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Biofuels - design, installation and air pollutionDecember 2008

In the second part of features on biomass boilers, BSRIA examines the operational performance of biofuel heating systems, their design and installation requirements, and the consequences for air pollution.

As reported in part one, there is a wide range of different sized biomass boiler systems, from small hand-fired domestic stoves to fully automated, self-igniting systems up to 100 MW.

Faced with a daunting range of options, building designers struggle with understanding the special requirements of different systems. In unravelling such complexity the best place to start is with the type of fuel. This needs to be decided prior to the design of the building, and before the boiler house is sized.

Fuels and their characteristics

Boilers run under very specific fuel conditions. Fuel is also the main operational cost. Building designers can identify the right amounts of fuel for a proposed project by asking manufacturers for their fuel consumption benchmarks. Manufacturers should also be able to supply detailed specifications on fuel consumption for different load conditions, such as peak, high, mid and slumber operation.

Designers need to decide from the start whether a development will run on a pellet boiler or wood-chip boiler. The two systems run under different conditions and require different spatial demands.

Generally, pellet-fuelled systems cannot take wood chips. The furnace is not designed to take them, and there would be issues in fuel transportation and combustion. On the other hand, wood chip boilers can use a mixture of different woody fuels at different levels of quality. They do not run solely on pellets, as they are too dry and mostly not suitable for the types of feeding mechanisms usually installed for wood chips.

A comparison between different types of biomass furnace

Biomass fuels have numerous characteristics that can affect the performance of a boiler system. The three that influence the performance the most (and which also affect costs) are calorific value, moisture content and particle size.

Wood pellet fuel is very dry, with 6-10 percent moisture content. It therefore has a higher calorific value compared to wood chips, which have a moisture content of 25-50 percent. As they are made from compressed sawdust, pellets are more homogeneous and therefore exhibit more consistent combustion characteristics.

The European Committee for Standardisation (CEN) is developing a standard (CEN/TC 335) that will specify typical standardised characteristics for a number of biofuels within the European Union. The quality of some fuel products, for example woody pellets, are already specified according to DIN 51731 or ONORM M7135.

A crucial consideration is the size of the fuel store and the volume of deliveries. This depends on the size of the system. Fuel for a wood-chip boiler system should be stored for one to two weeks. This allows flexibility in the supply chain.

The costs of running a biomass heating system can be significantly lower than that of a fossil-fuel based system. Arguably, this depends solely on the type of fossil fuel.

Wood-chip fuel is currently available at £75 - £90 per metric tonne, depending on the quality of the fuel. Generally, based on a 35 percent moisture content and an 85 percent boiler efficiency, this translates into 2 - 4 pence per kWh.

Air pollution

The critical reader will be concerned about the complex and environmentally-negative effects of wood-fuel fired systems - and not without reason. Most of the environmental issues of using wood as a fuel relate to the emissions resulting from their transportation and the air pollution that results from combustion.

Basically, due to its nature, biomass fuel is zero-rated for the EU Emissions Trading Scheme and the UK Climate Change Agreement. This means that the installation of a biomass system will contribute to a company's CO2 targets and environmental compliance.

In addition to this, local planning authorities should be informed about a potential biomass boiler project at the earliest possible stage. The planning authority will be able to advise on the requirements of the Building Regulations and planning permissions. This is particularly important for all non-domestic installations or installations above 45 kW, and environmental issues such as smoke control zones or air quality management areas.

The use of electrostatic filters is the best way of cleaning flue gases, with the highest separation of fine particulates below 30 mg/Nm3. Unfortunately, these filters are also one of the biggest items in terms of capital cost and physical size.

Sources of biomass fuel expressed in pence per kilowatt hour, applicable to most northern European countries. Footnotes: 1. Based on wood chips within 25 mile delivery redius and a 35 percent moisture content. 2. Based on pellets delivered as bulk no less than five metric tonnes. 3. The cost of electric heating covers a wide range. The lower end can be 11p/kWh using night tariffs, but average costs are usually much higher (19.29p is the maximum domestic rate.)

Working with a team of scientists, a number of biomass boiler manufacturers have found a way to provide and improve a flue-gas filter suitable for domestic boiler installations. The system works on a very similar principle to that of an electrostatic filter.

This is a breakthrough, as the separation efficiency for both fine particles and dust is greater than 60 per cent compared to unfiltered flue gases. This makes it an important step in efforts to improve the filtration of flue gases and to reducing the threat of increased dust emissions from biomass systems.

Maintenance and servicing

Maintenance and servicing is key to consistent performance of a biomass heating system. As ever, this comes at a cost.

Cost data that will help in the whole-life costing process are servicing, regular maintenance schedules, and electricity used by fans and pumps. That said, there is no easy way to benchmark the ideal levels of maintenance for systems such as a 900 kW wood-chip boiler unit with a walking floor.

Athough biomass power plants differ in servicing and maintenance requirements, 80 percent of the work is very similar across the biomass technologies, such as checking the auger, noise checks on fans, oil levels in aggregates, and the checks on the state of the heat exchanger in accordance with the manufacturer's operating manuals.

While the following list does not aim to be exhaustive, a typical weekly maintenance schedule should include:

  • checking flue-gas temperature and colour out of the stack
  • checking furnace temperature 
  • checking combustion area
  • checking oil levels on aggregates
  • cleaning the furnace and grates
  • checking safety temperature limiters
  • performing a function test of back-burn devices
  • performing a function test of negative pressure indicator
  • checking pressure switches
  • checking emergency stops for bunker and auger

Maintenance intervals will depend on the type of system. (Pellet systems strongly differ from wood chip systems.)

During the summer months every biomass boiler system should undergo a thorough annual service and maintenance. It is recommended that the boiler is shut down to allow it to fully cool, and to allow service teams full access inside the furnace for inspection. The annual service also gives time for heat exchanger tubes, flue tubes, and filter cyclones to be cleaned. Most manufacturers offer annual services along with an extension or renewal of warranties.