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.
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-topping boiler as well. Hence, it is usual for back-up boilers to be sized to meet the peak load.
We would recommend your biomass boiler has at least an annual maintenance service. Like anything if you maintain your boiler correctly it will last longer and perform better. 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.
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.
Almost all biomass materials can be burned in biomass boilers, but the majority of systems operate on woodchips or wood pellets.
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.
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.
On automatically fed boilers a fuel extract and fed mechanism is employed to move fuel from the silo to the boiler. The Automatically Fed Systems Guide, available from the BEC website, details the fuel storage, extraction and feed systems possible together with photographs of the systems. In summary the fuel extract options are walking floors, sweeping mechanisms and hopper bottoms, while fuel feed is by auger or ram stoker.
A well designed and operated biomass boiler burning fuel within the specification of the boiler should not produce any smoke, but black smoke may be produced if the fuel is too wet for the boiler. All biomass boilers produce some oxides of nitrogen (NOx), particularly nitrogen dioxide (NO2). While NOx emissions from gas boilers have been reduced significantly in recent years through the use of low NOx burners which burn at a temperature slightly below that at which NOx forms, the nature of biomass combustion, and the need to ensure the complete combustion of wood gases, means that combustion takes place at a temperature where atmospheric oxygen and nitrogen can chemically combine. The better the quality of the combustion control system on a biomass boiler, the lower the NOx emissions will be. In general, biomass boilers produce less NOx than oil boilers. Biomass boilers do not produce any oxides of sulphur (SOx).