Key Takeaways & Executive Findings
- •• • At a 5% sludge co-firing rate, fan flow rate decreased by 24.0%, fan power consumption dropped by 56%, ammonia consumption reduced by 31.4%, and lime consumption by 24.7%, while steam production fell by 15.1% compared to 0% co-firing, indicating a trade-off between operational efficiency and energy recovery. • • Increasing the co-firing rate from 5% to 10% degraded combustion conditions, leading to higher flue gas volume, fan flow, and reagent consumption, and further reduced steam production from 2.358 t/t to 2.117 t/t, demonstrating a non-linear penalty on system efficiency. • • Fly ash leaching toxicity tests showed that Zn leached concentration dropped from 31.325 mg/L at 0% co-firing to 2.513 mg/L at 5% and 0.876 mg/L at 10%, while Pb decreased from 0.05 mg/L to 0.03 mg/L and below detection at 10%, all well below GB 16889—2008 limits, confirming effective stabilization. • • Hg, As, Ba, and Se leached concentrations increased with co-firing rate (e.g., Hg from 2.61×10⁻⁴ to 3.88×10⁻³ mg/L; As from 2.23×10⁻³ to 0.14 mg/L), yet remained far below regulatory thresholds, indicating that sludge co-firing alters heavy metal speciation but does not compromise landfill disposal compliance.
Abstract
Co-firing municipal sludge with municipal solid waste (MSW) provides a viable solution for sludge disposal. This study evaluated the effect of sludge co-firing rate (0%, 5%, and 10%) on flue gas pollutant emissions, operational performance, and incineration byproduct characteristics in a waste-to-energy plant. The results showed that under all co-firing rates, the concentrations of SO2, NOx, CO, HCl, particulate matter, and dioxins complied with the limits specified in the Standard for Pollution Control on Municipal Solid Waste Incineration (GB 18485—2014). As the co-firing rate increased from 5% to 10%, concentrations of all flue gas pollutants except NOx and HCl exhibited an upward trend. The optimal operational performance under the test conditions was achieved at a 5% co-firing rate, when the flue gas volume, fan volume, and ammonia and lime consumption were minimized. Compared to the 5% rate, increasing the co-firing rate to 10% resulted in elevated flue gas volume, fan volume, and reagent consumption. Steam production decreased from 2.778 t/t (0% co-firing rate) to 2.358 t/t (5% co-firing rate) and 2.117 t/t (10% co-firing rate), indicating a reduction in power generation efficiency with increasing sludge co-firing rates. Leaching toxicity analysis of fly ash revealed significant reductions in the leached concentrations of Zn and Pb, while those of Hg, As, Ba, and Se showed slight increases, all remaining well below regulatory limits. This study confirms the technical feasibility of directly co-firing mechanically dewatered sludge (with a moisture content of 50% to 60%) without thermal drying. A 5% co-firing rate is identified as the optimal balance between operational economy and system efficiency.
1. Introduction
Municipal wastewater treatment plants generate substantial quantities of sludge that, if improperly managed, pose severe environmental risks due to heavy metals, pathogens, and organic pollutants. Conventional disposal routes such as mono-incineration, landfilling, and land application face critical bottlenecks: mono-incineration demands high energy input and complex flue gas treatment; landfilling consumes valuable land and risks long-term leachate contamination; and agricultural or construction-material reuse is constrained by market acceptance and heavy metal content limits. These limitations underscore the need for a disposal pathway that integrates waste reduction, energy recovery, and pollution control with minimal additional infrastructure investment.
Co-firing dewatered sludge with municipal solid waste (MSW) in existing waste-to-energy plants offers a pragmatic solution by leveraging the high calorific value of MSW to offset sludge's low energy content, while alkaline ash components capture acidic pollutants from sludge combustion. However, the optimal co-firing ratio remains contentious: excessive sludge addition can depress furnace temperature, increase flue gas volume, and elevate reagent consumption, thereby undermining operational economy and energy efficiency. This study systematically evaluates co-firing rates of 0%, 5%, and 10% in a full-scale plant, measuring flue gas pollutant emissions, steam production, and fly ash leaching toxicity to identify the technically and economically optimal blend ratio.
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WANG Hao, ZHONG Yang, XIAO Sihua, YUAN Weifang, SONG Xiaowei (2026). Effect of Co-firing Rate on Dewatered Sludge and Municipal Solid Waste Incineration: Pollutants Emissions, Operational Performance and Byproducts. Journal of Environmental Engineering Technology. https://doi.org/10.13205/j.hjgc.202607003
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Frequently Asked Questions
What is the maximum sludge co-firing rate that maintains compliance with GB 18485—2014 emission limits while preserving acceptable combustion stability?
Under the tested conditions, co-firing rates up to 10% maintained all flue gas pollutants (SO2, NOx, CO, HCl, particulate matter, dioxins) within GB 18485—2014 limits. However, at 10% co-firing, combustion conditions deteriorated, as evidenced by increased flue gas volume and reagent consumption, and steam production dropped to 2.117 t/t from 2.358 t/t at 5%. Therefore, 5% is recommended as the optimal rate for balancing emission compliance and operational efficiency.
How does sludge co-firing affect the leaching behavior of heavy metals in fly ash, and what mechanisms explain the divergent trends for Zn/Pb versus Hg/As/Ba/Se?
Zn and Pb leached concentrations decreased significantly with increasing co-firing rate (Zn from 31.325 mg/L at 0% to 0.876 mg/L at 10%; Pb from 0.05 mg/L to below detection at 10%). This is attributed to dilution by higher ash content, formation of stable silicate phases, and reduced chloride content that limits volatile metal chloride formation. Conversely, Hg, As, Ba, and Se showed slight increases (e.g., Hg from 2.61×10⁻⁴ to 3.88×10⁻³ mg/L; As from 2.23×10⁻³ to 0.14 mg/L) due to higher input from sludge and their volatile/semi-volatile nature, yet all remained well below GB 16889—2008 limits.
What is the impact of sludge co-firing on steam production and overall power generation efficiency?
Steam production decreased from 2.778 t/t at 0% co-firing to 2.358 t/t at 5% and 2.117 t/t at 10%, representing reductions of 15.1% and 23.8%, respectively. This decline is primarily due to the lower calorific value of the mixed fuel and incomplete combustion at higher sludge ratios, which reduces thermal energy recovery and thus power generation efficiency.
What operational adjustments are required to maintain optimal performance when co-firing sludge at 5%?
At a 5% co-firing rate, the plant achieved the lowest fan flow (24.0% reduction), fan power consumption (56% reduction), ammonia consumption (31.4% reduction), and lime consumption (24.7% reduction) compared to 0% co-firing. These improvements likely result from enhanced combustion stability and reduced flue gas volume, but operators must monitor steam production trade-offs and adjust feed rates to sustain efficiency.
Is thermal drying of sludge necessary before co-firing, and what moisture content is acceptable?
This study confirms that mechanically dewatered sludge with a moisture content of 50%–60% can be directly co-fired without thermal drying, as demonstrated by the successful operation at 5% and 10% co-firing rates. This eliminates the energy and cost penalties of drying, making the process more economically attractive.
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