explosion is also present during delivery. Ventilation of the store is essential to sustain
safety, especially in view of the fact that a CO2 alarm would need to be intrinsically safe and would be forever causing nuisance alerts because of the supply of CO2.
PERFORMANCE CHECKLIST
Systems that are well designed and properly integrated with the building or process, achieve the following:
The building uses at least 75% of the heat in the fuel
Subject to client needs, more than 90% of annual heat needs are supplied from biomass
Plant downtime (except for major services) is less than 150 hours per year
Flue gases are well dispersed and stay clear of adjacent buildings
Ash production is less than 0.5% of input fuel weight
Boiler tube cleaning is needed less than once per month.
Integration between the biomass boiler packaged controls and the BMS/direct digital controls (DDC) can be difficult. Communication protocols may not be easy to interface with building management systems, and suppliers may have limited product knowledge and understanding of effective control strategies.
What can be achieved The picture is not all bleak, as case studies written by the Carbon Trust show. For example, Lochaber leisure centre’s biomass system has been operating reliably for the last 15 months, supplying more than 95% of the total heat requirement. This installation also ticks nearly all of the boxes in the checklists included in the new CIBSE Application Manual, and this is by no means an isolated example. Successful installations have ensured
that the whole procurement process for biomass boilers is properly informed, with a fully trained professional engineer engaged at the earliest concept stage to define the chimney height or boiler-house location.
Where do we go from here? To ensure all those involved in the procurement process of a biomass boiler are adequately trained will be a mammoth task. Professional level engineering training is likely to require a total of five to seven days to complete, plus mentoring
28 CIBSE Journal December 2012
Equipotential cross-bonding of all metal parts of stores with the building’s electrical system, and earthing of delivery trucks and hoses with a minimum 4mm² copper cable before and during delivery, is essential to eliminate the occurrence of static discharge igniting either the explosively inflammable gases or the dust. This is now an official recommendation given by the German Energy Wood and Pellet Association and German Pellet Institute. Explosions due to both causes have taken place in Europe. The rate of the oxidative reaction is temperature dependent and tails off after six to seven weeks. Therefore pellet fuel hoppers/stores must have a notice placed prominently by any openings and must have the relevant hazard warning symbols displayed. l Jim Kinnibrugh is divisional director at Specialist Chimney Consultants
on the first three or four projects. Design technician training will take a minimum of three days, together with access to professional-level engineers. Then training will be needed for architects, civil engineers, cost consultants, town planners, project managers, construction managers, commissioning engineers and technicians, facilities managers, maintenance fitters and site staff. All of this will need to be built up from a very small initial skills base of people with knowledge of the biomass procurement process from start to finish. CJ
A guide to biomass heating The CIBSE Application Manual on Biomass Heating is due to be published in spring 2013, with 140 pages covering the whole procurement process, with advice for a wide range of audiences.
References
1 Safety Notice: Risk of carbon monoxide release during the storage of wood pellets, The Health and Safety Executive, November 2012
2 Biomass Heat Accelerator: overview and summary of output, Carbon Trust 2012
3 BS EN 13384-1 Chimneys. Thermal and fluid dynamic calculation methods. Chimneys serving one appliance.
Four experts contributed to this article: COLIN ASHFORD, consultant to the Carbon Trust and editor of the Application Manual on Biomass Heating; DAVID PALMER, the founding member of Campbell Palmer Partnership and lead technical author of the new manual; JIM KINNIBRUGH, divisional director of The Specialist Group; and GRAHAM SMITH, founding member of Birling Consulting.
www.cibsejournal.com
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