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Techno-Economic Analysis of Scenarios on Energy and Phosphorus Recovery from Mono- and Co-Combustion of Municipal Sewage Sludge
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0002-6141-7796
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0002-2319-0906
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Energy Science.ORCID iD: 0000-0002-4597-4082
2022 (English)In: Sustainability, E-ISSN 2071-1050, Vol. 14, no 5, article id 2603Article in journal (Refereed) Published
Abstract [en]

This study evaluates the techno-economic feasibility of energy and phosphorus (P) fertilizer (PF) recovery from municipal sewage sludge (MSS) through incineration in new combustion plants. We evaluated the economic impact of five critical process design choices: (1) boiler type, (2) fuel (MSS mono-combustion/co-combustion with wheat straw), (3) production scale (10/100 MW), (4) products (heat, electricity, PF), and (5) ash destination. Aspen Plus modeling provided mass and energy balances of each technology scenario. The economic feasibility was evaluated by calculating the minimum selling price of the products, as well as the MSS gate fees required to reach profitability. The dependency on key boundary conditions (operating time, market prices, policy support) was also evaluated. The results showed a significant dependency on both energy and fertilizer market prices and on financial support in the form of an MSS gate fee. Heat was preferred over combined heat and power (CHP), which was feasible only on the largest scale (100 MW) at maximum annual operating time (8000 h/y). Co-combustion showed lower heat recovery cost (19–30 €/MWh) than mono-combustion (29–66 €/MWh) due to 25–35% lower energy demand and 17–25% higher fuel heating value. Co-combustion also showed promising performance for P recovery, as PF could be recovered without ash post-treatment and sold at a competitive price, and co-combustion could be applicable also in smaller cities. When implementing ash post-treatment, the final cost of ash-based PF was more than four times the price of commercial PF. In conclusion, investment in a new combustion plant for MSS treatment appears conditional to gate fees unless the boundary conditions would change significantly.

Place, publisher, year, edition, pages
MDPI, 2022. Vol. 14, no 5, article id 2603
Keywords [en]
municipal sewage sludge, energy recovery, phosphorus recovery, techno-economic analysis, mono-combustion, co-combustion
National Category
Energy Systems Environmental Management
Research subject
Energy Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-89866DOI: 10.3390/su14052603ISI: 000770990400001Scopus ID: 2-s2.0-85125328366OAI: oai:DiVA.org:ltu-89866DiVA, id: diva2:1647306
Funder
Swedish Research Council Formas, 2018-00194Bio4EnergySwedish Energy Agency
Note

Validerad;2022;Nivå 2;2022-03-25 (hanlid)

Available from: 2022-03-25 Created: 2022-03-25 Last updated: 2024-04-15Bibliographically approved
In thesis
1. Integrated sewage sludge treatment scenarios – techno-economic analysis on energy and phosphorus recovery
Open this publication in new window or tab >>Integrated sewage sludge treatment scenarios – techno-economic analysis on energy and phosphorus recovery
2022 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Sewage sludge is a by-product of wastewater treatment that simultaneously gathers contaminants, valuable organic matter, and nutrients. The treatment of the increasing amount of sewage sludge is important from both pollution prevention and resource recovery perspectives as i) large shares of mineral phosphorus, listed as a critical raw material, terminate in the sewage sludge, and ii) energy recovery from sewage sludge can cover the energy-intensive demand of the treatment process. Previous research has identified sewage sludge combustion as a suitable treatment approach as it both addresses contaminant destruction and paves the way for efficient phosphorus recovery from the sewage sludge ash. The commercial development of this practice has, however, been slow. Therefore, this thesis aims to investigate the challenges in sustainable sewage sludge management, and to, in more detail, identify the economic viability of energy and phosphorus recovery from sewage sludge through combustion.

The thesis’ aim is divided into two objectives addressed in three papers. First, to investigate how different aspects of sewage sludge management, such as contaminants, economic efficiency, technical aspects, and legislation, evolve and interact. This has been done by a review of sewage sludge management research over fifty years (Paper I). Second, to investigate the economic viability of simultaneous energy and phosphorus recovery from sewage sludge by comparing different technology and market scenarios. This has been done for i) new sewage sludge mono-/co-combustion plants (Paper II), and ii) the integration of treatment technologies, mainly anaerobic digestion, hydrothermal carbonization, and combustion, in an existing wastewater treatment plant (Paper III). 

Results from the analysis of sewage sludge management research (Paper I) show a narrow-focused perspective that often excludes inseparable aspects such as combination of economic consideration and advanced extraction technology. The investment viability of a new mono-/co-combustion of sewage sludge (Paper II) is highly conditional on heat, electricity, and fertilizer price, and external financial support is often a crucial requirement. Sewage sludge co-combustion with potassium-rich biomasses improves sewage sludge quality and forms usable ash as fertilizer without further need for phosphorus recovery technology. In this case, the economic feasibility of the process is independent of usable ash revenue, which stimulates a competitive selling price for the ash, thereby improving the marketing of sludge-based fertilizer. Avoided disposal costs of sewage sludge for a retrofitted wastewater treatment plant by introducing hydrothermal carbonization (Paper III) shows good economic feasibility while recovering phosphorus. Integrating anaerobic digestion, hydrothermal carbonization, and combustion may also improve investment incentives by improving energy outputs and phosphorus recovery. The economic feasibility is contingent on product (hydrochar, heat, electricity) prices and sensitive to added equipment costs, and costs for sludge transportation and disposal. 

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2022
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
Keywords
Sewage sludge management, techno-economic analysis, phosphorus recovery, thermochemical treatment
National Category
Energy Systems
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-93662 (URN)978-91-8048-196-0 (ISBN)978-91-8048-197-7 (ISBN)
Presentation
2022-12-16, A117, Luleå tekniska universitet, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2022-10-21 Created: 2022-10-20 Last updated: 2023-09-05Bibliographically approved
2. Sewage Sludge Treatment Scenarios: Techno-Economic Analyses of Energy and Phosphorus Recovery Focusing on Implementation Challenges
Open this publication in new window or tab >>Sewage Sludge Treatment Scenarios: Techno-Economic Analyses of Energy and Phosphorus Recovery Focusing on Implementation Challenges
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Managing sewage sludge, an inevitable by-product of wastewater treatment processes rich in both contaminants and valuable resources, presents a dual challenge: ensuring pollution prevention by immobilizing or destroying contaminants, and facilitating resource recovery. Balancing these objectives is critical given the growing volumes of sewage sludge and the imperative to both protect the environment and recover valuable resources. The unknown risks of land application of sludge, the currently most common disposal method, make thermal conversion a promising alternative, as it enables energy recovery, the breakdown of potentially harmful organic compounds, and the formation of volume-reduced, sanitized products. Despite the technical feasibility, the commercial development of advanced recovery technologies has been slow. 

This thesis aims to expand knowledge on different sewage sludge treatment and disposal scenarios under varying conditions; thereby shedding light on implementation challenges, local opportunities, and the financial dynamics critical for phosphorus and energy recovery from the perspectives of wastewater treatment plants, investors, and policymakers. The aim is primarily addressed by performing techno-economic analysis of sewage sludge treatment scenarios, covering the entire sludge treatment process from sludge treatment to end products and disposal (Paper II-V). The techno-economic analysis is supplemented by a review of academic research on sewage sludge management from 1971 to 2019 (Paper I). 

Results from the analysis of sewage sludge management research (Paper I) show a narrow-focused perspective that often misses the broader, interconnected aspects of sewage sludge management, leading to research that, while detailed, fails to capture the complexity of the field. The investment viability of a new mono-/co-combustion plant for sewage sludge (Paper II) is highly conditional on heat, electricity, and fertilizer prices, and external financial support is often a crucial requirement. Cocombustion of sludge (in low ratios) with K-rich agricultural biomass requirement in and energy demand of a thermal dryer, and by yielding ash that contains phosphorus in a plant-available form. Utilizing existing heat facility (Paper III) and co-combustion to mitigate investment costs and energy demand in sludge management showed the potential to offer a cost-effective alternative to land application. However, the viability of co-combustion hinges on both a high heat market price (Paper II) and the proximity of affordable biomass resources (Paper III). Without these conditions, co-combustion may increase the financial burden of sludge management on wastewater treatment plants and policymakers. 

Retrofitting a wastewater treatment plant by integrating hydrothermal carbonization in sludge treatment (Paper IV), demonstrated good economic feasibility, primarily due to the avoided disposal costs, while also recovering phosphorus. However, integrating hydrothermal carbonization in a system designed for a thermal dryer may cause a significant reduction in electricity production.

Given the high moisture content of sludge and the low market prices for fertilizer, the potential revenue from energy and phosphorus recovery is inadequate to solely drive investment in advanced sewage sludge treatment technologies (Papers II-V). This issue is exacerbated by the fact that most wastewater treatment plants are small in scale. Collaborative sludge management across neighboring wastewater treatment plants (Paper V) increases phosphorus recovery capacity and leverages economies of scale, fostering investment in advanced technology. This strategy presents a significant opportunity to lower the treatment costs and offers a competitive alternative to land application, while encompassing energy and phosphorus recovery into the sludge treatment.

In conclusion, resource recovery and pollution prevention in sludge management is a complex task that necessitates simultaneous consideration of technical aspects, product quality, site-specific conditions, and profitability to ensure a comprehensive and viable approach. Leveraging local infrastructure and resources in sewage sludge management is crucial, highlighting the ecessity for strategies to be tailored to the local opportunities and limitations. Such an approach outperforms mono-combustion by eliminating the investment ensures that solutions are not only environmentally sustainable but also economically viable and socially acceptable. 

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2024
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
Sewage sludge management, techno-economic analysis, phosphorus recovery, energy recovery, thermochemical treatment
National Category
Energy Engineering Energy Systems
Research subject
Energy Engineering
Identifiers
urn:nbn:se:ltu:diva-105083 (URN)978-91-8048-534-0 (ISBN)978-91-8048-535-7 (ISBN)
Public defence
2024-06-14, A117, Luleå University of Technology, Luleå, 09:00 (English)
Opponent
Supervisors
Available from: 2024-04-15 Created: 2024-04-15 Last updated: 2024-05-24Bibliographically approved

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Bagheri, MarziehÖhman, MarcusWetterlund, Elisabeth

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