Using biomass fuels
in modern efficient heating systems is a well-established technique used in individual
buildings and district heating schemes throughout Europe and Scandinavia. There are a number of European standards
that cover both the design of biomass systems and the specification of the fuel used.
- Dry biomass can be grown on a large scale in the form of:
- oilseed cake
- or specific energy crops such as:
- sweet sorghum
- it can be also be taken from forest residues and certain wood wastes such as:
- forestry trimmings
- primary wood processing
- secondary wood and textile wastes
- and also clean residues from:
- furniture manufacturers
- timber processors
- recycled timber can also be suitable.
It is important to note that the installation of any boiler plant needs very careful consideration, when waste materials are a
source of fuel. Materials considered NOT to be waste are clean, uncontaminated materials such as those previously noted.
Installations burning waste, such as wood wastes from construction or demolition would be subject to both the Waste Incineration
Directive (WID) and the Pollution Prevention and Control Regulations 2000 (PPC).
The WID applies to all incineration and co-incineration plants. Co-incineration includes installations where waste is used as a
fuel where energy generation is the main purpose.
All co-incinerators that fall within the WID are also subject to control under the PPC. Larger plant will be regulated by the
Environment Agency under Part A(1) of Section 5.1 and smaller plant will be regulated by the local authority under Part A(2)
of Section 5.1. Part A(2) applies to plant burning non-hazardous waste with a capacity of less than 1 Tonne per hour.
If a plant or ‘installation’ comes under the PPC Regulations then there must be an application made to operate such a facility.
In addition it must be demonstrated that the installation uses the Best Available Techniques (BAT) in the design and operation
of the plant to prevent pollution.
Modern biomass boilers, are computer controlled for optimum efficiency and are easy to maintain and operate. Woodchips, pellets
and other fuel types are delivered on a regular basis into a fuel store and then automatically fed into the boiler when demanded.
Figure 1: Typical Domestic Wood Pellet Biomass Boiler Features
Biomass has, along with being a carbon neutral fuel and providing potential cost savings based on present-day prices, less tangible
wider benefits. The use of biomass fuels provides an incentive for the sustainable management of local woodland, it adds to the local
economy and it provides an opportunity for other buildings to follow suit through the establishment of a reliable supply chain.
Some of the disadvantages of biomass boilers are: the additional capital cost of the boiler and the need for additional facilities such as the
fuel store and fuel transfer machinery; the need for regular fuel deliveries via lorry; and the additional space required for the boiler and
store which is significantly larger than for a gas or oil-fired system.
Biomass heating systems are typically designed to replace either oil or gas fired boilers. Due to the nature of biomass fuels the boilers
tend to be physically larger than those for oil or gas, they require more space and need to be located in a position that is easily
accessible to fuel delivery vehicles. For state of the art boilers, maintenance and operating procedures are similar to oil or gas boilers
with the exception of some cleaning every one or two months.
All automatic biomass-fired boiler plants are essentially similar, consisting of four key elements: Fuel storage, fuel feed mechanism,
grate or hearth and heat exchanger.
2.1 Fuel Delivery and Storage
Fuel is either supplied by an external contractor or as a self supply operation, for example a farm or estate with its own wood harvesting
operation. There are a number of mechanisms to deliver the fuel to the bunker. The simplest option is to tip the chips or pellets directly
from a tipper truck or trailer into a subterranean bunker. Other options include dump bag systems, front-end loaders, purpose built
containers, or pneumatic delivery. The options for fuel delivery and storage are mostly determined by the position of the boiler and
the access to the premises being heated. The fuel delivery and storage system can account for a large proportion of the overall cost of
installing biomass heating systems. In new building applications, consideration to the location and operation of a biomass boiler and
store should be undertaken as early as possible to avoid additional costs in the future.
2.2 Fuel feed systems
Biomass fuels such as woodchip and oilseed cake are not free flowing materials and have a tendency to bridge, and so storage bunkers, or
silos, are fitted with some type of agitator system. Small systems use rotating sweeper arm devices that drag the material over an auger
as shown below.
Figure 3: Typical Sweeping Floor Systems
Larger woodchip storage systems require moving floors that move the material to an auger placed at the end of the bunker.
Figure 4: Typical Moving Floor System
Wood pellets flow very easily and do not need special agitator systems. The fuel is removed from the store by an auger which lifts
the fuel and deposits it in a drop cell which has three key functions:
- It prevents air being drawn into the boiler
- It measures the woodchips being fed onto the boiler stoker conveyor
- It assists in preventing burn back from the boiler
The precise arrangements of drop cells vary here are two typical examples:
Figure 5: Typical Pellet Store and Drop Cell Unit
Some of the smaller, simple woodchip systems units may dispense with the drop cell and use a single short auger to feed fuel
directly to the burner.
2.3 Grate (Hearth)
Comprises a short auger that feeds the fuel directly into the combustion zone. The type and design of the grate varies between individual
manufactures. ‘Underfed grates’ are typically used with wood-fuels with moisture contents of 30% or less, ‘Overfed grates’ are used for
small wood pellet burners and ‘Moving step grates’ are used for fuels with moisture contents of 30% to 60%.
Wood requires relatively little primary air (air supplied at the point of solids combustion, usually under the grate) but requires a good
supply of secondary air (over-grate) to ensure that complete combustion takes place. All modern woodchip and wood pellet burners have
provision for separate control of the primary and secondary air supply, in order that combustion can be properly balanced. The combustion
zone is usually insulated and comprises a primary and secondary chamber to ensure thorough mixing (turbulence) and time for the combustion
reactions to occur.
Figure 6: Typical Biomass Moving Floor Grate
Boilers are fitted with a number of control features. The inlet, outlet and room temperatures are monitored and linked to sensors that control
the feed auger and determine the rate that fuel is supplied to the combustion zone. Sensors in the exhaust, monitoring oxygen and carbon
monoxide levels (lambda sensors), are used to regulate the air supply and/or the fuel feed to maintain the correct air to fuel ratio for
clean combustion and low emissions. In general wood fuel is a low sulphur fuel but has higher NOx emissions than gas or oil boilers due
to the inherent high nitrogen content of the fuel.
2.4 Heat Exchanger
These are conventional heat exchangers similar to those used for gas or oil fired boiler systems. They comprise either a simple water
jacket or multi-pass fire tube positioned either horizontally or vertical. The boilers are highly efficient with typical efficiencies
of 80% to over 90% for woodchip and wood pellet systems.
There is a tendency for fly ash to accumulate in the fire tubes, especially those set in a horizontal position, which then require
cleaning at regular intervals dependent upon the fuel quality. A number of manufactures now offer automatic tube cleaning for larger
boiler plant as a standard feature. This is recommended for ensuring high efficiencies and low maintenance costs. The larger systems
are also fitted with cyclone separators to collect any fly ash passing through the boilers and into the environment.
Many boilers are also fitted with flue gas recirculation, which helps to control combustion temperatures and reduce NOx formation.
Figure 7: Typical 3 Pass Heat Exchanger with Automatic Cleaning
Compared to oil and gas, biomass fired systems require more specific maintenance procedures.
Wood combustion produces about 0.5 – 1.5% by weight of ash depending on fuel quality. This ash collects in an ash pan under the grate.
It has to be removed manually in regular intervals depending upon heating demand. Automatic de-ash systems into a large ash bin
(see Figure 7) reduce the amount of work required but the ash bin still requires removal and emptying periodically. This ash is safe
for use as a soil fertiliser.
Boilers that incorporate cyclone separators trap minute particles passing through the boiler. Depending upon the operating conditions
these separators will need cleaning every three to six months. For manually operated boilers the boiler plates and tubes will need to
be scraped clean every six months to a year.
Chimney flues will collect deposits of the tar and soot depending upon operating conditions. It is therefore advised that flues are
checked annually and swept clean as necessary which may be once a year or every two years.
Motor and auger bearings need checking twice every year and greased according to the manufacturers specifications.
Most burners are fitted with multiple safety features to prevent burn back which could result in fuel store ignition. The drop cell
or zone is fitted as a physical barrier that prevents a fire spreading to the fuel store. Up to two temperature sensors are fitted
along the burner feed auger to detect burn back and, if activated, can disconnect the power supply and release water (dousing system)
to extinguish the fire.
All biomass boilers share a common set of features as defined in the following sections.
High quality automatic wood-fuel boilers can operate between 20% and 100% of their maximum continuous rating (MCR) without a serious
loss of efficiency. This enables a single boiler to be operated over a range of outputs and follow the heat demand more closely.
5.2 Pause/Slumber Mode
When heating or hot water is not required most wood-fuel boilers switch into pause mode. This feature is similar to the pilot light
on a gas or oil boiler. During pause mode fuel is automatically fed, very occasionally, to the burner to maintain embers sufficient
to restart the burner when heating or hot water is required.
5.3 Automatic Ignition
Most boilers can be fitted with automatic ignition. Woodchip boilers typically use an electrical system that blows very hot air
into the fuel bed (similar to a paint stripper) until the fuel on the grate ignites. Wood pellet boilers often use an electric
coil to heat up pellets until they ignite. Larger boilers and those burning wet fuel use small gas burners for ignition (requiring
a bottled gas or piped gas supply).
5.4 Automatic Cleaning
As in all types of boilers the fire tubes will become encrusted with deposits of soot and will eventually affect the performance
of the boiler by reducing the power output and lowering the overall efficiency. It is necessary to periodically remove these
deposits to maintain the boiler performance. Industrial type boilers require periodic manual cleaning. The more modern boilers
with higher specifications often incorporate automatic cleaning systems, such as augers that turn periodically to scrape clean
the boiler tubes, compressed air or air recirculation jets.