Biomass Feasibility Study for Bristol City Council
Using local biomass to meet heat and power needs
Project duration: January 2002 to May 2003
To undertake a detailed feasibility study into using locally obtained biomass as a source of renewable energy to meet heat and/or power needs at Bristol City Council sites.
In the Bristol area, local biomass could be used solely as a heating fuel (as an alternative to coal, oil or gas, feeding a boiler in a central heating system) or to provide heat and power (if used as an energy source for a combined heat and power unit).
The existing woodland resource within Bristol (e.g. Ashton Court Estate, other council woodlands, street tree management, local agricultural and Forest of Avon resources) were thought to be able to provide sufficient existing biomass resource for heat and power supply to a limited number of suitable council-controlled sites.
The council was also restoring the mansions at Ashton Court and Blaise heritage estates in the city, presenting the opportunity to introduce local biomass as a renewable energy fuel as part of boiler replacement work at these sites.
The steps involved in delivering the feasibility study included:
- identification of potential sites
- technology options and specification
- options for biomass supply
- economic evaluation
- facilitating feedback
- final feasibility report and action plan
The study has now been completed. It identified two potential sites, which were explored in more detail. These were a social housing block in south Bristol and a plant nursery in the grounds of Blaise Castle estate.
Social Housing Block
- Initial estimates suggest a 700kW biomass boiler would be optimum size for the site. This would supply baseload heating needs and meet about 70% of the annual heat load, running for about 1640 equivalent full load hours. By displacing the use of mains gas, this would save about 266 tonnes of carbon dioxide (CO2) emissions per year
- The economics appeared favourable. Based on a grant level of 50% (readily achievable given current levels of Government support), and allowing for the avoided cost of installing a biomass boiler in place of a gas boiler, then the simple payback would be nine years, based on savings from reduced gas consumption
- The gas boilers at the block were due to be replaced in 2004 and there was a window of opportunity to include installing of a biomass boiler as part of this replacement
- The economics of installing a biomass system at this site were more marginal and depended on using cheaper arboricultural wood chip and the existing barn as the boiler house
- If this were possible, with a level of grant support of 50%, the simple payback time would be 13 years, due to savings in LPG consumption
- Initial modelling suggested that the optimum size of biomass boiler for Blaise nursery would be about 400kW maximum output, although more detailed modelling would be required. Typically, this would run for the equivalent of about 900-1000 full load hours per year and would save about 45,000 litres in LPG consumption per year. This would reduce CO2 emissions from the site by about 70 tonnes per year
- The quantity of green wood chip available at Ashton Court at the time was roughly estimated to be 378 oven dry tonnes (odt)
- Allowing for errors in this coarse estimate, this was almost certainly enough to supply a biomass boiler at Blaise nursery, which would require about 100 odt per year. It may also be enough to supply a boiler at the social housing block, which would require about 300 odt per year. However, based on the rough estimate, it probably would not be enough to supply the boilers at both the social housing block and Blaise nursery
- The survey of tree surgeons suggested in the region of approx. 482 odt additional supply of green wood chip could be available if tree surgeons were provided with a site where they could dispose of wood chip for free
- Before any of the arboricultural wood chip, either from Ashton Court or from tree surgeons, could be used, it would need to be screened to remove oversize slivers, foreign objects and green matter. Furthermore, it should also be kept in a covered barn, to prevent it from getting any wetter and to allow preliminary drying to take place
photo © Bristol City Council