Biomass

The good of wood

Solid biomass is the oldest source and the most utilised form of renewable energy in Europe. Most common examples of solid biomass are wood fuels such as woodchips, wood pellets and split logs. Solid biomass is burned in a boiler or stove to release its stored energy and converted into heat for hot water or space heating purposes. Modern biomass combustion units have much higher efficiencies than traditional fireplaces and stoves.

Solid biomass is suitable for use in remote areas that are not connected to the gas grid, where it can be deployed as a lower cost alternative to oil. Biomass is particularly attractive in rural areas where the biomass is readily available and alternative fossil fuels are expensive. Although the price of solid biomass fuel varies considerably, it is often cheaper than other heating fuels where biomass is regionally available.

Although solid biomass could potentially be termed “carbon neutral”, some greenhouse gas emissions (GHG) emissions will always occur during its cultivation, production and distribution. The more local the biomass is sourced and used, the lower the greenhouse gas emissions are.

The use of biomass for combustion emits local air pollutants, just like burning other solid fuels such as coal. The last decade has seen an improvement in the quality of residential and institutional heating units, many with efficiencies close to 90%. These units produce significantly less air pollution. In areas with much solid fuel combustion, local air quality however may become an issue and should be regulated appropriately.

Solid biomass is a form of biomass that does not compete with food, unless arable land is being converted to cultivating the biomass. Sustainability safeguards need to be in place to limit any potential negative impacts of using solid biomass. In Europe, stricter sustainability criteria are currently being defined by the European Commission.

Biomass boilers can readily replace oil and gas boilers in an existing heating system. Boilers are available in a range of sizes: from small domestic systems to large commercial systems that supply heat to multiple properties (via a district heat network) and/or industrial processes

General Info

What is it?

Solid biomass is by far the most utilised form of renewable energy deployed in the world (including Europe). It is commonly used to generate heat and increasingly also electricity in combined heat and power (CHP) systems.

There is a wide variety of solid biomass energy resources, including dedicated forestry, energy crops (such as short rotation coppice), forestry and agricultural residues, processing residues (such as offcuts and sawdust from the sawmill industry) and also wastes (such as waste wood). This factsheet focuses on the most common solid biomass fuel types that include wood chips, wood pellets and logs for heating purposes.

What are the benefits?

Low carbon and renewable: biomass is a low carbon fuel. The amount of CO2 that is released during the combustion of biomass is equal to the amount of CO2 that is sequestered from the atmosphere during its lifetime (see more in the section environmental impact below). It is an abundant and renewable fuel.

Regional availability: biomass has a large potential in areas where biomass is widely available.

Cost effective: solid biomass is a cost-effective renewable energy source. The costs for biomass are largely dependent on their availability and the distance over which the biomass must be transported. In areas with high shares of regionally available biomass is a good cost-competitive alternative to conventional (fossil) energy sources.

How does it work?

Solid biomass is burned in a boiler (or stove) to release its stored energy and converted into heat for hot water or space heating purposes.

Boilers are available in a range of sizes from small domestic systems (typically under 30kWth) to large commercial systems (over 5MWth) that supply heat to multiple properties (via a district heat network) and/or industrial processes. Modern systems often have partly or fully automated furnaces and boilers with electronically regulated biomass feeding and firing systems.

Before the biomass can be converted into useful heat, it is often pre-treated to facilitate its use. For example, wood is typically either chipped to make wood chips, or compressed into wood pellets (or briquettes). Wood pellets have a higher energy density compared to both wood chips and logs making them very suitable when the biomass needs to be transported over larger distances.

Suitability/applicability

Solid biomass is suitable for use in remote areas that are not connected to the gas grid, where it can be deployed as a lower cost alternative to oil. Biomass is particularly attractive in rural areas where the fuel is readily available and alternative fossil fuels are expensive. At larger scale and larger distances from biomass source, the costs for biomass will increase. Although the price of solid biomass fuel varies considerably, it is often cheaper than other heating fuels where biomass is regionally available.

Solid biomass can be used in a wide variety of applications. It is particularly suitable for use in remote areas that are not connected to the gas grid, where it can be deployed as a lower cost alternative to oil.

The availability of solid biomass and the distance from the source to the location of end-use is an important factor in its applicability. As solid biomass has a low bulk density, large transport distances can have a negative impact on its cost and also on its environmental performance (particularly if the biomass is transported by road).

Biomass boilers can readily replace oil and gas boilers in an existing heating system. However, a key consideration is that sufficient space is available for deliveries and also to store the biomass to keep it dry. The design of the storage system will depend on the fuel selected; logs can be kept in a simple shed, chips in a storage bay and pellets in an enclosed hopper. Access is needed for biomass deliveries and some means is needed to convey the fuel to the boiler on demand. A further consideration is that wood boilers are typically larger than their oil and gas equivalents.

The type of application has a bearing on the suitability of the fuel type. For example, pellet boilers are more suitable for the domestic market as they are easier to use compared to other wood fuel types, require around a third of the space of logs or chips (due to their higher energy density) and also burn cleaner (less ash). Wood chip boilers do not work effectively below 50kWth and are more suitable in larger buildings, community heating systems and industry. Most wood chip and pellet boilers now use automatic fuel feed-in systems, while log-burning boilers have to be filled by hand. An advantage of logs is that can often be cheaper than pellets or chips, if sourced locally.

Detailed Info

Costs, Savings, Earnings

The cost of solid biomass depends on a number of factors. These include the fuel availability, whether the fuel is pre-treated (i.e. logs are generally the cheapest wood fuel, while wood pellets will be most expensive), the quality of the fuel, the moisture content, and, more than conventional fuels, on the transport distance. Wood fuel can potentially be a ‘free’ resource if the end-user has access to its own local wood supply.

Biomass is particularly attractive in areas where the fuel is readily available and alternative fuels, such as oil, are expensive. At larger scale and larger distances from biomass source, the costs for biomass will increase.

Although the price of solid biomass fuel varies considerably, it is often cheaper than other fuels for heating where biomass is regionally available.

Environmental Impacts

The amount of CO2 that is released into the atmosphere after combusting the biomass is equal to the amount of CO2 that has been absorbed from the atmosphere during its lifetime. Although solid biomass could potentially be termed “carbon neutral”, some greenhouse gas (GHG) emissions will always occur during its cultivation, production and distribution. The more local the biomass is sourced from, the lower the distribution emissions generally are, although the mode of transport used also has a significant bearing on the overall GHG emissions (transport by sea is the most efficient mode, and transport by road the least). Producing wood pellets is an energy intensive process and can therefore have an impact on the overall GHG emissions of solid biomass.

The use of biomass for combustion emits (local) air pollutants, just like burning other solid fuels such as coal. Burning organic material emits particulate matter (PM), nitrogen oxides (NOx), carbon monoxide (CO), sulfur dioxide (SO2), lead, mercury, and other air pollutants. The last decade has seen an improvement in the quality of residential and institutional heating units, many with efficiencies over 90%, much higher than traditional fireplaces and wood stoves. Modern efficient biomass combustion units that burn wood pellets or wood chips, produce less particulate emissions and in particular, less products of incomplete combustion emissions, than traditional fireplaces and stoves. EU law does not set direct regulatory requirements for small and average size residential and institutional units (<50MW). However, air quality limit values established by the Air Quality Framework Directive1 have to be complied with by Member States. It is up to the Member State how this is delivered. In areas with much solid fuel combustion, local air quality may become an issue and should be appropriately regulated by (local) authorities.

Solid biomass is a form of biomass that does not compete with food, unless arable land is being converted to cultivating the biomass. Sustainability safeguards need to be in place to limit any potential negative impacts of using solid biomass. These include not sourcing wood from highly bio diverse woods and forests and other protected areas, and also to ensure that sustainable forestry management (SFM) best-practises are implemented in-line with local/national regulations. SFM best-practises include limits on the harvest rate and requirements to replant trees following harvest, as well as other aspects such as the preservation of soil quality. Stricter sustainability criteria are currently being defined by the European Commission.

Efficiency

The efficiency depends on the type of conversion technology that is used to convert it into heat and/or electricity.

Currently, much of the solid biomass used in Europe is burned in low-efficient stoves or fire places where a large part of the heat is lost through the chimney. The energy released when burning solid

biomass in modern heating systems is utilised very efficient – generally up to 90%.

The moisture content of solid biomass fuel affects the amount of energy available for conversion to useful heat and therefore has a direct effect on the efficiency. Wood pellets are a dry fuel (generally between 6% and 10% moisture content), while logs and wood chips are typically utilised between 20% and 35% moisture content.

Using fuel at the correct moisture content is a key factor in the efficient operation of a boiler. If the moisture content of the fuel is too high it can cause a boiler to shut down, while if fuel is too dry it will burn too quickly.

Commercial Maturity

Solid biomass is a commercial and mature fuel that is widely deployed in Europe, particularly for the supply of heat. The availability of biomass and the distance from the source to the location of use is an important factor that determines the eventual price to a large extent.

Solid biomass conversion technologies are also technically mature. Much improvement have been made in the areas of small-scale firing systems and intelligent control and management technologies, as well as in the development of convenient furnace feeding systems, achieving relatively high levels of efficiency and greatly reducing emissions.

Level of Maintenance

The level of maintenance is depending on the type of technology used to convert it into heat and/or power.

For example, modern biomass boilers burning even the most difficult materials can operate unattended, but regular inspection (weekly or bi-weekly) is required to carry out a visual inspection of the boiler and fuel feed system, to check the lubrication of bearings and to empty left-over ashes. Some boilers have a mechanism for compressing the ash. This reduces the number of times the ash bin needs emptying. If the boiler is not fitted with automatic flue cleaning, regular cleaning of the flue pipe and chimney is required (2 times per annum is advised).

Technical Details

Below is an overview of the typical specifications of wood fuels for a number of parameters.

Wood pellets

Bulk density: 600 kg/m3

Particle size (diameter): 6mm to 8mm for domestic and small scale systems, for larger systems

Moisture content: <10%

Heating value (NCV at 10% mc): 17 GJ/t

Wood chips

Bulk density: 250 kg/m3

Particle size (diameter): 20-50 mm Moisture content: <35%

Heating value (NCV at 30% mc): 12.5 GJ/t

Logs

Bulk density: 350-500 kg/m3 Particle size: >100mm

Moisture content: 20% (if air dried) Heating value (NCV at 20% mc): 14.7 GJ/t

Regional variations

There are significant differences between the availability among European countries and therefore is biomass in some regions more interesting than in other countries.

Key European markets for solid biomass include Austria, Denmark, Germany and Scandinavia.

Trade associations

European Photovoltaic Industry Association

www.epia.org