There is an enormous variety of wood boilers on the market. Most are fully automated and have sophisticated controls, however in principle there two basic choices;
An underfed hearth system
Underfed hearth systems tend to be smaller and less expensive. They can only use wood fuel up to 35% moisture content as there is no system of pre-drying prior to combustion. For most systems under 500kW it is usually more cost effective to select underfed hearth boilers.
A moving grate system
Moving grate designs shunt wood fuel along the combustion chamber and allow it to be dried prior to combustion. This means the boilers are larger and more expensive. However this allows the use of ‘wet’ fuels of up to 55% moisture content in some cases.
General Operation Overview
A typical wood chip energy system comprises a boiler, accumulator tank, controls and a fuel store containing some sort of mechanical device to move the chips to the feed auger, and usually a stirrer (left). The chips are then conveyed to the boiler by the auger completely automatically.
Automated wood chip boilers are generally only available in sizes from 25kW and above (50kW wood boiler below). Many systems install a heat meter on the output side of the boiler. This measures the flow and return of hot water and provides a read out of the output of the boiler in kWhs. This allows the user to purchase wood fuel on a kWh basis. This is usually known as a heat supply contract. These contracts provide a simple and clear way for users to purchase wood fuel and avoid the need for complicated measuring of the fuel and its moisture content.
Fuel suppliers will often own moisture meters that allow them to test their fuel and ensure it has reached the required moisture content. These devices can be simple probes that test the moisture content of round wood, or more complex electronic buckets that automatically measure the moisture content of a small load of wood chips. Independent fuel testing for moisture content, particle size and contaminants is now generally available in Ireland.
Modern wood boilers are provided with automatic ignition systems and electronic controls that enable the boilers to automatically feed wood fuel from a silo via augers or ram stokers. Fuel feed systems are generally designed and supplied as part of the boiler. Most boilers come equipped with automatic de-ashing systems that discharge all generated ash into moveable bins. Most boiler suppliers will provide linked fuel silo equipment or advice on how fuel storage and fuel feed mechanisms can be designed to operate with their boilers.
Flue gas re-circulation helps control emissions and along with electronic controls for combustion settings and fans the boilers will usually operate at over 90% efficiencies. Most boilers are operated from a sophisticated control panel and these are often installed with modem so that remote real time monitoring and control can be provided--by the user at their PC or by the supplier from their factory in another Country. Being fully controllable and entirely automatic, using programmable timers and zone thermostats, wood boilers can be easily incorporated into building energy management systems.
Wood boilers are almost always now used in conjunction with a heat store tank or ‘accumulator tank’ which helps to provide fast response and reduces fuel use at times of low demand.
The modern generation of wood boilers date from the late 1970s and have evolved steadily since then. Many of earliest examples are still in operation and the robust equipment should have a 20 year plus life if properly maintained. Typically Eastern European boilers are lower in cost and tend to be less automated, whereas Austrian and German boilers are more costly and automated.
Wood chip boilers are already installed in Clare County Council HQ, Olympus GMBH, Cahercalla Community Hospital and Torpey Wood Products. Further installations are in the pipeline for 2008. These wood heat systems are extremely reliable, technically robust and easy to maintain.
Proposed national exemptions for renewable technology will remove the requirement for planning permission in many cases.