Emission characteristics of polychlorinated dibenzo-p-dioxins and dibenzofurans from industrial combustion of biomass fuels

Dioxin concentrations and congener distribution in biomass ash from small to large scale biomass combustion plants

Biomass plays a pivotal role in global energy production, with a significant share allocated for industrial heat and power generation. The combustion of biomass generates biomass ash, which is widely utilized as a fertiliser. However, concerns arise regarding the presence of dioxins in biomass ash, which may limit its continued use. Dioxins are toxic compounds listed under the Stockholm Convention due to their persistence and detrimental effects on human health and the environment. This study investigates the dioxin content in biomass ashes produced in various combustion plants with a capacity of 1-50MWth in Estonia, where biomass is widely used for heating and power production. The research encompassed samples from nine biomass combustion plants with varying technical parameters considering both bottom and fly ash. Dioxin concentrations were determined for 7 polychlorinated dibenzo-p-dioxins (PCDDs), 10 polychlorinated dibenzofurans (PCDFs), and 12 dioxin-like PCBs (PCBs). The results indicate that dioxin TEQ content in all samples was well below the European Union's (EU) POP Regulation limit of 5 µg TEQ/kg, with most values being at least tenfold lower. However, some samples failed to meet the EU Fertilising Products Regulation's threshold of 20 ng TEQ/kg. Notably, fly ash exhibited higher dioxin concentrations than bottom ash. While PCBs were in significant concentrations, PCDDs dominated the overall dioxin TEQ content. This study provides essential insights into the dioxin content in biomass ash and its correlation with current EU regulatory limits. It also highlights the complex distribution of dioxin congeners, particularly PCBs, within biomass ash, emphasizing the continued research's importance.

Polycyclic aromatic hydrocarbons emissions from biomass-fueled boilers in China

Biomass serves as a sustainable energy source; however, the environmental risks associated with polycyclic aromatic hydrocarbons (PAHs) emitted from industrial biomass-fueled boilers are not well understood. This study analyzed 16 priority PAHs in both particulate and gaseous phases from 13 representative real-world industrial biomass-fueled boilers. Flue gas samples were collected from the stacks and analyzed using advanced techniques. Total PAHs concentrations ranged from 1.36 to 8870 μg m-3 (9 % O2 v/v), with benzo[a]pyrene emissions from certain boilers exceeding the allowable emissions standards for the coking chemical and petroleum refining industries in China. PAHs were predominantly found in the gaseous phase, with both gas and particle phases exhibiting similar toxicity. The average emission factor (EFmass) was 9.23 mg kg-1, while the toxicity-equivalent emission factors (EFCEQ, EFMEQ, and EFTEQ) were 1.96 × 10-2, 1.39 × 10-2 and 7.61 × 10-4 mg kg-1, respectively. It is estimated that annual PAH emissions from 2020 to 2050 will significantly decrease if biomass is used as industrial fuel in boilers (0.61 to 1.32 Gg y-1) instead of being openly burned in the field (3.39 to 7.21 Gg y-1). Overall, this study provides a comprehensive evaluation of PAH emissions from industrial biomass combustion, offering valuable data for future research and policy-making.

Emission and distribution characteristics of PCDD/Fs during the co-processing of various solid wastes in coal-fired boilers in China

The advancement of co-processing solid wastes in coal-fired boilers is significant for waste recycling and contributes to the sustainable development of the coal-fired power industry. However, concerns over the emission of dioxins during co-processing have prompted a comprehensive investigation into the dioxin emission properties. In this study, the PCDD/F emission concentrations of seven coal-fired boilers, including three pulverized coal boilers and four circulating fluidized bed boilers were examined. The results indicate that co-processing solid wastes in coal-fired boilers did not lead to an increase in the mass concentration of dioxins in either the flue gas or solid samples, and the international toxic equivalents (I-TEQ) of dioxins in the flue gas complied with prevailing emission standards (0.1 ng I-TEQ/Nm3) in China, proving that coal-fired boilers co-processing would not raise the emission risk of dioxins. The types of waste during co-processing had minimal effect on the I-TEQ of dioxins. A significant proportion of PCDD/Fs was observed in the ash samples, while only 13.0-25.7% and 0.7-6.8% of dioxins were distributed in the boiler slag and the flue gas, respectively. The emission factor of dioxins under the blank conditions ranged from 0.009 to 0.327 ng I-TEQ/kg-coal, while it ranged from 0.015 to 0.129 ng I-TEQ/kg-(coal+waste) under the co-processing conditions. The reduction of emission factor under co-processing condition could be attributed to the significant decrease of dioxins I-TEQ.

PCDD/F adsorption enhancement over nitrogen-doped biochar: A DFT-D study

Polychlorinated dibenzo-p-dioxin/furans (PCDD/F) have a great threat to the environment and human health, resulting in controlling PCDD/F emissions to regulation far important for emission source. Considering 2,3,4,7,8-pentachlorodibenzo-p-furan (PeCDF) identified as the most contributor to international toxic equivalent, 2,3,4,7,8-PeCDF can be considered as the target molecule for the adsorption of PCDD/F emission from industries. With the aim to in-depth elucidate how different types of nitrogen (N) species enhance 2,3,4,7,8-PeCDF on the biochar and guide the specific carbon materials design for industries, systematic computational investigations by density functional theory calculations were conducted. The results indicate pristine biochar intrinsically interacts with 2,3,4,7,8-PeCDF by π-π electron donor and acceptor (EDA) interaction, six-membered carbon rings of PeCDF parallel to the biochar surface as the strongest adsorption configuration. Moreover, by comparison of adsorption energy (-150.16 kJ mol-1) and interaction distance (3.593 Å) of pristine biochar, environment friendly N doping can enhance the adsorption of 2,3,4,7,8-PeCDF on biochar. Compared with graphitic N doping and pyridinic N doping, pyrrolic N doping biochar presents the strongest interaction toward 2,3,4,7,8-PeCDF molecule due to the highest adsorption energy (-155.56 kJ mol-1) and shortest interaction distance (3.532 Å). Specially, the enhancing adsorption of PeCDF over N doped biochar attributes to the enhancing π-π electron EDA interaction and electrostatic interaction. In addition, the effect of N doping species on PeCDF adsorbed on the biochar is more than that of N doping content. Specially, the adsorption capacity of N doping biochar for PCDD/F can be improved by adding pyrrolic N group most efficiently. Furthermore, pyrrolic N and pyridinic N doping result in the entropy increase, and electrons transform from pyrrolic N and pyridinic N doped biochar to 2,3,4,7,8-PeCDF molecule. A complete understanding of the research would supply crucial information for applying N-doped biochar to effectively remove PCDD/F for industries.

Emission reduction of PCDD/Fs by flue gas recirculation and activated carbon in the iron ore sintering

One of the main sources of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the environment is the sintering of iron ore. Both flue gas recirculation (FGR) and activated carbon (AC), which have the impact of decreasing both PCDD/Fs and conventional pollutants (NOx, SO2, etc.), are significant technologies for the abatement of PCDD/Fs from the sintering exhaust gas. This work involved the first measurement of PCDD/Fs emissions during FGR and a thorough analysis of the impact of PCDD/Fs reduction following the coupling of FGR and AC technologies. According to the measured data, the ratio of PCDFs to PCDDs in the sintered flue gas was 6.8, indicating that during the sintering process, the PCDD/Fs were primarily produced by de novo synthesis. Further investigation revealed that FGR initially removed 60.7% of PCDD/Fs by returning it to the high temperature bed, and AC further removed 95.2% of the remaining PCDD/Fs through physical adsorption. While AC is better at removing PCDFs and can efficiently remove tetra-to octa-chlorinated homologs, FGR is more effective at removing PCDDs and has higher removal efficiency for hexa-to octa-chlorinated PCDD/Fs. Together, they complement each other with a removal rate of 98.1%. The study's findings are instructional for the process design of combining FGR and AC technologies to reduce PCDD/Fs in the sintered flue gas.

Effects of increasing chlorine concentration in feedstock on the emission and distribution characteristic of dioxins in circular fluidized bed boiler

Field studies were conducted to study the emission and distribution characteristics of dioxins by elevating the chlorine concentration in feedstock in a circular fluidized bed boiler. The concentration and total equivalent quantity of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) in all flue gas, electrostatic ash, bag filter ash, and bottom ash samples under blank condition (i.e., feedstock was normal coal) and chlorine labeling condition (i.e., feedstock mixed with coal and chlorine-containing labeling agent) were analyzed. Results illustrated that the concentration of PCDD/Fs in all gaseous and ash samples increased with the addition of chlorine in feedstock, with the largest and least increment in dioxin concentration observed in electrostatic ash and flue gas. PCDDs were the predominate congeners in flue gas, accounted for 50.1-60.4% of the total PCDD/F concentration under chlorine labeling and blank conditions, while PCDD/F distribution changed from PCDD- to PCDF-predominate by increasing chlorine content in feedstock under all field test conditions: 46.6-92.9%, 34.0-76.1%, and 47.0-53.1% of PCDFs were distributed in electrostatic ash, bag filter ash, and bottom ash, respectively. Highly chlorinated PCDD/F congeners such as O₈CDD/F and 1,2,3,4,6,7,8-H₇CDD/F were the primary contributors to dioxin concentration in flue gas and bottom ash samples, whereas low-chlorinated 2,3,7,8-T₄CDF and 1,2,3,7,8-P₅CDF congeners became critically dominating in electrostatic and bag filter ash.

Characteristics and health impacts of PM2.5-bound PCDD/Fs in three Asian countries

Polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) were ubiquitous, persistent chemical compounds attached to particulate matter in the atmosphere. We aimed to study the characteristics of these pollutants in atmospheric PM_2.5 of three Asian countries, including Taiwan (Taipei), Thailand (Chiang Mai), and Vietnam (Hanoi). We carried out a source apportionment analysis to determine significant PCDD/F contributors in these areas. Multiple media model was conducted to access the health impact assessment. The PM_2.5 concentration in Taipei (n = 7), Chiang Mai (n = 20), and Hanoi (n = 10) were 18.4 ± 6.21 μg/m³, 133 ± 49.5 μg/m³, and 88.1 ± 12.6 µg/m³, respectively. The PCDD/Fs level in Hanoi was 92.4 ± 67.3 fg I-TEQ/m^3, and in Taipei and Chiang Mai was 5.01 ± 2.39 fg I-TEQ/m³ and 14.4 ± 13.1 fg I-TEQ/m³, respectively, which showed that the higher PM_2.5 concentration was not necessary to follow with higher PCDD/Fs level. In all three cities, the effect of traffic on ambient PCDD/F level was significant (23-25 %). However, we also observed the specific sources of PCDD/Fs in each city during the sampling periods, namely long-range transport (Taipei, 55 %), Biomass/open burning (Chiang Mai, 77 %), and industrial activities (Hanoi, 34 %). In the carcinogenic risk estimation, the highest median total carcinogenic risk was in Hanoi (5.87 × 10^-6), followed by Chiang Mai (1.06x10^-6), and Taipei (2.95 × 10^-7). Although diet was the major absorption pathway, the food contributor of exposure differed among the three areas due to the difference in food consumption composition.

Chlorine Corrosion in a Low-Power Boiler Fired with Agricultural Biomass

The selection of appropriate heat-resistant materials which are at the same time resistant to atmospheres rich in chlorine and its compounds is one of the most important current construction problems in steel boiler elements when using biomass fuels of agricultural origin. In the research presented here, an area was identified in the furnace of a 10 kW boiler where there was a potential risk of chlorine corrosion. This zone was determined based on numerical analysis of the combustion process; it is the zone with the highest temperatures and where the gas atmosphere conducive to the formation of chlorine corrosion centers. Subsequently, tests were carried out in the process environment of the combustion chamber of a 10 kW boiler (the fuel was barley straw) by placing samples of eight construction materials in a numerically-designated zone. These included samples of steel (coal boiler St41K, heat-resistant H25T and H24JS, and heat-resistant valve 50H21G9N4) as well as intermetallic materials based on phases (FeAl, Fe3Al, NiAl, and Ni3Al). The samples remained in the atmosphere of the boiler furnace for 1152 h at a temperature of 750–900 °C. After this time, the surfaces of the samples were subjected to SEM microscopy and scanning analysis. The results showed that the St41K boiler steel was not suitable for operation under the assumed conditions, and that a thick layer of complex corrosion products was visible on its surface. The least amount of corrosion damage was observed for the samples of 50H21G9N4 steel and intermetallic materials.