Synergistic in situ control of NO/SO₂ and selected heavy metals during co-combustion of sewage sludge with coal

Abstract

The co-combustion of sewage sludge (SS) with coal is a promising strategy for waste valorization; however, the impacts on gaseous pollutant emissions and heavy metal (HM) behavior remain inadequately characterized. This study investigates NO and SO₂ release, the retention and extractability of six selected heavy metals (Mn, Zn, Cr, Ni, Cu, and Pb), and ash fusion characteristics during co-combustion. Co-firing with SS promoted early ignition and enhanced the coal combustion reactivity through the rapid release of volatiles, which facilitated fixed-carbon combustion; however, excessive SS blending reduced heat release owing to its low calorific value and high ash content. Concurrent in situ reductions of NO and SO₂ were achieved. The enhanced combustion reactivity accelerated O₂ consumption and promoted CO formation, creating a locally reducing atmosphere that contributed to the suppression of NO emissions. SO₂ mitigation was associated with sulfur retention via the formation of stable sulfates by Ca-bearing minerals. Co-combustion enhanced the retention of Cr, Cu, Pb, and Zn in ash matrices while reducing their extractability, which is associated with HM-encapsulating silicate-, aluminosilicate-, and phosphate-rich phases. These refractory phases increased ash flow temperatures and inhibited liquid slag formation. Overall, SS/coal co-combustion reduces gaseous emissions and ash fusibility while stabilizing HMs in ashes.

Graphical abstract

Keywords

coal combustion / sewage sludge disposal / NO and SO₂ reduction / heavy metal retention / ash fusibility

References

[1] Zheng M , Hu Z T , Liu T , Sperandio M , Volcke E I P , Wang Z Y , Hao X D , Duan H R , Vlaeminck S E , Xu K N . et al. Pathways to advanced resource recovery from sewage. Nature Sustainability, 2024, 7(11): 1395–1404

[2] Nyitrai J , Almansa X F , Zhu K , Banerjee S , Hawkins T R , Urgun-Demirtas M , Raskin L , Skerlos S J . Environmental life cycle assessment of treatment and management strategies for food waste and sewage sludge. Water Research, 2023, 240: 120078

[3] Zhao L , Sun Z F , Pan X W , Tan J Y , Yang S S , Wu J T , Chen C , Yuan Y , Ren N Q . Sewage sludge derived biochar for environmental improvement: advances, challenges, and solutions. Water Research X, 2023, 18: 100167

[4] Feng J T , Burke I T , Chen X H , Stewart D I . Assessing metal contamination and speciation in sewage sludge: implications for soil application and environmental risk. Reviews in Environmental Science and Bio/Technology, 2023, 22(4): 1037–1058

[5] Pozzebon E A , Seifert L . Emerging environmental health risks associated with the land application of biosolids: a scoping review. Environmental Health, 2023, 22(1): 57

[6] Azarmanesh R , Zarghami Qaretapeh M , Hasani Zonoozi M , Ghiasinejad H , Zhang Y . Anaerobic co-digestion of sewage sludge with other organic wastes: a comprehensive review focusing on selection criteria, operational conditions, and microbiology. Chemical Engineering Journal Advances, 2023, 14: 100453

[7] Li W L , Gupta R , Zhang Z K , Cao L X , Li Y Q , Show P L , Gupta V K , Kumar S , Lin K A , Varjani S . et al. A review of high-solid anaerobic digestion (HSAD): from transport phenomena to process design. Renewable and Sustainable Energy Reviews, 2023, 180: 113305

[8] Liang Y , Xu D H , Feng P , Hao B T , Guo Y , Wang S Z . Municipal sewage sludge incineration and its air pollution control. Journal of Cleaner Production, 2021, 295: 126456

[9] Kwapinski W , Kolinovic I , Leahy J J . Sewage sludge thermal treatment technologies with a focus on phosphorus recovery: a review. Waste and Biomass Valorization, 2021, 12(11): 5837–5852

[10] Kossińska N , Krzyżyńska R , Ghazal H , Jouhara H . Hydrothermal carbonisation of sewage sludge and resulting biofuels as a sustainable energy source. Energy, 2023, 275: 127337

[11] Fan Y J , Hornung U , Dahmen N . Hydrothermal liquefaction of sewage sludge for biofuel application: a review on fundamentals, current challenges and strategies. Biomass and Bioenergy, 2022, 165: 106570

[12] Wang Y L , Jia L , Guo J R , Wang B R , Zhang L , Xiang J , Jin Y . Thermogravimetric analysis of co-combustion between municipal sewage sludge and coal slime: combustion characteristics, interaction and kinetics. Thermochimica Acta, 2021, 706: 179056

[13] Gao F Y , Zhou C C , Du J , Zhang Y L , Wu W T , Liu G J , Li D . Effect of gaseous agents on co-combustion characteristics of sewage sludge and coal. Journal of Environmental Chemical Engineering, 2021, 9(5): 106227

[14] Qu Z P , Wei X T , Chen W D , Wang F , Wang Y T , Long J S . Co-combustion characteristics of municipal sewage sludge and coal in a lab-scale fluidized bed furnace. Energies, 2023, 16(5): 2374

[15] Guo S , Gao J L , Zhao D , Zhao C C , Li X C , Li G Y . Co-combustion of sewage sludge and Zhundong coal: effects of combustion conditions on gaseous pollutant emission and ash properties. Sustainable Energy Technologies and Assessments, 2022, 54: 102836

[16] Gu T B , Ma W C , Guo Z N , Berning T , Yin C G . Stable and clean co-combustion of municipal sewage sludge with solid wastes in a grate boiler: a modeling-based feasibility study. Fuel, 2022, 328: 125237

[17] Aidabulov M , Zhakupov D , Zhunussova K , Temireyeva A , Shah D , Sarbassov Y . Thermal characterization, kinetic analysis and co-combustion of sewage sludge coupled with high ash ekibastuz coal. Energies, 2023, 16(18): 6634

[18] Liu C Y , Yue C T , Ma Y . Pollutant emissions and heavy metal migration in co-combustion of sewage sludge and coal. Energies, 2024, 17(11): 2457

[19] Zhao Z H , Wang R K , Wu J H , Yin Q Q , Wang C B . Bottom ash characteristics and pollutant emission during the co-combustion of pulverized coal with high mass-percentage sewage sludge. Energy, 2019, 171: 809–818

[20] Jadlovec M , Výtisk J , Honus S , Pospišilík V , Bassel N . Pollutants production, energy recovery and environmental impact of sewage sludge co-incineration with biomass pellets. Environmental Technology & Innovation, 2023, 32: 103400

[21] Lin X Q , Mao T Y , Ma Y F , Zou X B , Liu L J , Yin W H , Rao M M , Ye J , Chen C T , Yu H . et al. Influence of different catalytic metals on the formation of PCDD/fs during co-combustion of sewage sludge and coal. Aerosol and Air Quality Research, 2022, 22(9): 220268

[22] Zou X B , Ye J , Chen C T , Huang H , Yu W X , Ying Y X , Mao T Y , Ma Y F , Zhu B J , Lin X Q . Formation pathways and emission characteristics of PCDD/fs during co-combustion tests of sewage sludge in coal-fired power plants. Aerosol and Air Quality Research, 2023, 24(1): 230178

[23] Li C K , Wen C , Wang D P , Zhao C X , Li R . Emission characteristics of gaseous and particulate mercury from a subcritical power plant co-firing coal and sludge. Atmosphere, 2022, 13(10): 1656

[24] Yang X , Zhu W K , Li Z M , Xu L , Zhu S J , Tian J L , Wang Z Z , Shen B X . Investigation of combustion and NO/SO₂ emission characteristics during the co-combustion process of torrefied biomass and lignite. Molecules, 2024, 29(12): 2728

[25] Guo Y , Wu J J , Jia W K , Guo F H , Qiu G F , Wang R M , Zhang Y X , Dai B Q . Evaluation of the thermal behavior, synergistic catalysis, and pollutant emissions during the co-combustion of sewage sludge and coal gasification fine slag residual carbon. Catalysts, 2021, 11(10): 1142

[26] Fan H H , Li L , Li Z , Shang S . Structure of sewage sludge-clay multiscale composite particles to control the mechanism of SO₂ and H₂S gas release. Materials, 2022, 15(5): 1855

[27] Yan J C , Wu Y S , Zhang L , Huang S J , Lei Z P , Li Z K , Zhang W D , Ren S B , Wang Z C , Shui H F . Synergistic retention of heavy metals and in-situ reduction of NO and SO₂ by co-combustion of sewage sludge and coal gangue: a promising approach for contaminant management and emission reduction. Fuel Processing Technology, 2023, 252: 107984

[28] Yang W J , Zou L , Shao H S , Zhao Q X , Wang Y G . Research on thermal behaviors and NO_x release properties during combustion of sewage sludge, sawdust, and their blends. ACS Omega, 2022, 7(23): 20172–20185

[29] Yu S L , Tao R Z , Tan H Z , Zhou A , Deng S H , Wang X B , Zhang Q F . Migration characteristics and ecological risk assessment of heavy metals in ash from sewage sludge co-combustion in coal-fired power plants. Fuel, 2023, 333: 126420

[30] Wang Y L , Jin Z P , Li J K , Bai T , Chen Y . Heavy metals behavior of co-combustion ash from sewage sludge and coal slime: temperature and mixing ratio dependence, interactions and speciation. Journal of Cleaner Production, 2024, 434: 140435

[31] Yan M , Liu S , Zhang H H , Zheng R D , Cui J T , Wang D , Rahim D A , Kanchanatip E . Syngas production and heavy metal dynamics during supercritical water gasification of sewage sludge. Frontiers of Environmental Science & Engineering, 2024, 18(12): 149

[32] Wang J , Yan R , Liu Z Y , Wang J X , Zhang P L . Evolution behavior of mineral structure and trace elements in feed coals from six coal-fired power plants in China. Environmental Science and Pollution Research, 2023, 30(36): 85759–85771

[33] Liu H , Wang Y J , Chen J . Transformation of Zn and Cr during co-combustion of sewage sludge and coals: influence of coal and steam. Environmental Science and Pollution Research, 2023, 30(43): 98351–98361

[34] Qin S N , Jia L , Qiao X L , Wang C X , Guo B H , Chang X Y , Cheng P , Jin Y . Heavy metals migration and in-situ solidification in the co-combustion of solid waste fuels: rules and differences. Journal of Environmental Chemical Engineering, 2025, 13(1): 115097

[35] Wei D N , Han H W , Zhu Z Y , Wu J W , Wang T , Sun B M . Speciation distributions and risk assessments of trace elements in sludge and coal co-combustion ash. Energy, 2024, 292: 130633

[36] Shi Z C , Li R , Chen Y Q , Wang Z Y , Fan M J , Zhang J Y , Yu W , Liu X Q , Liu B , Mao N . et al. Co-combustion of coal gasification fine slag and sewage sludge in a two-stage preheating and combustion reactor: burnout, gas pollutant and heavy metal emissions. Sustainable Energy Technologies and Assessments, 2025, 82: 104473

[37] Qin S N , He X F , Li Z K , Jia L , Qiao X L , Chang X Y , Cheng P , Jin Y . Co-combustion of sewage sludge and high ash coal: thermal behavior, ash formation behavior, interaction mechanisms and economic analysis. Energy, 2025, 323: 135847

[38] Zhang X X , Huang L , Bai X W , Feng W , Bai J , Kong L X , Bai Z Q , Zhong M , Dai Z H , Li W . The effect of sewage sludge on Zhundong coal ash melting behavior under different atmosphere. Materials Chemistry and Physics, 2025, 333: 130396

[39] Wu Y S , Zhang L , Liu M X , Lei Z P , Li Z K , Zhang W D , Ren S B , Wang Z C , Shui H F , Yan J C . Synergistic reduction of NO/SO₂ through co-firing municipal sewage sludge with coal gangue. Chemical Engineering Journal, 2024, 496: 154025

[40] Yang Z Z , Zhang Y Y , Liu L L , Wang X D , Zhang Z T . Environmental investigation on co-combustion of sewage sludge and coal gangue: SO₂, NO_x and trace elements emissions. Waste Management, 2016, 50: 213–221

[41] Ni Z S , Zhang Y K , Liu X , Shi H , Yao Y R , Tian J J , Hu P , He L Q , Lin Q Z , Meng K S . Effect of furnace temperature and oxygen concentration on combustion and CO/NO emission characteristics of sewage sludge. Renewable Energy, 2024, 234: 121225

[42] Liu S J , Wang M Y , Zhang K X , Yu Z L , Yang S , Shangguan J , Zhang G Q , Du W G , Li J , Liu Y H . Sulfur retention efficiency of clean coke produced by co-pyrolysis of coal with CaCO₃ to substitute household coal. Carbon Resources Conversion, 2021, 4: 142–149

[43] Wu Y S , Zhang L , Zhang Z Z , Zhang W D , Lei Z P , Li Z K , Ren S B , Wang Z C , Shui H F , Yan J C . Co-firing of coal gangue and sewage sludge: combustion characteristics and ash fusion behavior. Process Safety and Environmental Protection, 2026, 215: 109232

[44] Ulusoy B , Anicic B , Lin W G , Lu B N , Wang W , Dam-Johansen K , Wu H . Interactions in NO_x chemistry during fluidized bed co-combustion of residual biomass and sewage sludge. Fuel, 2021, 294: 120431

[45] Aho M J , Hämäläinen J P , Tummavuori J L . Importance of solid fuel properties to nitrogen oxide formation through HCN and NH₃ in small particle combustion. Combustion and Flame, 1993, 95(1/2): 22–30

[46] Wargadalam V J , Löffler G , Winter F , Hofbauer H . Homogeneous formation of NO and N₂O from the oxidation of HCN and NH₃ at 600–1000 °C. Combustion and Flame, 2000, 120(4): 465–478

[47] Ulusoy B , Wu H , Lin W G , Karlström O , Li S G , Song W L , Glarborg P , Dam-Johansen K . Reactivity of sewage sludge, RDF, and straw chars towards NO. Fuel, 2019, 236: 297–305

[48] Ke X W , Jiang L , Li Y R , Yang H R , Lyu J F , Zhang M . A comprehensive study of char reaction kinetics: does CO always promote NO reduction on char surface. Fuel, 2022, 310: 122440

[49] Yu S R , Zhang B , Wei J X , Zhang T S , Yu Q J , Zhang W S . Effects of chlorine on the volatilization of heavy metals during the co-combustion of sewage sludge. Waste Management, 2017, 62: 204–210

Rights & permissions

Higher Education Press