FAQ

A biomass boiler is a boiler designed to burn solid fuels classed as biomass. Such boilers can be supplied to burn every form of biomass from woodchips, wood pellets or logs to waste agricultural materials such as straw and grain husks, olive kernels, rice and the dust from any of these including sawdust. Boilers need to be designed to burn specific materials, with any given boiler able to burn a limited range of biomass.

Yes, in much the same manner as any other boiler. However, as many biomass boilers operate at a higher temperature, and hence pressure, than fossil fuelled boilers, with some operating at above 100C, it may be necessary to interpose a plate heat exchanger between the biomass boiler and the existing heating system.

In many respects biomass boilers can be controlled based on heat demand just as with fossil fuelled boilers. However, the much slower response of biomass boilers to changes in load mean that up to three control loops are used to control the fuel feed rate, the primary and secondary air fans, and the delivery of energy to the load including the charging/discharging of the buffer vessel. The minimisation of emissions requires carefully controlled combustion for which a Lambda sensor in the flue monitors the excess oxygen level to enable combustion to be optimised.

Many situations require one or more back-up boilers, especially those involving residential accommodation. A properly designed and installed biomass boiler will be as reliable as a fossil fuelled boiler, and in this respect the usual rules for specifying back-up boilers should be applied. However, as a biomass boiler is rarely sized to meet the peak load, the back-up boiler usually acts as a peak-lopping boiler as well. Hence, it is usual for back-up boilers to be sized to meet the peak load.

Biomass boilers operate at their highest efficiency, and are most reliable, when operating continuously. Biomass boilers cannot be switched on and off like fossil fuelled boilers and need to operate in conjunction with a buffer tank if the boiler is to be able to handle modulating loads, particularly loads less than the minimum boiler output, whilst continuing to operate efficiently. Biomass boilers are ideally suited to meet the continuous heat loads of buildings such as swimming pools, hospitals and nursing homes, and industrial processes with a constant heat demand. Other buildings with long periods of daily heat demand like schools and hotels are also a good match to a biomass boiler when a buffer tank is used in conjunction with the boiler.

While fossil fuelled boilers usually require an annual maintenance visit only, biomass boilers require more attention. Biomass boilers burning even the most difficult of materials can operate unattended, but weekly inspection visits are required to carry out a visual inspection of the boiler and fuel feed system, to check the lubrication of bearings and to empty the ashbin. If the boiler is not fitted with automatic flue cleaning, regular cleaning of the flue tubes is required using a flue brush.

Summer hot water loads are usually very small in relation to the size of the boiler, typically 5% - 10% of the boiler rating and, as such, are always less than the minimum output of the boiler. The use of a correctly sized buffer vessel allows the boiler to be operated for short periods once or twice a week to charge the vessel. Hot water is then drawn from the buffer vessel as required to meet the load.

In addition to the boiler itself, a biomass system will require a fuel store (silo) and a mechanism to extract fuel from the store and to feed it into the boiler. The flue gases may require treatment and a cyclone grit arrestor is the most common flue gas cleaning device. Where a cyclone or other flue filtration system is fitted, an induced draught fan will be required on the flue. Finally, a buffer vessel will be required for the majority of boilers in the majority of circumstances.

All but the smallest of biomass systems can be configured to work fully automatically. Typical automated features include time switch or optimum start/stop, fuel feed, de-ashing and flue cleaning. The majority of boilers can operate for up to 1 week at a time without manual intervention.

The height of flue required depends on many factors including the boiler rating, whether an induced draught fan is installed and whether the boiler is to be installed in a Smoke Control Area. The Biomass System Feasibility Guide, also available on the BEC website, contains a Flue Selection Flowchart which leads the user through the regulations relating to flue height. This chart should be referred to for a definitive answer.

Currently several hundred systems, larger than domestic systems, are installed in the UK ranging in size from 50kW to the 220MW system installed at a power station. However, the number of installations is increasing rapidly as is the associated design and installation, and operating experience.

Almost all biomass materials can be burned in biomass boilers, but the majority of systems operate on woodchips or wood pellets. On a small scale almost any biomass material can be burned in a batch fed boiler including straw, wood offcuts, waste wood, and logs. It should be noted that the burning of waste wood and offcuts may be subject to the Waste Incineration Directive which prohibits the burning of materials contaminated by heavy metals (eg lead based paint) and halogenated organic compounds (eg some pesticides, sheep dip and similar materials). On a very large scale, as found in district heating systems, fully automatic boilers can burn straw, sawdust, nut husks, olive kernels, and oat, wheat and barley husks. Some of these latter materials are not recommended for burning on any but the largest scale because of the potential problems of slag formation, combustion control and emissions.

As with any heating system the amount of fuel required will depend on the size of boiler and the period for which it is operated. The amount of fuel required also depends on the calorific value of the fuel and its density. For example, a 100kW boiler operating at 85% efficiency for 10 hours a day for 7 days would require:
• Using wood pellets: 1.7 tonnes requiring 2.6m3 of storage
• Using woodchips @ 30% moisture content: 2.4 tonnes requiring 11.2m3 of storage
• Using woodchips @ 50% moisture content: 3.7 tonnes requiring 12.2m3 of storage

On a domestic scale fuel can be supplied as logs or as bags of pellets; above this size fuel has to be delivered by lorry. Woodchips are best delivered by tipping into a silo although they can be tipped into a trough from which they are blown into a silo. Wood pellets can be tipped or blown from the delivery vehicle, with specialist pellet delivery vehicles equipped with fans for this purpose. While other delivery methods are possible they are either slow or have health and safety implications, and are not recommended.

Fuel on a domestic scale, which is manually handled into a boiler feed hopper, can be stored in any dry shed or building. Most automatically fed boilers require a fuel silo attached to the boiler house, this silo requiring access for a delivery vehicle. Depending on the manner of delivery, discussed in the answer above, the vehicle will need to tip into the silo or fuel will need to be blown into the silo. In the case of blown fuel the silo does not necessarily need to be on the outside of the building and, in the case of pellets, fuel can be blown up to 20m from the delivery vehicle. On the smallest automatic pellet boilers, fuel be stored some distance from the boiler and extracted from the silo and delivered to the boiler by a vacuum conveying system. Finally, containerised silos are available where fuel is delivered in the container which forms the fuel store and is coupled up to the fuel extract mechanism on delivery.