1. INTRODUCTION
Combustion refers to the reaction of a material with oxygen in the air to release heat and light, which converts the chemical energy of the material into thermal energy (Tillman et al., 1981). Fuel is, therefore, required for combustion, and it is generally easier to combust liquids than solids, and it is easier to burn gases than liquids. For combustion, the temperature must be raised above the ignition point, and an ignition point is the temperature at which a fire is caused by heat without a flame. The certain amount of oxygen is needed for combustion.
When a substance is burned, a new one is produced. Fossil fuels such as petroleum, coal, and natural gas are converted to carbon dioxide and water vapor through combustion (Tillman et al., 1981). When sulfur, nitrogen, and phosphorus are contained in the combustion substance, sulfur oxide (SOx), nitrogen oxide (NOx), and phosphorus pentoxide (P2O5) are generated as combustion byproducts. When wood burns, it mainly produces water vapor and carbon dioxide. Besides, carbon monoxide (CO), NOx and SOx, hydrocarbons (HC), volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAH), and particulate matter (PM), are generated. Wood pellets are fuels for heat production or sustainable energy fuels with great potential as a substitute for electricity (REN, 2016). Thus, when wood pellets are burned, substances such as VOCs, PAH and PM, similar to when ordinary woods are burned, occur. Excessive exposure to these substances can cause cancer along with severe cardiopulmonary damage. In this regard, there are reports in Korea that cite data that are not fully verified, which causes controversy (Kim, 2017). This is because studies on wood pellets in Korea focus mainly on fuel characteristics (Kim et al., 2015) or boiler combustion performance, but there is little information on substances emitted from wood pellet combustion.
Some existing wood combustion systems bring about incomplete combustion due to combustion under inappropriate conditions, and therefore, CO, HC and soot are generated. However, biomass fuels such as wood pellets, whose production has increased significantly in recent years, have resulted in optimized combustion that can be controlled, reducing the incidence of substances incompletely combusted, and they have advantages such as cleanliness, dryness, and ease of injection suitable for being used as fuel. To use these fuels safely without affecting the human body negatively, however, specific information on various substances generated from the combustion of various biomass raw materials, regulatory standards and evaluation methods should be prepared. This article explains the types of emissions from wood combustion, how they are generated and the degree of harmful influence on the human body, and a comparison between domestic and overseas allowable standards regarding the safety of wood pellets, the allowable amount of emissions caused by combustion and so on was conducted.
2. POSSIBILITY OF EMISSIONS FROM WOOD PELLET COMBUSTION AFFECTING HUMAN BODY ADVERSELY
CO is a colorless, odorless gas which is mainly produced by oxygen deficiency or incomplete combustion and is contained in combustion gas of coal briquettes or exhaust gas of automobiles. Theoretically, when the excess air ratio for complete combustion is adjusted to 14.8% or more, no CO is generated (McDonald, 2009). CO itself is not toxic, but it binds to hemoglobin in the blood within the lungs, thereby interfering with the ability to supply oxygen into the body, and therefore, it causes oxygen deficiency in tissue cells, eventually resulting in addiction symptoms. When the inhalation of CO results in insufficient oxygen supply, the central nervous system (brain, spine), which is sensitive to oxygen deficiency, is affected. This can lead to headaches, dizziness, tinnitus, heart palpitations, increased pulse, and vomiting. In the worst case, CO poisoning may cause anesthesia. Especially if carbon monoxide-hemoglobin concentration is more than 60%, people lost their consciousness, and if this condition persists for a long time, people may die because oxygen supply discontinues.
NOx is produced by the reaction of oxygen and nitrogen at high temperatures, and in the case that its concentration is high, it causes oxygen deficiency, hypofunction of the central nervous system. It is also a major cause of optical smog. Being emissions that occur when fuels containing sulfur are burned, SOx occurs mainly in forms of SO2 (sulfur dioxide) and SO3 (sulfur trioxide). On the other hand, it is known that SO2 is mainly generated in exhaust gas, and when wood is burned, its trace amounts are generated (Obernberger et al., 2006). NOx and SOx act as precursors of ultrafine dust that is secondly generated, and therefore, they are regarded as harmful compounds to be noted.
HC refers to organic compounds composed of carbon and hydrogen and exists in natural gas, petroleum, natural rubber, terpenes contained in plants, etc. (Boman et al., 2005). HC is generated mainly by incomplete combustion of fuel, and it has the least amount around the excess air ratio for complete combustion in a combustor, but when the excess air ratio for complete combustion is exceeded, the dispersion of the flame is stopped, resulting in incomplete combustion and increasing the amount generated. Among the HCs, alkenes, olefinic HCs or saturated aliphatic HCs are the causative agents of photochemical smog since they react with ozone and NOx in the atmosphere. On the other hand, it is known that when the concentration of HC increases, it stimulates the mucous membrane and destroys the cell tissue, and it is reported that it is deposited on the surface of the particles and penetrates into lungs along with heavy metals.
VOCs are a generic term for liquid or gaseous organic compounds that evaporate readily into the atmosphere. It is a substance that causes a photochemical reaction in the atmosphere, or generates photochemical oxidizing substances such as ozone, thereby causing photochemical smog (US-EPA, 2017). VOCs cause air pollution and are carcinogenic, and since it is one of the sources of global warming, every country controls it politically to reduce their emissions. There are a variety of VOCs ranging from solvents used in industries to organic gases emitted from chemical and pharmaceutical plants or plastic drying processes. Liquid fuels with low boiling points, paraffin, olefins, and aromatic compounds are the commonly used VOCs in daily life (Perzon, 2010). In Korea, substances and products such as benzene, acetylene, and gasoline are currently regulated under the Clean Air Conservation Act (ME, 2017a). The emission sources of VOCs include natural sites such as soil, wetlands, plants and trees, and grasslands. In addition, VOCs are also artificially discharged in such locations as facilities using organic solvents, paint facilities, laundries, storing places for oil, gas stations, and exhaust gases from various transportation means. Globally, most of the emissions are from the facilities using organic solvents and such mobile sources of pollution as automobiles. As these VOCs have a great impact on the environment and the human body, most countries are making policy efforts to reduce emissions (ME, 2018).
A compound having a condensed ring in which two or more rings each have two or more atoms shared is referred to as a polycyclic compound, and a carbocyclic compound having a benzene nucleus is referred to as an aromatic compound. Examples of the substances belonging to PAH include aromatic hydrocarbons such as naphthalene and anthracene, and organic compounds including derivatives thereof. These PAHs, as persistent pollutants, do not decompose naturally when released into the environment and exist for a long time in atmospheric, soil or aquatic environments (Riva et al., 2011). PAH, as carcinogenic or mutagenic substances, is known to have adverse effects when ingested by humans.
Particulate matter (PM) is a fine substance in a solid or liquid state which is generated by mechanical treatment such as crushing and sorting of materials or combustion, synthesis, and the like. The size is larger than a single molecule with a diameter of 0.0002 μm and smaller than 500 μm. On the other hand, a PM having a size of 0.001-100 μm is called an aerosol, and it is generally classified into PM1, PM2.5, and PM10 depending on its size. These PMs stay in the atmosphere for several seconds to several months, while particles smaller than 0.1 μm in diameter undergo free Brownian motion in the atmosphere and collide with each molecule. It is divided into primary and secondary particles depending on the generation process. The primary particles are emitted directly from the source to the atmosphere, such as sea salt, soils, and combustion particles (Kim et al., 2011). Secondary particles are the ones formed by the photochemical and thermochemical reactions of gaseous materials released into the atmosphere from the source, and their constituents are mainly sulfate, nitrate, ammonium salts and organic substances. Depending on their size, source and shape, large and small particles that are present in the atmosphere are classified into particles, aerosols, dusts, droplets, fly ash, fog, fumes, mist, smoke, smog, and soot (Burkhard & Russel, 2009).
The sizes of particles present in the atmosphere greatly affects the properties of the emissions, and when they are present in the atmosphere, fine dust particles can enter the lungs and respiratory system of the human body and cause catastrophic damage as well as visibility obstruction. Also, when they accumulate on the leaves of plants, they block the sunlight and interfere with photosynthesis, which has a bad influence on growth, and when harmful chemical components are deposited on plants, they cause indirect damages to animals feeding on these plants.
Boman et al. (2005) conducted a literature review of the various emissions that occur when wood is burned and reported the results of the toxic effects of emissions.
In general, PM10 with a diameter of less than 10 μm was found to affect asthma, cardiopulmonary disease, and so on. In addition, Boman et al. have challenged the assumption that wood burning in houses is a major cause of PM and that the impact of PM in the area contaminated by wood burning emissions on the human body is not significant compared with other regions. However, these results were attributed to the use of firewood stoves.
After comparing the amount of emissions according to the combustion conditions of the firewood stoves, various types of emissions were detected, and the amount of emissions varied according to species (birch, conifer) and water content. Large amounts of incomplete combustibles occurred in firewood stoves combusting conifers with high water content, but it was possible to control the emission level by applying appropriate technological operating methods. There was room for further improvement through additional development and optimization. Thus, using high quality solid biofuels in pellet stoves with adjustable combustion conditions offers many advantages over using firewood stoves, which are conventional combustion devices.
A comparison of PM emissions with a firewood stove or a pellet stove indicated that the pellet stove was effective in reducing PM emissions, and their emissions were about 37-160 mg/MJ and 15-46 mg/MJ, respectively. The size of PM generated in the pellet stove was mostly 1 μm, but the PM in the firewood stove was found to vary in size. On the other hand, in a large capacity pellet stove, a small amount of PM was generated, and the sizes of PM were also almost the same, and the size and the shape of the pellets affected the size and distribution of PM. Finally, a comparison of the amount of emissions harmful to the human body to fuel input in a pellet stove showed that the emission factor of CO and PAH (emissions relative to fuel consumption) was higher under low fuel conditions. The results indicate that, under properly controlled combustion conditions, high quality wood pellet combustion offers advantages such as improved thermal efficiency and reduced emissions compared to conventional wood burning such as firewood.
3. COMPARISON OF DOMESTIC AND OVERSEAS ATMOSPHERIC ENVIRONMENTAL STANDARDS RELATED TO WOOD PELLET COMBUSTION
Domestic regulations related to emissions from combustion of wood pellets include the Clean Air Conservation Act (ME, 2017a). For the emissions from large combustion plants of a certain scale or more, the current Clean Air Conservation Act regulates 1) among biomass and wood pellet manufacturing facilities, the ones with a fuel consumption of 30 kg or more per hour, a volume of 3m3 or more, or a power of 3hp (20hp for crushing facilities) or more, 2) screening, drying and heating, crushing, compression and molding facilities, 3) among facilities using biomass and wood pellets, the ones with fuel consumption of more than 200kg per hour, but facilities that burn other fuels with wood pellets are excluded (ME, 2017a).
In North America, the emission of wood pellets is not only regulated, but also the total combustion emissions are regulated (Bäfver et al., 2011). For example, the US Environmental Protection Agency (EPA) has strictly regulated emissions through the National Ambient Air Quality Status (US-EPA, 2017) by setting the types and allowable emissions of combustion emissions. Canada, on the other hand, manages emissions from wood pellets through the Environmental Management Act established by the Ministry of Environment (BC-ME, 2011).
Korea's atmospheric environmental standards are little or no different from most European and Japanese standards. On the other hand, it is higher than North American and Asian standards (Villeneuve et al., 2012). Next, after comparing the emission allowance standards of domestic and overseas wood pellet manufacturing and use facilities, we found that Korea set and managed the standards related to CO, NOx, and PM. On the other hand, in the United States, PM is regulated based on the visible distance of PM, and Canada has also set and managed standards of PM emissions from drying, heating, and use facilities. These have been found to be very low compared to Korean standards (Pa et al., 2011; Pa et al., 2013). It was also found that the limits for CO and NOx in Europe and North America were based on general atmosphere environmental standards. However, atmosphere environmental standards related to the combustion emissions of wood pellets have been found to be affected by wood pellet combustors rather than wood pellets themselves.
4. COMPARISON OF EMISSION ALLOWANCE STANDARDS FOR WOOD PELLET BURNERS
Regulations for wood pellet boilers for houses are being implemented both domestically and internationally as rules relating to the combustion emissions of wood pellets. In Korea, KSB 8901 manages CO, NOx, and soot emissions from wood pellet boilers of 58 kW or less (KS, 2017). In addition, the Korea Forest Service has been implementing a pellet boiler supply project for residential use (KFS, 2018). In addition, as described earlier, the industrial scale pellet burners are managed by applying regulated values for CO, NOx, and PM.
In the United States, there are no allowance standards for emissions from biomass combustors (Sjoding et al., 2013). However, the top 25% of US biomass combustors were measured in terms of combustor capacity and emission amount. CO emission amount was in the order of stoves, hot water stoves, boilers and combined boilers, and PM emission amount was also in the order of stoves, combined boilers/boilers and hot water stoves (Burkhard and Russel, 2009). There were no significant differences in the NOx emissions among the combined boilers, hot water stoves, boilers, and the NOx emissions are the lowest in the stove. On the other hand, the emission amount was examined according to the types of biomass fuels. As a result, firewood combustors were the highest, and there was no difference between wood chips and wood pellets. According to the results, considerable amounts of CO and PM are generated in stoves among combustors using wood pellets as fuels.
Europe has set the standards for emissions from pellet boilers (EN 303-5) to minimize the possibility of air pollution from emissions from pellet combustion (ECS, 2012). Standards are set for fuel injection method (manual and automatic) and capacity, respectively, and according to the results of the measurement, grades ranging from 1 to 3 were set. Grade 3 means the best quality pellet boiler. Before these standards were set, each country in Europe used its pellet boiler specifications and standards to reduce air pollution and improve the quality of pellet boilers. For example, the UK has minimized emissions of CO, organic gaseous compounds, and PM (BSI, 2007) through the allowance standards of emissions of pellet boilers. According to these standards, boilers are divided into five grades ranging from 1 to 5, and grade 5 means the highest-grade pellet boiler.
We compared each country's standards for emissions from solid fuels and pellet boilers, including emission limits for pellet boilers. In Korea, standards for CO, NOx, PM, etc. exist as items for the permissible amount of each substance discharged from pellet boilers. On the other hand, in the EU countries such as Germany and Scandinavian countries, where wood is used as a fuel, organic gaseous compounds are included in the standards (Olsson et al., 2003; SSCI, 2010; Schmidl et al., 2011).
Next, we compared the domestic and overseas allowance standards for PM. In Korea, a 24-hour emission amount of PM10 is measured, but in Japan, a strict control is exercised on combustion materials by setting the emission limits per hour. On the other hand, North America, Australia and the World Health Organization have set environmental standards in which up to PM2.5 is allowed and considered environmental hazards and human health risks caused by microparticles (US-EPA, 2012). Korea is also continuing to work on the emission standards and measurement regulations in consideration of such overseas situations (Choi et al., 2006; Hong et al., 2011), planning to include PM2.5 emissions in conjunction with PM10 in the emission allowance standards through various research and data collection.
Lastly, according to the PM measurement method of each country examined to date, the amount of TSP (total suspended particle) in the emissions from flues is measured according to the standard methods for the measurements of air pollution in Korea (ME, 2017b). The US is conducting an analysis of PM (PM10, PM2.5) in the same way as the Korean method based on the EPA 201A standards (US-EPA, 2017). In Europe, PM is measured using the dilution tunnel method (NS 3058-2) in Norway, the electrostatic dust filter method (BS 3841-1) in the UK and the heated filter method (DIN+) in Germany. Residential solid fuel burning appliances - Emission test methods (DD CEN / TS 15883: 2009) have been determined and are now in use (Qiu, 2013).
5. CONCLUSION
The combustion of wood-based fuels releases substances such as CO, NOx, SOx, HCs, VOCs, PAHs, and PMs in addition to water vapor and carbon dioxide. These emissions are hazardous to the human body and the environment, and through overseas case studies, PM has been identified as a major cause of asthma and cardiopulmonary disease. However, it has been found that the degree of PM emissions can be controlled by combusting the dried fuel, such as wood pellets, using appropriate techniques.
In Korea, the safety of the emissions of wood pellet combustors for residential and industrial use is set by the KS and the Clean Air Conservation Act. In Europe and North America, not only the emissions from wood pellets but also the overall combustion emissions are controlled. Regarding the emissions from wood pellet combustion, subjects of the current Clean Air Conservation Act is no different from or slightly lower than European and Japanese standards and higher than North American and Asian standards. Germany, Scandinavian countries and Japan, which use wood as a main fuel, are managing the combustion substances by strictly setting the allowance standards.
