Wood-chip heating systems are very similar to fossil-fuel heating systems in many ways. However, organisations who treat them as the same will encounter difficulties. Oil, liquefied petroleum gas and gas are uniform fuels. Wood chip is not. The two main variables are moisture content and chip size. Calorific value usually has a linear relationship with moisture content. Freshly felled trees have a moisture content of 55-60%. Logs will dry at a rate dependent on the dimensions of the stack and air circulation. Two examples of poor locations for log drying we have seen are in the forest and in an old quarry. Logs stored correctly will typically reach 25-30% moisture content (%MC) after nine months of air drying. Chip size is governed by the chipping process. The main factor is the size of the screen on the outlet of the chipper. This can be changed to produce larger/smaller chip. Sharpness of cutting knives in the chipper also contributes to the quality of chip produced. While wood chip is not a uniform fuel, careful management of the log drying and chipping process is necessary in order to provide a fuel that is as uniform as possible. Wood-chip boilers are set up to burn a specific moisture content wood chip at a specific size. This is referred to as a recipe. These settings can be changed if required. Wood chip and maintenance contracts can be combined in a heat contract, wherein the fuel and maintenance provider is paid to provide kWh of heat, measured on a heat meter after the boiler. This incentivises the service provider to provide the correct fuel for the boiler and to maintain such that boiler efficiency is maximised. Where wood chip and boiler maintenance are not combined, it can arise that the incorrect specification wood chip is delivered. This may be perfectly good wood chip, but not that required by the boiler in question. More fundamentally, poorly produced wood chip may be too wet (or too dry, but this is very rare) and either too big or too small for any normal application. Common issues caused by poor or incorrect specification wood chip are:

  • Black smoke;
  • Ash clinker in the grate;
  • Loss of power (boiler not producing rated output);
  • Dust particles/sparks out of flue.

Wood chip versus oil

Wood-chip heating systems require regular observation and maintenance. Wood chip is a bulky fuel. A typical calorific value of 3,500kWh/tonne and a bulk density of 250kg/m3 lead to an energy density of 875kWh/m3. Oil, for comparison, has a calorific value of 10kWh/L or and energy density of 10,000kWh/m3. So oil is about 11 times more energy dense than wood chip. It is clear, therefore, that storage volume and rates of wood chip transfer are key considerations. The requirement to feed wood chip into a boiler at 11 times the rate of the equivalent oil boiler means that arguably the hardest working element of a wood-chip heating system is the wood-chip handling equipment. The next hardest working element is the boiler itself. The production of up to 2% wood ash is a key difference from oil or gas boilers. The most important objective of maintenance must be to keep the boiler free of ash. The boiler grate and various ash augers must be monitored closely. Clearly, ash bins need to be emptied periodically. Regular maintenance by trained, experienced professionals is required. A four- to six-week cycle is ideal. Common issues caused by poor maintenance are:
  • Grate clogged with ash;
  • Inlet air holes clogged up causing incomplete combustion;
  • Heat exchanger clogged up causing flue gas temp rise and system inefficiencies;
  • Failure of components going unnoticed and reducing boiler efficiency.
Design of wood-chip heating systems benefits greatly by taking feedback from the fuel supplier and the maintenance provider on a project by project basis. Where these three functions are not incentivised to work together, the end user may be left with a poorly operating heating system and a lack of clarity of the cause. Common issues caused by poor design are:
  • Inadequately sized wood-chip stores;
  • Poor access to key components within wood-chip store or boiler house for maintenance;
  • Wood-chip store that constrains wood-chip delivery (wood-chip unit cost will vary with size and speed of delivery).

Clearpower Forest to Heat System

Clear power offers bespoke design of biomass heating systems on a case-by-case basis, taking account of site and customer-specific requirements and applying best practice. The designs are carried out by degree-qualified engineers. With regard to procurement and project management, established relationships with key suppliers ensure value for money and short lead times. We prepare works specifications to issue to civil engineers and maintenance and engineering subcontractors. We apply our experience of project management to avoid any pitfalls of biomass heating systems. Our technical services team have over thirty years of experience between them of installing, commissioning and maintaining biomass heating systems. They have received installation, commissioning and maintenance training from biomass equipment manufacturers, Safepass, confined-spaces entry and risk-assessment training. In addition, one is a qualified electrician and the other a qualified plumber. We purchase raw material/pulpwood from a combination of public and privately owned sustainable forestry. Pulpwood is delivered from the forest to any one of our five depots around the country, where it is seasoned to the required moisture content. The logs (usually 3m in length) are stacked rows up to 6m high, exposed to the wind to reduce the %MC. Typically, the %MC will reduce from 55-60% to 25-30% over a nine-month period. When the logs reach the required %MC, they are chipped in the field by mobile chipper and transported to a storage shed. Chip is then delivered directly to the contracted end user via tractor and trailer or walking floor lorry. Clearpower has been certified by the Wood Fuel Quality Assurance (WFQA) scheme for the production of air-dried wood chip. We sell to end users as fuel (tonnes) or as metered heat (kWh). The former means the end user is responsible for boiler efficiency. The latter means Clearpower assumes responsibility for system performance (labour included, spare parts excluded). The rate for heat depends on the size of installation and heat output. We can also offer a fully funded ESCO solution with no capital outlay by the end user. We offer various levels of maintenance package to our clients. This can be as little as an annual visit to reinforce the on-site maintenance function up to an all-inclusive offering where we do monthly planned maintenance and all emergency maintenance required. All maintenance is carried out by our in house team. Many of the boilers we have installed have the facility to login remotely to their control system. This permits us to identify, diagnose and sometimes resolve problems without visiting site. Out-of- hours support is included with all maintenance contracts. To easily demonstrate our ‘Forest to Heat’ system, we commissioned the construction of a model showing the process from forest through to heat end users – including logging, seasoning, chipping, delivery and combustion at end user sites. This has proven very useful when describing our offering to new customers. Author: Ciaran Miller, design manager, Clearpower Ltd. Contact: info@clearpower.ie, (01) 462 5000 or 087 4153159. Clearpower was established in 2002. We have installed over 100 commercial-scale biomass heating and CCHP systems with a total thermal output of almost 50,000kW. End users include hotels, hospitals, public-sector organisations, leisure centres, schools and district heating systems. In 2016, we delivered 15,000 tonnes of biomass (wood chip and pellets). There are two manufacturers of wood pellets on the island of Ireland. We purchase from both and sell direct to end users. Clearpower Registrations, Certifications & Affiliations
  • Accredited to ISO9001, ISO14001 and ISO18001;
  • Wood Fuel Quality Assurance (and EN17225-4) certified for the production and supply of air-dried wood chip;
  • OFTEC registered for service and commissioning of oil boiler systems;
  • RGI registered gas installer;
  • Corporate partner member of Engineers Ireland.