Table of Contents
Introduction
Bioenergy is often described as carbon neutral, but in reality its effects on air quality and climate depend strongly on what is burned, how it is converted to energy, and where and at what scale it is used. This chapter focuses on the specific emissions from biomass and biofuels, how they influence local and regional air quality, and what can be done to reduce negative impacts while keeping the climate benefits that sustainable bioenergy can offer.
Types Of Emissions From Bioenergy
When biomass is burned or converted into fuels, several substances can be released into the air. These emissions fall into two broad categories. Some affect climate over years to centuries, and some primarily affect human health and ecosystems over hours to days through air pollution. Bioenergy produces both kinds, although in different proportions depending on the technology.
At the simplest level, complete combustion of pure, dry biomass would produce carbon dioxide and water. In real systems, combustion is never perfectly complete, and biomass contains nitrogen, sulfur, minerals, and trace elements. As a result, practical bioenergy systems can emit fine particles, carbon monoxide, unburned organic gases, nitrogen oxides, and other pollutants. Understanding these substances is essential to compare bioenergy fairly with fossil fuels and to design cleaner systems.
Carbon Dioxide And Carbon Neutrality
Biomass releases carbon dioxide, or CO₂, when it is burned or converted into biogas or liquid biofuels and then used. This CO₂ is part of the natural carbon cycle. Plants absorbed it from the atmosphere as they grew. If the same amount of biomass grows back and is managed sustainably, the CO₂ released can be taken up again over time.
However, the timing and completeness of this cycle matter. If forests are cleared and not regrown, or if soil carbon is lost, bioenergy can lead to a net increase in CO₂ in the atmosphere. There is also a delay between emission and regrowth, known as a carbon payback period. During that period, atmospheric CO₂ can be higher than it would have been without the bioenergy use. In addition, harvesting, processing, and transporting biomass uses energy, which can add fossil CO₂ emissions if not powered by renewables.
From an air quality perspective, CO₂ itself is not directly harmful at the concentrations typically found outdoors, but it is the main long lived greenhouse gas from combustion. For bioenergy, the main air quality issues are linked to other pollutants, even when the CO₂ balance is favorable.
Particulate Matter And Smoke
The most visible emissions from many biomass systems are smoke and soot. The technical term for these is particulate matter, which includes tiny solid and liquid particles suspended in the air. Two size ranges are especially important. PM₁₀ refers to particles with a diameter up to 10 micrometers, which can enter the upper respiratory tract. PM₂.₅ refers to particles up to 2.5 micrometers, which can penetrate deep into the lungs and even enter the bloodstream.
Incomplete combustion of wood, crop residues, dung, and other solid biomass can produce large amounts of PM₂.₅ and PM₁₀. Open fires and simple traditional stoves, particularly for cooking and heating, are major sources of fine particles indoors and outdoors in many parts of the world. Modern biomass boilers and power plants operate at higher temperatures with controlled air supply and can include filters that capture most particles, leading to much lower emissions per unit of energy.
Fine particulate matter from biomass smoke contains organic carbon, black carbon, ash, and sometimes trace metals depending on the feedstock. These particles reduce visibility and contribute to haze. They are linked to respiratory and cardiovascular diseases, premature deaths, and can aggravate asthma and other chronic conditions.
Fine particulate matter, especially PM₂.₅ from biomass smoke, is one of the most harmful air pollutants for human health and must be minimized through better technologies and practices.
Black Carbon And Other Short Lived Climate Pollutants
Black carbon is the dark component of soot produced when solid fuels do not burn completely. It is part of PM₂.₅ and has both air quality and climate implications. As a strong absorber of sunlight, black carbon warms the atmosphere and can accelerate melting when it settles on snow and ice.
Traditional biomass use for cooking and heating, agricultural residue burning in fields, and poorly controlled small boilers are important sources of black carbon in many regions. Modern, well designed combustion systems with sufficient oxygen supply and high temperature can greatly reduce black carbon formation. Filters such as electrostatic precipitators or fabric filters can capture most remaining particles in larger installations.
Other short lived climate pollutants related to bioenergy can include organic carbon aerosols and some volatile organic compounds that participate in ozone formation. These pollutants stay in the atmosphere for days to weeks, which means that reducing them can have a relatively rapid effect on both climate forcing and local air quality.
Gaseous Pollutants From Biomass Combustion
Beyond particles, biomass combustion can emit several gases that affect air quality. The most common of these are carbon monoxide, nitrogen oxides, and volatile organic compounds. Their levels depend heavily on combustion conditions.
Carbon monoxide, or CO, forms when combustion is incomplete, that is, when there is not enough oxygen or the temperature is too low. High CO levels are especially dangerous indoors and can be deadly in enclosed spaces. Traditional biomass stoves, poorly vented heaters, and malfunctioning boilers are typical sources. Improved stove designs and proper maintenance reduce CO emissions by promoting more complete burning.
Nitrogen oxides, often written as NOₓ, include nitric oxide (NO) and nitrogen dioxide (NO₂). They form when nitrogen in the air or in the fuel reacts at high temperatures. NOₓ contributes to ground level ozone and smog formation and can irritate the lungs. Large biomass power plants and district heating boilers can emit NOₓ, but combustion control and flue gas treatment technologies, such as low NOₓ burners and selective catalytic reduction, can reduce these emissions significantly.
Volatile organic compounds, or VOCs, are unburned or partially burned hydrocarbons that evaporate into the air. They include a wide variety of gases and vapors. In combination with NOₓ and sunlight, VOCs contribute to the formation of ground level ozone, which is harmful to both plants and humans. Incomplete biomass combustion, especially at low temperatures, can emit many VOCs, including irritants and carcinogens.
Indoor Air Pollution From Traditional Biomass Use
One of the most serious air quality problems linked to biomass is indoor air pollution from traditional cooking and heating. In many low and middle income countries, households burn wood, charcoal, crop residues, or animal dung in simple stoves or open fires inside homes or semi enclosed spaces. These systems often lack chimneys or adequate ventilation, so smoke accumulates indoors.
As a result, people inhale high concentrations of PM₂.₅, CO, VOCs, and other pollutants for many hours each day. Women, children, older people, and those who spend more time near the fire are especially exposed. Health impacts include respiratory infections in children, chronic lung diseases, heart disease, stroke, eye irritation, and increased risk of certain cancers. Indoor air pollution from solid fuels is one of the leading environmental health risks worldwide.
Efforts to reduce these hazards focus on access to cleaner fuels, such as biogas, ethanol, or electricity, and on improved biomass stoves that burn more efficiently and vent smoke outside. These measures can drastically lower pollutant concentrations inside homes, even when biomass continues to be used. It is important that improved technologies are accepted by users, affordable, and properly maintained to deliver real benefits.
Outdoor Air Quality And Biomass Burning
Biomass use can also affect outdoor air quality. In rural areas, smoke from household use, brick kilns, small industries, and open burning of agricultural residues can combine to create regional pollution episodes, especially under stagnant weather conditions. In urban areas, small biomass boilers, charcoal use, and some food industries can add to pollution from traffic and other sources.
Outdoor emissions from large scale biomass facilities, such as power plants or district heating systems, are usually concentrated at higher stacks and can be regulated and controlled more effectively. With proper design, they can release much less pollution per unit of energy than many small, scattered, unregulated sources.
Uncontrolled open burning of crop residues and forest biomass, whether intentional or accidental, is particularly damaging for outdoor air quality. Such fires emit large amounts of smoke, PM₂.₅, black carbon, and ozone forming gases. They can affect cities hundreds of kilometers away and contribute to regional haze and cross boundary pollution.
Biofuels And Tailpipe Emissions
Liquid biofuels, such as bioethanol and biodiesel, are used in transport, often blended with gasoline or diesel. Their combustion in engines produces exhaust emissions similar in type to those from fossil fuels, but the amounts and composition can differ.
In general, high quality bioethanol blends can lead to lower carbon monoxide and some hydrocarbon emissions compared with pure gasoline, because ethanol contains oxygen in its molecule and can promote more complete combustion. However, some blends may change the pattern of VOC emissions and can affect evaporative emissions from fuel systems.
Biodiesel blends in diesel engines often result in lower particulate mass and lower emissions of some toxic hydrocarbons compared with pure diesel. At the same time, they can increase nitrogen oxide emissions slightly in some engine types. Modern engines with advanced controls and after treatment, such as particulate filters and selective catalytic reduction, can manage these emissions for both fossil diesel and biodiesel blends.
From an air quality perspective, the overall benefit of biofuels depends on fuel quality standards, engine technology, and the presence of emission control systems. Poor quality fuels, or use in older uncontrolled engines, can still produce significant pollution even if the fuel is renewable.
Biogas, Biomethane, And Combustion Quality
Biogas is produced by anaerobic digestion of organic materials and contains mostly methane and CO₂, along with smaller amounts of other gases such as hydrogen sulfide. When purified to biomethane, it can have similar properties to natural gas and can be injected into gas grids or used in gas engines.
Combustion of clean biogas or biomethane in well designed burners and engines usually leads to low particulate emissions, because these gases do not contain solid particles. However, if hydrogen sulfide and other contaminants are not removed, they can lead to corrosion and formation of sulfur oxides, which affect air quality and equipment lifetime.
Compared with traditional solid biomass use, biogas used in improved stoves or gas burners can greatly reduce indoor and outdoor air pollution. Achieving this benefit requires proper gas cleaning, leak prevention, and safe handling to avoid methane losses and safety risks. In transport applications, dedicated gas engines with appropriate controls can manage NOₓ emissions and keep air pollution low.
Factors Affecting Emission Levels
Many factors influence how much pollution a bioenergy system emits. The type and quality of biomass feedstock is important. Dry, well seasoned wood burns more cleanly than wet, freshly cut wood. Pellets produced under controlled conditions have more predictable combustion behavior than loose residues. High ash or high mineral content fuels can increase particulate emissions and cause operational problems.
Combustion technology and design also play a crucial role. Simple three stone fires or unvented stoves have very poor combustion efficiency and release large amounts of smoke. Improved cookstoves, gasifiers, modern automated biomass boilers, and combined heat and power plants achieve higher combustion temperatures, better mixing of air and fuel, and more complete burning, which leads to lower emissions.
Operation and maintenance are equally important. Even a well designed system can emit excess pollution if it is poorly operated or not maintained. Incorrect fuel feeding, blocked air inlets, dirty heat exchanger surfaces, or missing filters can all increase emissions. User training and regular inspection help keep systems clean and efficient.
Finally, emission control technologies can significantly reduce pollutant releases from larger installations. Cyclones, electrostatic precipitators, and fabric filters capture particulates. Flue gas desulfurization reduces sulfur oxides. Catalytic systems can cut NOₓ and some organic emissions. These systems add cost and complexity, but they are common in modern industrial and district scale biomass plants in regions with strict air quality regulations.
Comparing Bioenergy With Fossil Fuels On Air Quality
From a climate point of view, sustainable bioenergy can replace fossil fuels and reduce net CO₂ emissions. From an air quality point of view, the comparison is more nuanced. Traditional solid biomass use without controls can be much dirtier than fossil fuels, especially in terms of PM₂.₅, CO, and VOCs. In contrast, modern, well controlled biomass combustion can have air pollutant emissions similar to or lower than some fossil fuel systems.
For example, a modern pellet boiler with filters can have particulate emissions far below those of an old coal stove. Biogas used in clean cookstoves can provide air quality benefits comparable to liquefied petroleum gas. On the other hand, open burning of crop residues is generally worse for air quality than using the same residues in a well controlled power plant.
Bioenergy only delivers air quality benefits compared with fossil fuels when it uses clean technologies, appropriate fuels, and effective emission controls. Traditional, uncontrolled biomass burning is a major source of harmful air pollution.
Strategies To Reduce Emissions From Bioenergy
Several strategies can align bioenergy use with air quality goals. The first is to shift from open fires and simple stoves to cleaner cooking and heating solutions. These include improved biomass stoves, pellet stoves, biogas systems, and, where possible, electricity or other clean fuels. This shift directly reduces exposure for households.
Another strategy is to favor centralized, well controlled biomass facilities over many small, uncontrolled sources, especially in densely populated areas. Larger plants can invest in advanced combustion technologies and pollution control equipment and can be located and managed to limit exposure to emissions.
Improving feedstock quality, such as using dry, standardized fuels like pellets, also helps. Avoiding the burning of contaminated biomass, waste with plastics or treated wood, and improperly stored fuel prevents the release of additional toxic substances and heavy metals.
Regulations and standards for emissions from biomass installations, fuel quality, and stove performance can drive the adoption of cleaner technologies. Monitoring, enforcement, and access to finance for cleaner equipment are important for these measures to work in practice. Public awareness of health risks and benefits can encourage households and communities to demand and adopt cleaner options.
Balancing Climate Benefits And Air Quality
Bioenergy policies often focus on greenhouse gas balances and renewable energy targets, but air quality must be considered at the same time. In some cases, a bioenergy option might reduce fossil CO₂ emissions but worsen local air pollution, particularly if it relies on low quality combustion in populated areas. In other cases, carefully designed bioenergy systems can provide both climate benefits and cleaner air, especially when they replace coal or traditional biomass burning.
Decision makers need to consider local conditions, including existing pollution levels, population density, and availability of clean technologies. Life cycle thinking helps to capture both climate and air quality impacts across production, conversion, and end use stages. Integrating air quality goals into renewable energy planning can lead to choices that support both public health and climate action, rather than trading one against the other.