Fuel
Wood chips (30% MC)
Wood pellets
Heating oil (for comparison)
Net calorific value (CV) by mass GJ/tonne
12.5 17 42.5
Net calorific value (CV) by mass kWh/kg
3.5 4.8 11.8 Figure 1: Typical calorific values of ‘woody’ biomass fuels4
present safety issues. For example, wood chips with high moisture content may degrade and decompose in storage, losing calorific value. In certain conditions, there can be mould growth that – in common with wood dust – if inhaled can cause respiratory problems. When wood pellets are blown into a store upon delivery, a proportion of ‘break up’ can happen, leading to a suspension of dust in the air that can pose an explosion risk. Furthermore, in the case of pellets, it has recently been reported that stored fuel can release carbon monoxide (CO), making a robust safety procedure essential when working on or inspecting enclosed fuel storage areas – including provision for ventilation and supervision. Established guidance is provided by the UK Health and Safety Executive (HSE) (
www.hse.gov.uk/confinedspace/), while the Dangerous Substances and Explosive Atmospheres Regulations 20025
(DSEAR)
provides further information on control of such risks. (Also, see Further reading section below.)
Combustion process Figure 2 shows a diagrammatic arrangement of the combustion process of an example mid- scale biomass boiler, incorporating a moving
grate fuel bed and cyclone flue-cleaning device. 1Fuel is automatically fed into the combustion chamber of the boiler. The fuel is ignited by a blown hot air ignition system.
2The automated grate movement evenly spreads the fuel at the point of combustion for optimum burning efficiency.
3Relatively small amounts of ash residue (from 0.5% to 2% dry fuel weight) will remain
when the fuel has been fully burnt. The grate movement propels ash to a mechanical screw that removes the ash into the ash box.
4When the unit is first ‘fired’, hot air is blown in at the fuel entry point. The hot air ignition system turns off when the flame is established.
5The hot gases and particulates are drawn through the boiler through a multi-cyclone collector to remove the majority of the particulates (roughly 2% of the total ash will be carried along with the combustion gases6
),
which are collected in the ash box at the base of the collector. The remaining flue gases are discharged to the atmosphere. The fuel feed rate will modulate according to
the heating system demand. Typically controlled by a ‘lambda sensor’, the rate of excess air supplied into the combustion process is critical to derive the highest efficiency, together with minimising the production of CO, NOx soot.
and
The combustion of biomass fuel typically generates a higher level of polluting emissions than that of a gas fired boiler – although not as high as other solid fuels – so compliance with local requirements on clean air and particulate emissions is critical. The Clean Air Act (1993) regulates particulate emissions from residential and industrial combustion sources, and allows areas to be designated as smoke control areas (SCAs). Under the Environment Act 2005, local authorities throughout the UK have a statutory duty to review and assess air quality in their area against the objectives set for nitrogen
System hydraulics and buffering Biomass boilers are typically less responsive than modern condensing boilers, with less effective modulation and lower turn-down ratios. For this reason, it is recommended that the biomass boiler is sized to cover the base heat load of the building, with condensing gas boilers meeting peak load requirements. This enables the biomass boiler to run for longer periods without load cycling; this is important, as multiple boiler starts over a short time period can result in incomplete combustion and a rise in pollutant emissions. A buffer vessel is essential in most installations – firstly, to allow longer run periods and smooth out peaks in system demand, and secondly, because when the boiler stops firing there can be a great deal of residual heat in the fuel bed and refractory lining that must be dissipated, which is typically undertaken by employing a pump overrun control. Sizing of the buffer store will vary by boiler design, fuel type and the system it is serving – manufacturers’ advice will be required. Boilers with a lower water content and good modulation may only need 10 litres per kW output, whereas larger chip-fed appliances may require upwards of 50 litres per kW. Figure 3 shows a typical bivalent system
layout with a buffer vessel, peak load boilers and a low-loss header. With this arrangement, it is typical for the condensing boilers to operate with a return temperature of around 50°C to ensure optimum efficiency. Many biomass boilers will require a return temperature of between 60 and 65°C, dependent upon fuel moisture content, to prevent corrosion of the heat exchange surfaces. Therefore, a back end protection loop – comprising a three-port control valve and shunt pump circuit – is a common arrangement, thereby bypassing the buffer vessel until the boiler loop is up to an acceptable temperature.
Figure 2: Typical biomass boiler combustion process (Source: Potterton Commercial BBS)
Renewable Heat Incentive for biomass Payments are based on kilowatt hours (kWh) of metered generation for heat produced by the biomass plant that is used for an eligible purpose (space heating, hot water production and some processes). Tariff levels are split by the installed thermal output (kWth) of the biomass plant, as detailed in Figure 4.
46 CIBSE Journal August 2013
www.cibsejournal.com
Bulk density kg/m3
250 650 845
Energy density by volume MJ/m3
3,100 11,000 36,000
Energy density by volume kWh/m3
870 3,100 10,000
dioxide (NO2), particles measuring 10µm or less (PM10), sulphur dioxide (SO2
) and other gases. To enable the wider adoption of biomass
heat, boilers can be approved for operation in designated SCA and, subject to the maximum concentrations of particulate matters and NO2 not being exceeded in a given area, approval for a biomass installation should be possible.7
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