Comparison of emissions from smokeless coal combustion in a household heating boiler used in Central Europe

Vol.:(0123456789) 1 3 International Journal of Environmental Science and Technology https://doi.org/10.1007/s13762-021-03649-9 ORIGINAL PAPER Comparison ofemissions fromsmokeless coal combustion inahousehold heating boiler used inCentral Europe P.Knigawka1· P.Pianko‑Oprych1 · K.Krpec2· L.Kuboňová2 Received: 21 April 2021 / Revised: 16 August 2021 / Accepted: 3 September 2021 © The Author(s) 2021 Abstract The objective of this work was to evaluate the relationship between the fuel quality and the gaseous and particulate pollutant emissions generated from a hot-water boiler during the combustion of different types of innovative processed fuels: smokeless coal, smokeless briquettes 1–3, smokeless pellets and unprocessed hard coal. The aim of our research was to prove the presumption that smokeless processed coals produce less gaseous and particulate emissions. By using modern fuels in already used and manufactured older boilers, there is a possibility to significantly reduce emissions of organic gaseous compounds (OGC) and polycyclic aromatic hydrocarbons (PAHs). The emission situation in the heating season can be significantly improved even without costly production, and thus consumption of natural resources and energy, and installation of modern boilers. Physical and chemical characterization of solid-fuel samples, including determination of moisture content, ash, volatile organic content, calorific value and elemental composition analysis, were performed. Fuels were burned in one type of hot-water boiler class 1 according to EN 303-5 to determine the impact of applied fuel types on pollutant emissions. The pollutant emissions were characterized by the contents of gaseous components: nitrogen oxides NOx, sulfur dioxide SO2, carbon monoxide CO, carbon dioxide CO2, organic gaseous compounds OGC and particle components: total suspended particles TSP, particulate matter less than 2.5µm and 10µm (PM2.5 and PM10, respectively) and polycyclic aromatic hydrocarbons PAHs in both phases. The emission factors from six types of fuel were compared with applicable European standards. The lowest NOx content was observed for smokeless briquette 1, while the lowest SO2 content was observed for smokeless pellets. The emission of CO was at a similarly low level of 200g/kg for smokeless briquette 1, smokeless briquette 2 and hard coal. Gaseous and pollutant emissions described by PM2.5 and TSP were observed to be the lowest for smokeless coal, smokeless briquette 1 and smokeless briquette 2. Keywords Boiler· Coal· Smokeless· Combustion· Emission· Particulate· Polycyclic aromatic hydrocarbons Introduction The last census in the Czech Republic in 2011 showed that from a total of 3.6 million households, 9.2% were heated by coal, and its combustion decreased by 41% within 2001–2011 (Horak etal. 2017). The next census in the Czech Republic is in 2021. In Poland, 36.5% of the total of 14.4 million households in 2018 were heated by coal, and its combustion decreased by 14.5% within 2009–2018 (GUS 2020). The energy sector in Poland and the Czech Republic has been changing by acts and directives of the European Union. As reported by Manowska and Rybak (2018), the energy demand in Poland from hard and brown coals should decrease by 15% and 80%, respectively, by 2050 in comparison with 2010. This energy policy includes steps such as improving energy effectiveness, increasing energy security, developing renewable energy sources, developing competitive fuels and energy markets and reducing the impact on the environment. To decrease emissions from solid-fuel combustion in households and commercial buildings, many countries in Europe were actively encouraged to implement Editorial responsibility: Shahid Hussain. * P. Pianko-Oprych [email protected] 1 Department ofChemical Engineering andProcesses, Faculty ofTechnology andChemical Engineering, West Pomeranian University ofTechnology, Szczecin, al. Piastów 42, 71-065Szczecin, Poland 2 Energy Research Center, Technical University ofOstrava, 70800Ostrava,Poruba, CzechRepublic International Journal of Environmental Science and Technology 1 3 residential heating with wood and other types of biomass. Biomass is a renewable fuel of utmost promise. However, the process of biomass combustion is quite complicated and consists of devolatilization, gas-phase combustion of volatiles and char combustion (Chen etal. 2010), which cause pollutant emissions. Research efforts should be focused on the development of high-quality, innovative fuels to reduce the concentrations of atmospheric aerosols. Particularly during winter, pollutant emissions due to solid-fuel combustion for residential heating are much higher in smog episodes (Mikuška etal. 2015). These new types of low emission fuel can be safely used for energy purposes for the environment and public health in accordance with the best available technology (BAT) practices (Kubica etal. 2007). For this reason, data must be collected, and knowledge must be developed about emissions from standard combustion installations, in which innovative types of fuel are tested. As presented in the WHO document (WHO 2015. ISBN 978 92890 50760), the latest WHO indoor air quality guidelines strongly advise against the residential use of unprocessed or raw coal. This coal requires higher ignition and combustion temperatures and is a source of SO2 and NOx emissions due to its higher content of sulfur and nitrogen than biomass. Coals may contain toxic elements (such as fluorine, arsenic, selenium, mercury and lead). A research focus on the content and exposure of these pollutants and toxic contaminants from the use of clean or smokeless processed coal is recommended by the WHO. Emissions from the combustion of new solid smokeless coal fuels in residential heating units have rarely been published previously. Ściążko etal. (1993) showed that smokeless coal fuel exceeded the combustion efficiency by 20–30%, e.g., benzo[a]pyrene resulted in 90-fold smaller emission. Matuszek etal. (2016) found that emissions of CO, dust, organic pollutants, benzo[a]pyrene and PAHs in the flue gas from the combustion of smokeless fuel were much lower than those from the combustion of coal (nut assortment) and other wood waste materials. Smokeless coal has already been described by Lasek etal. (2019) and Matuszek etal. (2016) when a significant decrease in gaseous and particulate emissions was obtained even in outdated construction boilers. The measured average pollutant emissions from combustion of smokeless coal at 10% O2 were as follows: (1) CO 4.64 × 103mg/m3, (2) NOx 180mg/m3, (3) SOx 220mg/m3, (4) OGC 94mg/m3, (5) PM 86mg/m3, (6) benzo[a]pyrene 0.02mg/m3 and (7) 16 PAHs in total 0.45mg/m3 (Lasek etal. 2019) and (1) CO 4.78 × 103mg/ m3, (2) NOx 89mg/m3, (3) OGC 64mg/m3, (4) PM 62mg/ m3, (5) benzo[a]pyrene 0mg/m3 and (6) 16 PAHs in total 0.59mg/m3 (Matuszek etal. 2016). Compared to hard coal, the emission factors from the combustion of smokeless coal were on average sixfold lower for OGC, 8.5-fold lower for TSP, 23-fold lower for 16 PAHs and 25-fold lower for benzo[a]pyrene (Lasek etal. 2019). The advantages of smokeless coal are described online as well (Smokeless 2020). This type of low-carbon fuel has a high calorific value, less ash and lower gaseous and particulate emissions which are the advantages of smokeless coal combustion. The disadvantage could be the price of smokeless coal which is about 20–30% higher than of conventional hard coal. Currently, in Central and Eastern European countries, an extensive change of old combustion devices for the improvement of air pollutant emissions is underway. The replacement of old boilers by new boilers leads to air quality changes; however, the process of boiler replacement could take a long time. The use of smokeless fuels could lead to a rapid improvement in the local air quality during the first winter heating period after the fuel change. Recent studies (Křůmal etal. 2019, 2021) combusted the same type of boiler (overfire boiler) fuel, which is regularly used in practice for heating. Knowledge is lacking about gaseous and particle emissions for the combustion of fuels for which an overfire boiler is certified (such as coke). Our research aimed to present the results showing a decrease in pollutant emissions (CO, OGC, PAHs, TSP) from the combustion of fuels similar to coke (such as smokeless coal). Research efforts should focus on high quality, innovative fuels due to demands for pollutant emissions decrease. Application of new smokeless fuels in household boilers could increase the local air quality after the fuel change. It is important to show the data about pollutant emissions from innovative fuels. This paper compares the chemical composition of the emissions from combustion in a household boiler of five types of new smokeless coal fuel prepared by Polish producers. Smokeless solid fuels were obtained in the process of thermal hard coal devolatilization conducted in a dedicated furnace. The fine fractions of the smokeless coal were used to form briquettes and pellets. For comparison, the test was also performed for unprocessed hard coal, which was used as a reference in this study. The different shapes and forms of these types of fuels have not yet been compared and described. There is a potential to find the most cost effective and the most environmentally friendly coal fuel which could be processed and used in practice. The type of reactor is not novel as we applied the old-type boiler which is, however, widely used in households and is suitable for tested coal fuels similar to coke in their parameters. The emissions were characterized for the content of gaseous pollutants: NOx, SO2, CO, and OGC, as well as CO2, TSP, PM10, PM2.5 and PAHs. The obtained values were discussed and compared with data in European standards to verify the potential use of these fuels for heating. The research was conducted in the Department of Chemical Engineering and Processes, West Pomeranian University of Technology in Szczecin, Poland, and the Energy Research International Journal of Environmental Science and Technology 1 3 Centre, VSB-Technical University of Ostrava, Czech Republic, within the period of February–October 2019. Materials andmethods Experimental setup forthecombustion tests The same design of a hot-water overfire boiler was used for all combustion tests. An overfire boiler was used because smokeless coal fuels cannot be used in any other boiler type (automatic, gasification, downdraft boiler). The reason is that the tested fuels have very low contents of volatile matter (except for hard coal, which was used for the comparison), and the characteristics of smokeless coal fuels are similar to those of coke. A schematic diagram and photograph of the boiler are shown in Figure S1 in the supplementary file. The boiler consists of cast iron cells connected to form a combustion chamber, an ashpan and a double-pass exchanger heater. The boiler was connected to the water-cooling system. The fuel was placed in one large batch onto a solid grate. The boiler was equipped with primary and secondary combustion air supplies. The primary combustion air, FA1, was supplied through a water-cooled grate to the fuel layer and was automatically regulated by a thermostatic valve, while the secondary combustion air, FA2, was supplied in front of the first exchanger draft. The burning fuel, FG, was in direct contact with a heat exchanger, which formed the boiler walls. Cooling water was supplied to the heat exchanger, FW1, from which hot water, FW2, was discharged into the water-cooling system. According to the boiler manufacturer’s characteristics, the boiler was designed for coke, hard coal and wood burning in the conventional burn-through process. The boiler was produced in output from 27 to 32kW based on fuel and with 5 cells. The technical parameters of the boiler declared by the manufacturer are presented in TableS1 in the supplementary file. The scheme of the measuring apparatus is shown in Fig.1. The boiler was connected to the insulated chimney in accordance with EN 303-5:2012 (CEN 2012), which was used for testing the fuels. Flue gas removal was ensured by the dilution tunnel, and the dilution air was sucked from the laboratory. The temperatures of the input and output heating water were measured using a PT100 resistance sensor, while the flue gas temperatures behind the boiler and in the dilution tunnel were measured using a K-type thermocouple. The water flow was measured using a Krohne electromagnetic flowmeter. Characteristics ofthefuels Six types of solid fuels (Tables1, 2) consisting of standard and innovative smokeless fuels were examined in this study. Smokeless solid fuels were obtained in the process of hightemperature pyrolysis of noncaking coal in a grate furnace. Fig. 1 Scheme of measuring apparatus International Journal of Environmental Science and Technology 1 3 For the production of smokeless fuel, an installation for the production of carbon reductants is used. During the production process, smokeless fuel or carbon reductant is the main product, and heat is also generated, which is recovered in a heating boiler and sold as superheated steam to the neighboring chemical plant. The detailed method of coal modification cannot be introduced because of the manufacturer’s know-how protection. To compare the properties of fuels, briquetting and pelleting were used to prepare samples made of smokeless char fines. Both technologies of agglomeration are applicable for Polish manufacturers. The samples were named smokeless coal (SC), smokeless briquette 1 (SB1), smokeless briquette 2 (SB2), smokeless briquette 3 (SB3), smokeless pellets (SP) and unprocessed hard coal (HC). Smokeless briquettes 1, 2 and 3 differed among themselves in the content of one key component. Smokeless briquette 1 was modified by polymer—phenolic resin, smokeless briquette 2 was modified by starch and smokeless briquette 3 was modified by carboxymethylcellulose (CMC). The smokeless pellets contained starch. All types of fuel were air-dried on the metal plate in the testing laboratory prior to the measurement and analysis. Fuel analyses were performed in accordance with the following standards: (ISO 2006, 2009, 2010a,b,c, 2011, 2013). Different shapes and sizes of solid fuels can be observed in the pictures presented in Figure S2 in the supplementary file. The smallest fuel particle diameters start from 7mm for smokeless coal, while the largest diameters start from 90mm for hard coal. Smokeless briquettes had the most regular shape, while the fractions of smokeless coal and hard coal had the least regular shape. Smokeless coal had smaller grains and the additional grate had to be inserted for no falling down of smokeless coal to ash tray. The exception was smokeless coal briquettes. Gaseous andparticulate matter emission measurements Concentrations of the following components, CO, CO2, NO (the catalytic convertor changed NO2 to NO), O2, OGC, PAHs, PM10 and PM2.5, were measured during tests. A Table 1 Results of physicochemical analysis of six tested fuels (raw state) Unit SC SB1 SB2 SB3 SP HC Carbon % 87.6 79.3 78.5 76.9 79.9 68.0 Hydrogen % 1.06 1.67 1.90 1.63 2.14 4.33 Nitrogen % 1.45 1.36 1.41 1.31 1.43 1.69 Sulfur % 0.33 0.54 0.51 0.83 0.42 0.34 Oxygen % 1.60 3.61 5.90 4.75 6.18 10.1 Ash % 4.94 8.26 7.55 9.99 5.15 6.60 Total moisture % 2.99 5.29 4.19 4.56 4.78 9.03 Analytical moisture % 2.55 4.70 3.80 4.06 4.75 8.41 Volatile matter % 4.65 9.92 13.3 11.2 12.9 32.8 Fixed carbon % 87.42 76.53 74.96 74.25 77.17 51.57 Net calorific value MJ/kg 30.7 28.3 28.3 27.6 28.9 26.3 Gross calorific value MJ/kg 31.0 28.8 28.8 28.1 29.5 27.5 Table 2 Results of physicochemical analysis of six tested fuels (dry basis) Unit SC SB1 SB2 SB3 SP HC Carbon % 90.3 83.7 82.0 80.6 83.9 74.7 Hydrogen % 1.09 1.76 1.98 1.71 2.25 4.76 Nitrogen % 1.49 1.44 1.47 1.37 1.50 1.86 Sulfur % 0.34 0.57 0.53 0.87 0.44 0.37 Oxygen % 1.65 3.81 6.16 4.98 6.49 11.1 Ash % 5.09 8.72 7.88 10.5 5.41 7.26 Volatile matter % 4.79 10.5 13.8 11.8 13.6 36.1 Fixed carbon % 90.1 80.8 78.3 77.8 81.0 56.7 Net calorific value MJ/kg 31.8 30.0 29.7 29.1 30.5 29.2 Gross calorific value MJ/kg 32.0 30.4 30.1 29.4 31.0 30.2 International Journal of Environmental Science and Technology 1 3 nondispersive infrared (NDIR) analyzer was used to determine the concentrations of CO, CO2 and NO. A paramagnetic analyzer was applied to estimate the concentration of O2. The concentration of the organic gaseous compounds was measured by a flame ionization detector (FID). The gas sampling was continuous with a time interval of 1s. All of the analyzers used were calibrated prior to taking measurements using calibration gases. To estimate the PM (TSP), PM10 and PM2.5 distributions, flue gas samples were taken in the dilution tunnel at sampling point 2, as shown in Fig.1. Multistage impactors with ranges below PM2.5, PM2.5–PM10 and above PM10 were used for sampling. The impactor is shown in Figure S3 in the supplementary file. Flue gas samples were taken in the dilution tunnel by isokinetic sampling in the middle of the flue gas stream. Measurements of PAH concentrations were taken in an accredited laboratory by the GC-HRMS method in accordance with applicable standards and regulations (EPA 1997, 1999, 2012; ISO 2003) and with the help of the sampling system (filtration-condensation apparatus) shown in Figure S4 in the supplementary file. The average standard deviations of emission factors of gaseous pollutants were for CO2 (1.8%), CO (20.9%), NOx (33.3%), SO2 (29.8%), OGC (33.5%), particulate pollutants PM (31.2%) and organic compounds (39.0%) (Křůmal etal. 2019). Test conditions Combustion tests were performed in a hot-water overfire boiler. There was no reason to conduct the tests with different types of boilers because tested fuels (coke-type fuels) are intended only for overfire boilers. The boiler was already presented in “Experimental setup for the combustion tests” section. The boiler was operated according to the operating manual and the requirements of the standard EN 303-5:2012 (CEN 2012). A sufficient amount of the fuel containing beech wood and the chosen type of briquette was burned to create a suitable basic layer of glowing ash and to heat the boiler to its operating temperature of approximately 50°C. Boiler controls were set to achieve a nominal heat output. The combustion air intake was controlled automatically by a simple thermoregulator, which reacts to the water temperature in the boiler. After the preheating period, a fuel charge sufficient for the maximum filling height of the boiler was added into the combustion chamber. The differences in fuel loads were caused by different bulk densities and granulometries of the tested fuels. The combustion period was finished each time when the fuel layer reached the level of the basic layer of glowing ash before the combustion period. The test duration and heat output depended on the fuel masses added into the combustion chamber and the pressure loss of the fuels. Results anddiscussion Coal fuels composition The results of analyses performed in the laboratory for six solid-fuel samples are summarized in Table1 (raw state) and Table2 (dry basis). The lowest carbon content was found in the case of hard coal (68.0%), for which the hydrogen content of 4.33% was the highest. The nitrogen content in all solid fuels was at a comparable level. Significant differences in oxygen content were noted for the tested fuels. The lowest amount of oxygen was found for smokeless coal (1.60%), while the highest was found for hard coal (10.1%). Quite significant differences in the amount of sulfur were also found, and a particularly high level (0.83%) was noted for smokeless briquette 3, which also had the highest ash content (9.99%). As expected, the highest number of volatiles was measured for the hard coal. The content of volatiles in the case of hard coal was threefold greater than that in the other tested fuels. The moisture content was 2.99% for smokeless coal, approximately 4–5% for smokeless briquettes 1–3 and 9.03% for hard coal. In general, for comparison, wooden fuels are considered dry with a moisture content of 15%, while higher moisture contents represent wet fuel. The highest net calorific value was 30.7MJ/kg obtained for smokeless coal, while the lowest value was 26.3MJ/kg obtained for hard coal. The coal fuel parameters were compared with those of other similar fuels published previously—hard coal (Křůmal etal. 2021) and smokeless coals (Ścicčżko etal. 1993; Lasek etal. 2019) in Table3 (dry basis). The hard coal parameters of this study and Křůmal etal. (2021) show quite good matches. Smokeless coal analysis from the literature (Ścicčżko etal. 1993; Lasek etal. 2019) is limited because elemental analysis is missing. The calorific values of our smokeless coal (SC) are similar to those reported by (Lasek etal. 2019). Gaseous emissions Conducted combustion tests of six solid fuels in the overfire boiler provided extensive measurement data (also in TableS2 in the supplementary file). The general characteristics of the operational conditions of the combustion process are summarized in Table4. The collected measurement data allowed us to assess the boiler efficiency International Journal of Environmental Science and Technology 1 3 Table 3 Results of physicochemical analysis of published coal fuels (dry basis) HC hard coal, BC blue coal (smokeless coal), SB smokeless briquettes Ref Křůmal etal. (2021) Lasek etal. (2019) Ściążko etal. (1993) Unit HC HC1 HC2 BC1 BC2 SB Carbon % 75.19 – – – – – Hydrogen % 5.14 – – – – – Nitrogen % 1.46 – – – – – Sulfur % 0.62 – – – – – Oxygen % 9.59 – – – – – Ash % 8.00 4.2 2.9 4.0–6.4 4.1–5.1 13.7 Volatile matter % 37.36 32.7 32.2 3.94–10.54 3.32–8.53 8.3 Fixed carbon % 54.64 63.1 64.9 – – 78 Net calorific value MJ/kg 29.79 30.80 31.85 30.69–31.64 31.26–32.02 27.84 Gross calorific value MJ/kg 30.89 31.84 32.89 31.03–31.96 31.59–32.32 28.86 Table 4 Operational conditions of the combustion process for six tested fuels Unit SC SB1 SB 2 SB 3 SP HC Combustion period h 4.4 4.2 4.1 3.8 3.7 5.3 Atmospheric pressure mbar 985 988 991 990 979 989 Ambient air temperature °C 30.5 27.9 29.3 26.8 24.4 27.9 Ambient air relative humidity % 43.5 26.6 38.1 50.0 55.9 54.3 Heat output kW 20.2 22.5 21.2 19.8 17.0 15.2 Fuel consumption kg/h 3.87 4.24 4.69 4.03 3.84 2.69 Water flow temperature °C 58.8 60.4 60.4 58.0 54.9 52.1 Water return temperature °C 47.1 47.5 48.1 46.6 45.1 43.4 Flue gases temperature °C 244 313 295 282 222 230 O2 concentration in dry flue gases % 8.1 8.5 7.8 7.5 9.2 9.1 Air excess – 1.62 1.69 1.59 1.56 1.78 1.77 Fig. 2 Comparison of boiler thermal efficiency for six tested fuels. The loss through radiation, convection and conduction has been estimated in the laboratory based on their experience 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% SC SB1SB2 SB3SPHC 61.1 66.8 57.6 63.8 55.0 77.0 9.9 15.8 11.6 12.8 9 12.4 16.2 7.60 5.50 18.1 16.6 7.30 2.5 2.5 2.5 2.5 2.5 2.5 10.3 7.30 22.8 2.80 16.9 0.80 Loss through unburned fuel in ash Loss through radiaon, convecon, conducon Loss through incomplete combuson Loss through sensible heat of the products of combuson Boiler efficiency International Journal of Environmental Science and Technology 1 3 depending on the type of fuel used and to determine the characteristic concentrations of pollutants: CO2, CO, NOx, SO2, OGC and PAHs. The obtained results are presented in Fig.2 and Tables4 and 5. The longest combustion time was found when using hard coal (5.3h), while the shortest time was found when using smokeless pellets (3.7h). Notably, Table4 shows that the lowest fuel consumption and heat output were recognized for the hard coal, equal to 2.69kg/h and 15.2kW, respectively. The operational conditions of the combustion process for the six tested fuels are shown in Table4. The highest boiler efficiency of 77% was found for hard coal, which resulted from the lowest total losses (23%) achieved with this standard type of fuel. The highest total losses (45%) were recorded when smokeless pellets were used. The loss through sensible heat of the products of combustion is directly related to the amount of flue gas produced (oxygen excess) and the temperature of flue gases (the difference between the temperature of flue gases and the laboratory temperature). The higher the excess O2 and the temperature are, the higher the resulting loss will be. The loss through incomplete combustion depends on the amount of flue gas and the concentration of CO in the flue gas. The loss through radiation, convection and conduction depend on the size of the surface of the individual boiler walls and the difference between the boiler temperature surfaces and the laboratory temperature. The temperature of the boiler surfaces is given by the temperature of the water in the boiler and the quality of insulation used. The loss through unburned fuel in ash is an analysis of the mass of solid residues after combustion (ash) and the C content in these residues (combustible matter). The procedures referenced to the test standard EN 303-5:2012 were applied. The emissions of CO, CO2, OGC, NOx, SO2, TSP, PM10 and PM2.5 for all types of fuels are presented in Table5. The highest CO emission was found in the case of smokeless pellet combustion. Moreover, equally high emissions of CO were also noted for smokeless briquette 3 and smokeless coal. The lowest value of CO emission among smokeless fuels was recognized for smokeless briquette 2. Major factors influencing the complete combustion of carbon include the fuel reactivity, fuel fineness and particle size, fuel/air mixing efficiency, excess air available for complete combustion, residence time and temperature profile inside the boiler (Tawil 2021). No obvious correlation holds between flue gas temperatures and CO emission factors; however, by excluding HC from the data set, CO decreased with increasing flue gas temperature for smokeless coals (Figure S5 in the supplementary file). As reported in studies of biomass combustion (Vakkilainen 2017), combustion at a high furnace temperature and a long residence time decrease CO emissions. There is no direct correlation between fuel characteristics such as volatile matter and CO emissions factors (Figure S6 in the supplementary file). From the comparison of the emissions of NOx, SO2 and OGC, the worst result was obtained for unprocessed fuel—hard coal. The amount of NOx emission for smokeless briquettes 1–3, smokeless coal and smokeless pellets was twice as low as the emission from hard coal on average. The emission of SO2 was on average 25% lower for smokeless fuels except smokeless briquette 3 in comparison with that from hard coal. In general, the lower sulfur content in fuels tested during the combustion process also resulted in a lower SO2 emission and a lower SO2 concentration in exhaust gases. Similar conclusions were drawn by Křůmal etal. (2019), who showed that combusted coal also contained a much higher amount of sulfur, ash and nitrogen than, for example, biomass (wood or wood pellets) and caused higher emissions of sulfur dioxide for a given boiler type. Yang etal. and Zhao etal. (2016) proved that the SO2 and NOx emissions were closely related to Table 5 Emissions of CO, NOx, SO2, OGC, CO2, PM2.5, PM10 and TSP for the six tested fuels NOx is expressed as NO2 Unit SC SB1 SB2 SB3 SP HC CO g/kg 623 238 206 553 670 200 NOxg/kg 1.69 1.20 1.25 1.21 1.46 2.82 SO2g/kg 3.80 4.33 4.54 5.95 2.48 4.69 OGC g/kg 1.05 5.15 3.45 1.72 12.4 72.6 CO2g/kg 2.67 × 1032.63 × 1032.58 × 1032.19 × 1032.34 × 1032.31 × 103 PM2.5 g/kg 0.275 1.75 0.776 9.18 4.74 30.0 PM10 g/kg 0.351 1.88 1.07 9.71 5.42 31.1 TSP g/kg 0.427 2.18 1.48 10.0 6.19 31.6 International Journal of Environmental Science and Technology 1 3 the sulfur and nitrogen contents of the fuel. Both of these components remain in the boiler as ash or are released by the chimney as emission. The largest differences were noted in OGC emissions. The emission of OGC was 69-fold lower during smokeless coal combustion compared to hard coal combustion. Moreover, the OGC emissions were 42-, 21-, 14and sixfold lower during combustion of smokeless briquette 3, smokeless briquette 2, smokeless briquette 1 and smokeless pellets, respectively, compared to hard coal combustion. PM emissions The dominant source of PM2.5 emissions in the EU-27 from small sources (the domestic sector) is the combustion of solid biomass, which contributed nearly three quarters of the total emissions, while the share of hard coal was approximately 25%. Sixty-nine percent of PM2.5 emissions came from residential stoves, followed by single-family house boilers (16%) and fireplaces (11%) (Cofala and Klimont 2012). Therefore, the determination of PM10 and PM2.5 contents was important for the six solid fuels chosen in this study. The values of PM10 and PM2.5 were over 100-fold lower for smokeless coal, 35-fold lower for smokeless briquette 2, 18-fold lower for smokeless briquette 1 and sixfold lower for smokeless pellets than for hard coal combustion. Only threefold lower values of PM10 and PM2.5 were obtained during smokeless briquette 3 combustion. Considering the current legislation in the EU, PM2.5 emissions from small sources must decrease by more than 20% until 2020, by 40% by 2030 and by 50% by 2050. Smokeless briquettes 1 and 2, as well as smokeless coal and smokeless pellets, meet these conditions. Dust in flue gases can be removed by electrostatic precipitators from small-scale combustion devices. Filters and catalysts are not suitable for the adsorption of dust as underburned products, as they would be clogged after a short time. Electrostatic precipitators are now being used in small-scale combustion devices, especially in automatic boilers, but not in manually batched boilers. For our tested boiler (overfire), the price of the electrostatic precipitator would include a notable part of the boiler price. Gaseous and particle emissions were studied previously, e.g., by Křůmal etal. (2019). Herein, wood biomass (dry and wet) combusted in the overfire boiler had lower values than coal in the studied parameters (such as SO2, CO, OGC and PM). However, OGC and TSP emissions are higher from combustion of biomass than from combustion of our smokeless coal samples. Sixteen EPA PAHs were described in the study of Křůmal etal., but benzo[a]pyrene emissions were also higher from biomass combustion than from smokeless coal combustion. The gaseous and particle emissions of hard Table 6 Comparison of the emissions of NOx, SO2, CO, CO2, OGC, TSP and BaP for six tested fuels with different solid fuels (Křůmal etal. 2019, 2021) B1 overfire boiler, WW wet wood logs, DW dry wood logs, BC brown coal, BCB brown coal briquettes, HC hard coal, BaP benzo[a]pyrene g/kg Křůmal etal. (2019) Křůmal etal. (2021) Smokeless coal (overfire boiler) Reduced output (40–60%) Reduced output (average = 60%) Reduced output (average = 67%) B1-WW B1-DW B1-BC B1-BCB B1-HC B1-BC B1-HC SC SB1 SB2 SB3 SP HC NOx0.405 1.16 1.2 1.06 2.36 2.02 2.43 1.69 1.2 1.25 1.21 1.46 2.82 SO2nd nd 9.97 3.7 10.2 1.7 9.24 3.8 4.33 4.54 5.95 2.48 4.69 CO 74.7 112 419 239 260 105 253 623 238 206 553 670 200 CO21.01 × 1031.53 × 1031.53 × 1031.79 × 1032.52 × 1031.79 × 1032.46 × 1032.67 × 1032.63 × 1032.58 × 1032.19 × 1032.34 × 1032.31 × 103 OGC 27.6 33.9 85.5 48.8 149 20.2 57.7 1.05 5.15 3.45 1.72 12.4 72.6 TSP 15.4 18.7 81.6 28.7 89.4 3.13 17 0.427 2.18 1.48 10 6.19 31.6 BaP 1.04 2.75 5.2 3.27 8.55 0.83 6.37 0.017 0.439 0.229 0.037 1.29 5.35 International Journal of Environmental Science and Technology 1 3 coal in an overfire boiler with similar heat output, as presented in Table5, are compared with Table6 (Křůmal etal. 2019, 2021). The emissions from the combustion of hard coal in the overfire boiler are comparable except for SO2, while our tested coal contained less sulfur in fuel. CO and OGC are products of incomplete combustion. They are quite well correlated with TSP (Figure S7 in the supplementary file), except CO emissions from HC. This result proves that the quality of combustion is dependent not only on the type of combustion unit but also on the type of fuel. The tested coal fuels SB1–SB3 have the same size. The smallest coal fuel is SC, followed by SP. The largest coal fuel is HC; however, it is a mixture of large and small pieces, which makes the fuel inhomogeneous. This causes the use of large coal fuels (HC, SB1–SB3) in the overfire boiler to not increase the pressure drop so much. The airflow is not limited significantly, which leads to lower CO concentrations. This corresponds to the results in Table5, except SB3, which corresponds to higher TSP concentrations. The volatile matter in coal fuels (Tables1, 2) influences the OGC concentrations, which was confirmed for HC (the highest volatile matter with the highest OGC) and SC (the lowest volatile matter with the lowest OGC). Volatile matter is also related to PAH concentrations. PAHs content For the overall quality assessment of smokeless fuels, the emission of PAHs is important. In Tables S3–S4 in the supplemental file, all emission factors of PAHs for all tested Fig. 3 Emission of benzo[a] pyrene, 4 PAHs LRTAP and 16 PAHs for six tested fuels