Abstract
Cement manufacturing and the treatment of sludge are considered both energy-intensive industries and major greenhouse gas (GHG) emitters. However, there are still few studies on comprehensive carbon footprint analysis for adding municipal sludge in the cement production. In this study, the lime-dried sludge blended with calcium oxide at the mass mixing ratio of 10% was utilized as raw material for the preparation of Portland cement. The chemical and physical properties of sludge were analyzed. A set of carbon footprint calculation methods of lime-drying treatment of sludge and reuse in cement kilns was then established to explore the feasibility of coprocessing lime-dried sludge in cement kilns. The results showed lime-dried sludge containing CaO, SiO2, Al2O3, and Fe2O3 was ideal for cement production as raw material. However, the water content of lime-dried sludge should be strictly limited. The lime-drying process presented the biggest carbon emission (962.1 kg CO2-eq/t sludge), accounting for 89.0% of total emissions. In the clinker-production phase, the lime-dried sludge as raw material substitute and energy source gained carbon credit of 578.8 and 214.2 kg CO2-eq/t sludge, respectively. The sludge used for producing cement clinker could reduce carbon emissions by 38.5% to 51.7%. The addition ratio of lime and stacking time in the sludge lime-drying process could greatly affect the carbon footprint of coprocessing lime-dried sludge in cement kiln.
Highlights
Due to rising urbanization and the increase of wastewater treatment ratio in China, a large amount of activated sludge has been generated as byproduct during the wastewater treatment process [1]
According to the sludge test standard in China (CJ/T 221-2005), organic matter was measured by weight method and total nitrogen (TN) was analyzed by alkaline potassium persulfate digestion followed by UV-vis spectrophotometry
Compared with the conventional landfilling of sludge with emission in the range of 1564 to 1992 kg CO2-eq/t sludge in China [34,35], the sludge in this study used for producing cement clinker could reduce carbon emissions by 38.5% to 51.7%
Summary
Due to rising urbanization and the increase of wastewater treatment ratio in China, a large amount of activated sludge has been generated as byproduct during the wastewater treatment process [1]. The treatment of sludge has become one of the significant challenges for wastewater treatment plants (WWTPs) [1]. Near 80% of WWTPs in China are equipped with sludge-handling facilities, there are still over 80% of municipal not treated well [2]. The sludge has not been well stabilized due to the lack of anaerobic digestion facilities. Many sanitation agencies have invested sludge-drying facilities to achieve lower water content and volume. Some applications in Xiaohongmen WWTP and Fangzhuang WWTP in Beijing, and Jiading WWTP in Shanghai, are using a large amount of lime for further sludge dewatering
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