THE GREATEST CO 2 SAVING IS FROM CHP FUELLED BY BIOMASS
The second part of our renewables series explores biomass and combined heat and power (CHP). An extended version of this article appears in the sustainability section of www. ajplus. co. uk. A study of photovoltaics also appears on the website Biomass and CHP do not necessarily go together. Today, CHP is typically gas-fired, while a biomass boiler usually generates just heat. But there may be an overlap in multi-fuel projects and also in the future we may see biomass-fired CHP, operating at a larger scale with the heat going to district heating. So looking at them together now seems sensible.
Both biomass and CHP are affected by continuing changes in Building Regulations - particularly the energy requirements in new-build and refurbishment projects. Lower energy design is no longer defined solely by insulation and airtightness standards. This is partly because going beyond current building fabric standards will significantly increase the cost of projects. Furthermore, fabric measures only impact on one part of energy demand, namely space heating. Other energy uses, such as hot water, lighting and appliances now have bigger CO 2 impact than space heating. Low and zero-carbon technologies, such as CHP, and renewable energy sources, such as biomass fuels, will increasingly be used to improve building performance beyond the current Building Regulations.
In addition, local authorities are beginning to introduce their own requirements, which may be more demanding than the Building Regulations. Notably, the Greater London Authority (GLA) now requires the use of CHP and district heating in projects involving more than 500 dwellings, or in large projects ranging from 15,000 to 30,000m 2, depending on location. And there are some 160 local authorities at various points between having plan requirements and thinking about requiring 10 per cent of energy to come from on-site renewables.
However, using CHP and biomass together can be problematic. In cases like those in the GLA area where CHP is a planning requirement, this may limit the viability of biomass energy sources. To make the most economical use of heat from CHP (CHP systems are normally heat-led in design, not powerled) the system is designed to supply at least the base heat loads year round. In the majority of projects this will make the use of biomass more difficult and potentially less cost-effective, as these effectively compete with the heat supplied by CHP. And while the greatest CO 2 saving is obtained from CHP fuelled by biomass, this is not yet commercially viable for any but the largest of projects where they supply a district heating network.
When considering the use of either biomass boilers or CHP, the scale of the project should be considered. Biomass boilers are more suitable for smaller sites where there is good delivery vehicle access and space to store fuel. On larger sites, CHP units will be more efficient and can be combined with renewable sources such as solar PV or wind power where renewable energy provision is sought or required.
Another factor is plant-space demands. Both CHP plant and biomass boilers require more space than conventional boilers, and there is additional space required for thermal storage and, for biomass, also for fuel storage.
All these changes require the architect and engineer to work more closely together, taking a holistic approach.
THE ROLE OF CHP A common question today is: 'What is the most cost-effective way to reduce CO 2 emissions to achieve the target level?' The higher capital costs of using CHP are easier to justify if system running hours are maximised (4,000-5,500 hours per year). The greatest potential is for using the heat output from CHP to supply district heating to large, mixed-use developments where the different demand patterns of commercial, retail and residential spaces can complement each other. Hospitals and leisure centres also provide good baseloads for CHP. High heat-load densities make distribution of heat from CHP more efficient through reduced infrastructure costs and reduced heat losses.
In this respect, we should be seeking to include more existing buildings in district-scale heating and, where appropriate, making provision during design for connection to future district heating schemes. Using wet systems with centralised plant for space heating of the whole building or development makes it relatively easy to connect to a district-heating scheme in future or to change the energy source, such as to biomass or gas-engine CHP.
BIOMASS FUELS While there are a number of potential biomass fuels available, most UK biomass projects so far have opted for wood in various forms. Other options include straw and elephant grass (hybrid Miscanthus), both of which are currently limited in the UK.
Wood is plentiful and, from a sustainability point of view, the priority is to make more efficient use of clean waste wood from sources such as wood-processing industries and waste from the construction industry - much of which currently goes to land-ll.
Forestry has the capability to contribute a further 3 million tonnes (approx. ) a year by 2010 and many farmers are adopting short-rotation coppice plants, such as willow and poplar, which can usually be harvested on a three-year cycle.
So, while biomass is currently in its infancy in the UK, compared to many other parts of the world, there is the potential for a secure supply of fuel well into the future.
Much clean wood waste could be processed into wood chips or wood pellets. Wood chips are produced by chipping or shredding various sources of wood, including traditionally managed woodlands and short-rotation coppice. Pellets are manufactured from sawdust, shavings and finely reduced wood waste, some of which comes from further processing of wood chips.
A consideration with chips is that they vary in size and moisture content and this affects the consistency of combustion.
Wood-chip fuel stores also need more management, compared to pellet stores, to prevent decomposition.
Wood pellets from accredited sources are now processed to provide consistency in compliance with CEN 335 Solid Biofuels, so the heat output is more easily controlled. Historically, pellets have cost more than gas for the same thermal output, but as gas prices rise and pellet production increases this may be reversed.
BIOMASS STORAGE Important considerations for heating with biomass fuels include the size of plant-room space and storage facilities, frequency of fuel deliveries and access for large delivery vehicles (see table above).
As they require less processing, wood chips cost less than pellets, but they also require more storage space. For example, a 500kW boiler providing about 1,500 MWh/year will use about 530 tonnes of chips - or 350 tonnes of pellets. And 1m 3 of storage space will hold 650kg of pellets, but only 225kg of chips. So chips require both more storage volume and more deliveries or bigger vehicles.
If you design less storage space, this will necessitate more frequent deliveries. And, as delivery vehicles use fossil fuels, the frequency of deliveries and the distance the vehicles travel inuences the potential carbon-neutrality equation. Part L requires the use of a CO 2 emission factor for biomass which takes account of this factor and other energy used in the production cycle.
On the plus side, as the biomass industry becomes more organised and supplies more customers, companies are switching to larger vehicles that are able to make several deliveries in one trip.
MULTIFUEL PROJECTS In some cases there are strong advantages to combining biomass with gas-fired boilers to exploit the optimum performance of both.
At the new Hadley Learning Community near Telford, West Midlands, for example, the constant base heating load for the swimming pool and hydrotherapy pool is met by an 850kW biomass boiler using a mixture of wood pellets and chips sourced from a local short-rotation coppice plantation. Space-heating requirements are met by a gas-fired condensing boiler serving an underoor heating system, making use of the low return-water temperature to maximise condensing operation. From an energyefficiency point of view this is clearly a very good solution, but it does have implications for plant-room space and the overall project costs.
The best solution in terms of CO 2 reduction would be to use biomass to fuel a CHP plant whose heat output serves a district heating scheme. Current technology restricts commercially viable combined biomass/CHP to projects over around 10MWe. But there is much current research into this and a number of experimental projects are under way. As technologies for smaller projects are evaluated and fne-tuned there is a strong likelihood that biomassfuelled CHP will become viable for a wider range of projects over the next decade.
Claire Bonham-Carter and Paul Woods both work for engineering firm Faber Maunsell
BIOMASS HEATING OF A 160-DWELLING DEVELOPMENT All heat from a 150kW wood pellet boiler.
80m 2 of plant space for:
fuel store; and 20,000 litre buffer (two tanks 2.5m diameter x 2.9m high).
two 10-12T trucks/month in winter; and one 10-12T truck/month in summer.
boiler with automatic de-ashing; and ash disposal, 1-2 per cent by volume (may be saleable as fertiliser).