Commercial Wood Chip and Wood Pellet Biomass Boilers
WoodiePod™ biomass solutions feature a pre-fabricated plant room with an integral fuel store, they include the boiler, buffer/s, plumbing, heat meter, electrical wiring with control systems and remote monitoring pre-installed ready for connection to an external heating system and the internet.
The following guidelines are for automatically fed commercial wood chip and wood pellet biomass boilers of 50 kW and above.
Biomass Fuel Types and Storage
Biomass fuel can be stored in a range of ways, including silos, hoppers, containerised stores, flexible fabric silos, and in sheds above fuel extractor systems in large installations.
Pellet systems will usually have an automatic hopper-fed fuel system. The hopper can be either built-in, in the case of some smaller systems, or a detached separate unit. Pellet systems are generally the most responsive of the biomass boilers, have the simplest controls and are the closest to fossil fuelled boilers in terms of maintenance and operation, although there can be large variations between systems from different manufacturers in terms of sophistication and features.
Wood pellets are usually stored in a hopper bottom store. Providing the angle of the floor is greater than 40°pellets will flow down into the extractor auger. They can be filled by pneumatic delivery where one of the two hoses receives the pellets and the other provides pressure relief and recovers the dust produced to the delivery vehicle. While these silos can be fitted with auger outfeed equipment, pellets can also be extracted by a vacuum pump to the boiler’s dosing silo. Whilst pneumatic delivery of wood pellets is now routine, both chips and pellets can be delivered by tipping into a silo, for example where this is constructed underground or where ramped access is provided.
The safety risk with wood pellets relates to the dust produced during delivery by pneumatic conveying. When pellets are blown into a silo a proportion break up and produce the sawdust from which they were manufactured. If the dust concentration is not controlled it is possible for an explosive mixture of dust in air to be created which could explode if an ignition source is present. Buying quality pellets with a low dust content and good mechanical durability is important. Abrasion and impact during delivery of pellets can generate fine dust. This can be prevented by using smooth metal delivery pipes, with any bends of large radius, and a yielding impact baffle opposite the point of exit from the delivery tube to ensure that pellets are not shattered on impact with the opposite wall of the store. The store should be regularly checked for build up of dust and periodically cleaned out.
Another risk is from the potential for wood pellets to outgas CO in storage. This gas is both toxic and flammable. In particular nobody should enter an enclosed pellet store until it has been thoroughly ventilated, and only under the supervision of another person, outside the store.
Biomass Wood Chips
Wood chip boilers are fueled by an automatic feed of chipped wood, which can be supplied with moisture contents from 15% to 50%. Boiler responsiveness is determined partly by the fuel moisture content which the boiler is designed to accept; in general the wetter the fuel, the less responsive the boiler.
Wood chips are often stored in a silo with a sloping floor, and extracted using an outfeed mechanism known as an agitator which prevents bridging within the stack and ensures a continual feed of fuel. This usually comprises two sweeping arms which push wood chip onto the extract auger. Alternatively, wood chips with a moisture content less than 35% can be stored in a hopper bottom incorporating a fuel extractor mechanism.
Both chips and pellets can be delivered by tipping into a silo, for example where this is constructed underground or where ramped access is provided. Articulated containers can deliver up to 25 tonnes of wood chip at a time and may either be delivered using a walking floor or pneumatically. Some wood chip delivery companies are offering blown fuel delivery, allowing less accessible stores to be reached. For wood chips blown delivery is slow compared to tipping, and this will add to the cost of fuel delivery.
Unless wood chips are reasonably dry, typically less than 30% moisture content, they may degrade in storage. Microbial activity can lead to piles of wood chips generating heat as they decompose. The loss of calorific value via decomposition has been measured at 1% per month. Under the right conditions fungal spores will develop in the chip pile and these spores will be released when the pile is disturbed; if inhaled they can cause an incurable disease commonly known as ‘farmers lung’. To avoid the formation of spores moist or wet wood chip should be stored for as short a period as possible, and should be used within 1 month of being chipped.
Biomass Feed Systems and Combustion
Fuel is usually fed into the boiler by either a rotary auger or a hydraulic ram-stoker. Auger feed mechanisms are the most common feed systems in use on automatic feed biomass boilers. Although generally made of steel, flexible plastic augers may be found on small pellet fired installations, and can minimise costs and weight.
Fuel from the extractor auger is dropped onto the top of a rotary valve which meters fuel onto the boiler feed auger below. The segmented design of the rotary valve enables it to provide a positive seal between the two augers every time it rotates, physically preventing the fire from travelling to the fuel silo.
When there is no longer a demand for heat from a biomass boiler, the boiler continues to produce heat for some time. Fuel on the grate will need to be burned off and, depending on the type of boiler, the fuel in the feed system may need to be emptied onto the grate and burned as well. The time taken for a boiler to stop producing hot water will vary between 15 minutes and 2 hours, and the heat produced during this period must be absorbed or dissipated. The usual practice is to store the heat in a large water-filled buffer vessel, sized to the specific boiler output and thermal mass.
Biomass Boiler Maintenance
Biomass boilers have greater maintenance requirements than fossil fueled boilers. The boiler manufacturer’s representative or boiler installer will usually carry out an annual maintenance which will include a full internal and external inspection of the boiler, the replacement of worn components (particularly grate components on moving grate boilers), lubrication and cleaning.
The main maintenance interventions required by the user at regular intervals are a weekly visual inspection, emptying of the ash bin, greasing of induced draught fan bearings and manual brushing of the flueways. If automatic flue cleaning is installed a significant reduction in boiler downtime and in maintenance time is possible, reducing manual flue cleaning from a weekly to a 6-monthly exercise. The cleaning system comprises a series of compressed air jets or nozzles installed on the ends of the flueways which are pulsed in succession at regular intervals to blow soot from the boiler. This happens automatically while the boiler is operation. The system requires a small compressed air supply in the boiler house.
Biomass Boiler Controls
A frequently encountered problem with biomass boiler installations to date is the low seasonal efficiency of biomass systems, with many users reporting values of as little as 50%. Recent work undertaken in the UK has identified oversized boilers, undersized buffer vessels and inadequate boiler control systems as commonly found, preventable reasons for low system efficiency.
Reference: Biomass heating: a guide to medium scale wood chip and wood pellet systems (PDF 12MB).