9.3 Solid Biomass For Heat And Power

Introduction

Solid biomass is one of the oldest energy sources used by humans. It includes materials of biological origin that are burned or otherwise converted to provide heat and, in many cases, to generate electricity or combined heat and power. In modern energy systems, solid biomass can act as a renewable alternative to coal or fuel oil, but its sustainability and environmental performance depend strongly on how it is sourced and used.

This chapter focuses on how solid biomass is used specifically for heat and power, how these systems work in practice, and what distinguishes traditional from modern applications in this area. Broader definitions of biomass and other bioenergy routes are addressed in their respective chapters.

What Counts As Solid Biomass For Heat And Power

Solid biomass for heat and power usually refers to plant-based materials and some organic residues that can be handled, stored, and burned in solid form. Common examples are wood logs, wood chips, pellets, agricultural residues such as straw or husks, and solid fractions of organic waste that have been processed into fuels.

The same material can be used in very different ways. A wood log burned in an open fire for cooking provides basic heat with low efficiency. The same wood, processed into pellets and burned in a modern boiler, can provide clean and efficient space heating for a building or fuel a combined heat and power plant. The specific form of the fuel, its moisture content, and how uniform it is make a large difference to performance.

Key Properties Of Solid Biomass Fuels

For heat and power applications, several fuel properties are particularly important. Moisture content affects how easily the fuel burns and how much useful energy it provides. Fresh wood can contain more than 50 percent water by weight, which reduces the net energy released during combustion because a part of the energy is used to evaporate water. Properly dried firewood or industrial wood chips have significantly lower moisture content, which leads to higher efficiency and cleaner combustion.

Energy content is commonly expressed as the lower heating value or higher heating value, in units such as megajoules per kilogram. Solid biomass typically has a lower energy density than fossil fuels on a mass and volume basis. This means that more fuel volume is needed to deliver the same amount of energy, which affects storage and transport requirements.

Ash content and composition matter for boiler design and maintenance. Agricultural residues often have higher ash content and different inorganic components compared to wood. These can lead to slagging, fouling, or corrosion in combustion systems if not managed properly. Uniform particle size, especially for pellets, improves feeding and combustion control, and is one reason why pelletization is widely used as a fuel upgrade step.

Solid Biomass For Heat

Heat production from solid biomass can be grouped into three main scales. At the household scale, solid biomass is used for cooking and space heating. In traditional systems like open fires or simple stoves, efficiency can be very low and emissions very high. Modern stoves and residential biomass boilers, especially those designed for pellets, can reach much higher efficiencies and significantly reduce indoor air pollution, as long as fuel quality and maintenance are adequate.

At the building and commercial scale, biomass boilers are used to provide hot water, space heating, and sometimes steam. For instance, schools, hospitals, or apartment buildings can be heated by a central biomass boiler that replaces or supplements oil or gas. Here, the design focuses on automatic fuel feeding, consistent combustion conditions, and good flue gas treatment to meet emission standards.

At the district or industrial scale, biomass district heating networks or industrial boilers provide heat or steam for multiple users or for industrial processes. In a district heating system, hot water or steam heated by solid biomass is circulated through insulated pipes to buildings in a neighborhood or town. Industrial plants, such as food processing or paper mills, may use their own residues as fuel, which reduces fuel costs and waste disposal needs.

Because biomass is often locally available, using solid biomass for heat can reduce dependence on imported fossil fuels and support local economies. However, local air quality regulations, fuel logistics, and seasonal heat demand patterns have strong influence on where such systems are appropriate.

Using Solid Biomass In Power And CHP Plants

Solid biomass is also used to generate electricity, either in biomass-only power plants or in plants that combine heat and power generation. In a dedicated biomass power plant, solid biomass is burned in a boiler to produce hot flue gases that heat water into steam. The steam then expands through a turbine connected to a generator, producing electricity. After passing the turbine, lower pressure steam can be condensed back to water and recirculated, forming a closed steam cycle.

When both electricity and useful heat are produced, the system is usually called a combined heat and power plant, or CHP. Instead of condensing all the steam after the turbine, some of the heat is recovered and used for district heating or industrial processes. This can significantly increase the overall efficiency of fuel use, since more of the fuel’s energy is turned into useful products rather than being rejected as waste heat.

Biomass can also be co-fired with coal in existing coal power plants. In co-firing, a fraction of the coal is replaced by solid biomass such as wood pellets. This approach can reduce net greenhouse gas emissions compared to pure coal combustion and can make use of existing infrastructure, but the exact emission and sustainability performance depends on the biomass source and co-firing ratio. Adjustments to the fuel feeding system, boilers, and emission control equipment are often required.

Electricity generation from solid biomass generally has lower conversion efficiency compared to heat-only applications, because of limitations in thermodynamic cycles and the relatively low energy density and combustion temperature of biomass fuels. This is why biomass is often considered especially valuable for applications where both heat and power are used or where high temperature process heat is needed.

Thermal Conversion Pathways For Solid Biomass

Solid biomass for heat and power is mainly converted through thermal processes. Direct combustion is the most common. In combustion systems, biomass reacts with oxygen to release heat, water vapor, and carbon dioxide, along with small amounts of other gases and particulates that must be controlled.

Combustion systems for solid biomass use different combustion chamber designs, such as fixed bed grates, moving grates, or fluidized beds, to improve mixing of fuel and air and to manage ash removal. Fluidized bed boilers are particularly suited for mixed or lower quality fuels because they provide uniform temperature distribution and good combustion with different particle sizes and moisture levels.

Gasification is another thermal route that is relevant for heat and power. In gasification, solid biomass is converted, in a limited oxygen or oxygen free environment, into a combustible product gas that contains carbon monoxide, hydrogen, and methane. This producer gas can then be burned in a boiler or used in gas engines and turbines. Gasification systems can achieve more flexible and sometimes more efficient power generation, especially at smaller scales, but they are more complex and sensitive to fuel quality and gas cleaning requirements.

Pyrolysis, where biomass is heated in the absence of oxygen, primarily produces a liquid bio oil and a solid char, plus gas. Although pyrolysis is often discussed in the context of fuels and negative emissions, its direct use for heat and power is less widespread than combustion. However, the char and gas fractions from pyrolysis can be used as fuels, and there is growing interest in integrated systems where these streams are recovered for energy.

Efficiency Considerations For Solid Biomass Systems

The overall effectiveness of solid biomass use depends strongly on system efficiency. For heat-only systems such as modern biomass boilers, thermal efficiency, defined as the useful heat output divided by the energy content of the fuel, can be relatively high. For electricity generation, electrical efficiency, the ratio of electric output to fuel energy input, is lower than in large fossil fuel plants.

A simple way to express efficiency in such systems is:

$$ \eta = \frac{\text{useful energy output}}{\text{energy input in fuel}} $$

where $\eta$ is the efficiency, a number between 0 and 1 or between 0 and 100 percent. For combined heat and power systems, it is common to distinguish between electrical efficiency, heat efficiency, and total efficiency, which includes both electricity and useful heat.

Other factors that influence efficiency include proper sizing of boilers and turbines relative to demand, maintaining optimal combustion conditions, regular removal of ash deposits, and using fuels with appropriate moisture content. Slow response and partial load operation can reduce efficiency, so system design and integration with heat demand are crucial.

Fuel Preparation, Handling, And Logistics

Practical use of solid biomass for heat and power requires careful attention to fuel preparation and logistics. Raw biomass often needs to be dried, chipped, ground, or pelletized before it is suitable for combustion in modern equipment. Drying reduces moisture and increases net energy content. Chipping or grinding reduces particle size to make feeding and combustion more uniform. Pelletization compresses fine biomass into dense, regular pellets, which have advantages in storage, handling, and combustion control.

For large installations like industrial boilers or CHP plants, a continuous supply of fuel is needed throughout the operating period. This introduces logistical challenges such as securing reliable feedstock contracts, managing seasonal variations in biomass availability, and providing adequate storage capacity to handle fluctuations in supply and demand. Bulk transport by truck, train, or ship may be required, particularly for international trade in wood pellets.

At smaller scales, such as residential or commercial buildings, fuel handling is also a design concern. Automatic feeding systems for pellets rely on screw conveyors or vacuum systems, while manual handling of logs or chips can place limits on convenience and usage patterns. Storage must protect biomass from moisture, pests, and fire risks.

Because biomass has lower energy density than fossil fuels, transport over long distances can be relatively costly and energy intensive. This is one reason why solid biomass systems are often organized around local or regional supply chains, especially where residues from forestry or agriculture are available nearby.

Air Emissions And Combustion Quality

Combustion of solid biomass produces air emissions such as particulate matter, nitrogen oxides, carbon monoxide, and volatile organic compounds. The exact composition and quantity of emissions depend on fuel characteristics, combustion technology, and maintenance practices. Traditional stoves and poorly operated boilers can emit large amounts of smoke and harmful pollutants, while modern devices with good combustion control and flue gas treatment can meet strict emission limits.

Combustion quality is crucial. Sufficient air supply, correct mixing of fuel and air, appropriate combustion temperature, and adequate residence time of gases in the combustion zone all contribute to more complete burning of fuel. This reduces emissions of unburned hydrocarbons and carbon monoxide. In larger plants, flue gas cleaning systems such as cyclones, electrostatic precipitators, or fabric filters are used to capture particulate matter before it is released to the atmosphere.

From a climate perspective, solid biomass is often considered renewable when it is sourced from well managed systems where regrowth of biomass balances the carbon dioxide released during combustion over an appropriate time frame. However, this climate performance is separate from local air quality impacts, which are immediate and can be severe if combustion is poorly controlled. Therefore, diffusion of modern, efficient biomass technologies is essential when replacing traditional uses.

Applications And Integration With Energy Systems

Solid biomass for heat and power can be integrated into energy systems in several characteristic ways. One common pattern is the use of biomass as a central fuel in district heating networks, particularly in regions with cold climates and access to forest resources. In such systems, solid biomass can gradually replace fossil fuels in existing networks or become the primary source for new systems, often in CHP configurations that supply both electricity and heat.

Another application is in industries that generate their own biomass residues, such as sawmills, pulp and paper mills, or sugar and food processing plants. These facilities can install boilers or CHP units that use their residues to cover internal heat and electricity needs and sometimes export surplus electricity to the grid. This reduces waste and improves self sufficiency.

At smaller scales, solid biomass technologies can complement or back up other renewable sources. For example, a biomass boiler may serve as a backup for a solar thermal system, providing heat during periods of low solar radiation. In regions with electric grids dominated by variable renewables like wind and solar, dispatchable biomass power plants or CHP units can contribute to system flexibility by providing controllable generation, within the limits of fuel availability and environmental constraints.

Because the design of these systems intersects with sustainability issues such as land use and air quality, careful planning and assessment are required before scaling up solid biomass use. The specific role of solid biomass in a given region depends on available feedstocks, alternative renewable options, existing infrastructure, and regulatory frameworks.

Conclusion

Solid biomass remains an important source of renewable heat and power, particularly where local resources and existing industrial or district heating infrastructure can be leveraged. Modern technologies and practices offer substantial improvements in efficiency and air emissions compared to traditional uses, especially when coupled with fuel preparation and appropriate combustion and flue gas treatment systems.

At the same time, successful use of solid biomass for heat and power requires attention to fuel properties, system design, logistics, and emission control. When these elements are aligned, solid biomass can provide reliable energy services and support the transition away from fossil fuels, especially in sectors and regions where other renewable options are technically or economically less suitable.