Biomass Heating Systems Explained

Modern, efficient biomass heating systems are common in individual buildings and district heating schemes throughout Europe and Scandinavia. European and UK standards are in place for the design of biomass boilers and the specification of the fuel used.ETA Cut Away

Modern biomass boilers are computer controlled for optimum efficiency and are easy to maintain and operate. Wood chip, wood pellets and other fuels are automatically fed into the boiler on demand.

Biomass boilers tend to be physically larger than oil or gas boilers; they require more space and need to be located in a position that is easily accessible to fuel delivery vehicles. The quality boilers supplied by Asgard Biomass have maintenance and operating procedures similar to oil or gas boilers.

Cut Away of ETA boiler showing primary and secondary burning chambers, ash removal augers and automatic heat exchanger cleaning mechanism.

Key Elements

Biomass boilers consist of a number of key elements: Fuel storage, fuel feed mechanism, grate or hearth and burning chambers, heat exchanger, and flue.

Fuel 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 is to tip the chip or pellet directly from a tipper truck or trailer into a subterranean bunker. Others 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. When designing an installation the location and operation of the biomass boiler must be fully considered to avoid unnecessary additional costs in the future.

Fuel feed systems

Wood Chip

Biomass fuels such as wood chip are not free flowing materials and have a tendency to bridge, and so storage bunkers, or silos, are fitted with an agitator system. Small systems use rotating sweeper arm devices that drag the material over an auger.

Rotating Agitator

Larger wood chip storage systems require moving floors that move the material to an auger placed at the end of the bunker.

Wood Pellet

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 attached to the boiler. Alternatively fuel is augered from a store or gravity fed from a silo  and then vacuumed to a day hopper attached to the boiler from which the fuel is then augered through safety devices to the primary burning chamber.

Grate (Hearth)

Comprises a short auger that feeds the fuel directly into the primary combustion zone. The type and design of the grate varies between individual manufactures.

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 in the secondary (gasification) zone.

All modern wood chip and wood pellet burners have provision for separate control of the primary and secondary air supply in order that combustion can be properly balanced.

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 low in sulphur fuel but has higher nitrogen oxide (NOx) emissions than gas or oil boilers due to the inherent high nitrogen content of the fuel. Boilers

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 over 90% for wood chip 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.

All wood chip and wood pellet boilers supplied by Asgard Biomass offer automatic tube cleaning as a standard feature. This is recommended for ensuring high efficiencies and low maintenance costs. 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.

Heat Exchanger Automatic Cleaning System

Maintenance 

Ash production in a biomass boiler burning wood chip or pellets is about 1.0% by weight 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 are available and reduce the amount of work required but the ash bin still requires removal and emptying periodically. The ash is an excellent 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 swept annually.

Motor and auger bearings need checking twice every year and greased according to the manufacturers specifications.

Safety

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.

ETA patented 7-chamber rotary valve burn back protection

Common Biomass Boiler Features

All biomass boilers share a common set of features as defined in the following sections.

Turndown

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.

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.

Automatic Ignition

Most boilers can be fitted with automatic ignition. Wood chip 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).

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.