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1. INTRODUCTION
Korea Forest Service (KFS) have subsided the distribution and installation of industrial wood pellet boilers from 2011 to 2015 by expanding wood pellet consumption. This has helped step up the domestic industrial wood pellet boiler technology as it resulted in installing 76 industrial wood pellet combustors during the same period. For the effective distribution and management of the industrial wood pellet boilers, KFS established the Industrial Wood Pellet Boiler Subsidy Standards (KFS, 2015) and the Construction Inspection Standards for Industrial Wood Pellet Boilers (KFS, 2013). The target industrial wood pellet boilers were steam boilers in 0-5 to 7.0 ton/h or hot water boilers and heaters in 230kW or over. 63 of them were steam boilers, 7 of them were hot water boilers, and 6 of them were hot wind heaters. To receive subsidies for wood pellet boilers, the following conditions need to be met: 1) The boiler heat efficiency at over 85% based on the lower heating value; 2) the preparation of safety device like a backfire prevention device; and 3) the CO emissions, one of the air pollution substances, being below 200 ppm.
In Korea, air pollutants related to the combustion of wood pellets are regulated by the Clean Air Conservation Act (ME, 2017). The target missions generated by combustion facilities at or over a certain size according to the Clean Air Conservation Act are 1) among wood pellet manufacturing facilities (screening, drying and heating facilities, crushing and grinding facilities, compression and molding facilities), facilities in which fuel consumption is over 30kg per hour, the capacity is over 3m2 or more, or power capacity is 2.25 kW (15 kW for crushing and grinding facilities) or more and 2) among the facilities that use wood pellets, those in which the usage of fuel products is over 200kg per hour (excluding those that use other fuels together with wood pellets) (ME, 2017).
The allowable criteria of the air pollutants applied since 2015 shows that the CO concentration in a facility that uses wood pellets should be at or below 200 ppm (12% of the actually measured oxygen concentration). Nitrogen oxide (NOx, NO2) should be at or below 100 ppm for drying and heating facilities of wood pellet manufacturing facilities, or at or below 150 ppm for facilities that use wood pellets (12% of the actually measured oxygen concentration). Fine dust should be at or below 50 mg/Sm3 for both wood pellet manufacturing facilities or those which use wood pellets (12% of the actually measured oxygen concentration) (ME, 2017). With very low sulfur oxide and heavy metal content in wood pellets due to the fuel characteristics, there is no allowable criteria for those substances.
At the Parliamentary Inspection of the Administration in 2017, it was argued that the wood pellet combustion test showed that it produced 20 times as much nitrogen oxide, which generates ultrafine dust, as coal briquette, and there was a social controversy as the contents were quoted and reported without verification. (Kim, 2017). This is because while there has been research on the manufacturing characteristics of wood pellets in Korea (Kim et al., 2015; Yang et al., 2017), there has been little on air pollutants during wood pellet combustion.
The study aimed to measure air pollutants generated during the combustion of wood pellet boilers subsided by KFS, through which it verified whether industrial wood pellet boilers conformed the allowable criteria of air pollutants by the Ministry of Environment. Furthermore, based on the emissions of air pollutants, it determined the emission factors of industrial wood pellets boilers, which was compared to the NOx emitted from low-NOx certified burner.
2. MATERIALS and METHODS
There were 9 KFS subsidized industrial wood pellet boilers (steam boilers, hot water boilers, hot wind heaters) in 2011, 27 in 2012, 21 in 2013, 10 in 2014, and 9 in 2015. 63 boilers excluding hot wind heaters and some distributed earlier were the target for this study. Shown in Fig. 1 is the current installation conditions by region. Over 50% of the subsidized industrial wood pellet boilers were installed in Gyeonggi-do and Gyeongsangnam-do. It is believed that this is because there are many companies in industrial regions where there is high demand on industrial boilers. It also shows that Gwangju and Daegu among the metropolitan cities have many such boilers.
Fig. 2 shows the capacity of the installed industrial wood pellet boilers. The average capacity of these boilers is 3.0 MW (2,600,000 kcal/h), which is about 4.2 ton/h (wood pellet consumption is about 740 kg/h based on the low-level heat generation of wood pellets at 4,100 kcal/h and the boiler efficiency at 85%) if converted into the amount of steam generated. With low capacity boilers, 1.5 to 2 MW wood pellet boilers have been widely installed, as well as 5.0 MW boilers have also been installed in many places. Converted into steam generation, these capacities are about 2.5 ton/h as well as 7.0 ton/h.
The study conducted the performance test of the KFS subsidized industrial wood pellet boiler by installing it based on the “Construction Inspection Standards for Industrial Wood Pellet Boilers” (KFS, 2013). When the boiler operation stabilized, the O2, CO, and NOx concentration were measured for one hour using a portable multifunctional combustible gas meter (Testo 350), the results of which were averaged. Testo 350 can analyze various combustible gases (O2, CO, NOx and SO2, etc.) and allows for large amount of data records linked to a PC. The null adjustment was performed using the standard gas before the test to acquire the data reliability. The measurement scope of O2 is 0-25% at which the accuracy level is ±0.01%, and that of CO is 0-10000 ppm, and the accuracy level between 200-2000 ppm is ±5%. The measurement scope of NO is 0-4000 ppm, and the accuracy level between 100-1999 ppm is ±5%. Finally, the measurement scope of NO2 is 0-500 ppm, and the accuracy level between 100-500 ppm is ±5%. The measured carbon monoxide and nitrogen oxide were converted into the value of the state of 12% of emission gas O2.
3. RESULTS and DISCUSSION
Fig. 3 shows the carbon monoxide (CO) emissions from the installed industrial boilers by the year of installation. As has been shown, the average CO emission concentration from the installed industrial wood pellet boiler is 56 ppm when the oxygen concentration in the emissions was 12%. The maximum CO emission among the measured boilers was 220 ppm. High concentration of CO would lead to CO poisoning, causing nausea and in serious cases, death. There may be incomplete combustion or air is lacking in boilers. As shown in the figure, the CO emissions change by installation year. To clearly understand such a tendency, Fig. 4 shows the average CO emissions by taking five values based on the time of the CO emissions. Since 2012, the average CO emissions have decreased until 2015. While in 2012, it was within 50 - 100 ppm, in 2015, it decreased to 20 - 30 ppm. This is considerably below the allowable criteria of the air pollutants stipulated by the Enforcement Regulations of the Clean Air Conservation Act (at or below 200 ppm for facilities using wood pellets). Generally, CO emissions decrease by reducing incomplete combustions through the optimization of the combustion system. It is believed that such CO emission reduction was due to the improvement of boiler efficiency and CO emissions reduction by boiler manufacturers since 2011 when industrial wood pellet boilers were distributed by KFS.
The study calculated the emission factor using the CO emission from the boiler. The emission factor for CO or NOx (g/kg_fuel) is defined as the weight of the pollutant (g) emitted by the combustion of 1kg of fuel, which can be determined by the following equation.
In the equation above, the pollutant concentration is calculated by the emission concentration of each pollutant (ppm) measured at the time of the actual combustion of wood pellets, and the fuel consumption is the amount of fuel injected during the actual boiler operation. Finally, the gas emissions are based on the oxygen concentration in the emissions gas measured during the actual combustion of the boiler. Fig. 5 shows the CO emission factor for the industrial wood pellet boiler. The average CO emission factor of the distributed industrial wood pellet boiler is 0.73 g/kg. The average CO emissions of the industrial wood pellet boiler installed in 2015 was 25.7 ppm, which is considerably lower than 62.5 ppm, that of the wood pellet boiler installed in 2012. Such a trend can be verified in Fig. 5 which showed the CO emission factors by the time of installation where the CO emission factors decrease by the time of installation. The average CO emission factor of five industrial wood pellet boilers installed latest was very low at 0.36 g/kg.
Fig. 6 shows the NOx emissions based on the nitrogen content contained in various types of wood chip fuel including the wood pellets used in this study (referring to the data by Kang et al., 2017). The figure shows the maximum and minimum values of NOx emissions measured when the nitrogen content of wood pellets used in the industrial boiler installed by the KSF's subsidy project was 0.3%. It shows the tendency that the NOx value increases linearly based on the nitrogen content included in the fuel. Even the same wood fuels will have different NOx emissions based on the contained nitrogen content. However, wood pellets usually have nitrogen content below 0.5%, and thus, the NOx content will become 100 ppm or below. The study results on the NOx emissions characteristics based on the types of coal (bituminous coal and subbituminous coal) from a 500 MW coal-fired boiler showed that the nitrogen content in coal is 0.76 - 1.7 %. As such, it was also confirmed that the emissions from a coal-fired plant were mainly thermal NOx emissions at high temperature, but when coal with high nitrogen content was used, the NOx generation increased.
Shown in Fig. 7 and Fig. 8 are the average of the five values of NOx emissions from industrial wood pellet boilers. The average NOx emissions measured from the boilers is 76 ppm at 12% of the oxygen concentration. The NOx emissions were between 44 - 128 ppm, which is considerably lower than the allowable standards of the air pollutants stipulated in the Enforcement Regulations of the Clean Air Conservation Act (at or below 150 ppm of NOx for facilities that use wood pellets). At this point, the emission factor of NOx is 1.63 g/kg. The two figures do not show the tendency of the decrease in the emissions by the time of installation, as opposed to the CO distribution. This is because the CO emissions can be reduced by providing the optimal combustion conditions by improving the burner. However, for NOx, wood pellets contain nitrogen by about 0.1 to 0.5%, as opposed to LNG which does not contain nitrogen (N). Such nitrogen is oxidized via the combustion at high temperature and turned into NO and NO2 among others. Such generated NOx is called fuel NOx, which is fundamentally difficult to decrease, and due to such characteristics, NOx emissions tend not to decrease.
Shown in Fig. 9 are the NOx emissions on domestic low- NOx certified products. As shown in the figure, the average NOx is 35 ppm. The low-NOx burner criterial for industrial boilers is below 40 ppm at 4% of oxygen concentration. Generally, the LNG burner that satisfies the low-NOx burner criteria is around 35 ppm at 3% of oxygen concentration.
Shown in Table 1 are the average NOx emission factors under the normal combustion of boilers with LNG, wood pellets, and coal (or bunker-C). In case of wood pellets, the average NOx value emitted from the industrial wood pellet boiler, measured previously used. Coal-fried or bunker-C-fired boilers run with NOx at or below 70 ppm (facilities installed before December 2014, Clean Air Conservation Act). Assuming that such a condition was the representative situation for each fuel, the NOx emission factors could be calculated, which are shown in Table 1. When the NOx emission index of LNG is set to 1, that of wood pellets is 1.90, and coal or bunker-C is 2.01. For coal or bunker-C, it was assumed that there would be post-processing facilities to reduce NOx such as SCR (selective catalytic reduction) at the end of the boiler. Otherwise, the emission index would have been higher.
| Fuel type | NOx (ppm) | O2 (%) | NOx emission factor (g/kg) | Based on LNG |
|---|---|---|---|---|
| LNG | 35 | 3 | 0.96 | 1.00 |
| Wood pellet | 67 | 13 | 1.82 | 1.90 |
| Coal or bunker-C with de-NOx facility | 70 | 3 | 1.98 | 2.06 |
4. CONCLUSION
The CO and NOx measurement values of 63 KFSsubsidized industrial wood pellet boilers between 2011 and 2015 all satisfied the allowable criteria by the Ministry of Environment. Based on the CO and NOx emissions values, the emission factors of industrial wood pellet boilers were calculated and compared with NOx emitted from the low-NOx certified burner. The analyzed results are summarized as follows.
The average capacity of the KFS-subsidized industrial wood pellet boilers was 3.0 MW, and the wood pellet consumption was about 740 kg/h.
The average CO emission from the industrial wood pellet boilers distributed for the five years was 56 ppm (@O2 12%), which can be converted into the emission factor at 0.73 g/kg.
The average CO emission from the industrial wood pellet boilers installed in 2012 during the early period of the subsidy program was 70.3 ppm (@O2 12%), but that from the boilers installed at the ending year of the subsidy program was 28.9 ppm. Such a result is determined to be due to the optimization of the burner and boiler system through technical development.
The average NOx emission from the industrial wood pellet boilers installed for the five years was 76 ppm (@O2 12%), which can be converted into the emission factor at 1/63 g/kg.
NOx did not show the tendency of reduction by time. It is determined that this is because fuel NOx generation did not decrease along with the nitrogen content included in the fuel.
The NOx emission factor from the industrial wood pellet boilers was compared to that from low-NOx certified LNG burner and coal (bunker- C)-fired burners. The former was about 1.90 times higher than that of the latter and was about 0.92 times lower than the coal- or bunker-C-fired boilers with denitrification facilities.
