污水處理廠設置沼氣熱電共生系統之財務效益研究

Abstract

目前我國公共污水處理廠之處理程序大部分採二級處理，部分處理水量較大之廠別則採一級處理，無論在一級或二級處理程序中，經初沈池或二沉池之沉降程序後皆會產生污泥，而一般污水處理廠在污泥產生後通常經過濃縮及脫水之程序，將污泥含水率降至80%後即委託外運處理，其污泥處理費經常佔整體操作營運費之40%以上，然而污泥本身尚含有豐富之有機質，如可透過厭氧處理程序產生沼氣，並再利用沼氣加以發電利用，則對於污水處理廠之經濟性將有提升之可能。 本研究以我國北部某污水處理廠(本研究稱A污水廠)為基礎進行沼氣發電系統之經濟效益評估，探討在既有處理量體及處理程序之條件下，設置沼氣發電系統對於整體污水處理廠營運上之財務效益。研究內容主要將沼氣發電系統區分為兩種，分別為往復式內燃機(Reciprocating Engines)以及微渦輪發動機(Microturbines)兩種形式之熱電共生(CHP)系統，並依據其相異之特性參數、設置成本、營運成本等進行財務分析，財務評估期程部分，工程興建期為1年，營運年期為20年，共計21年期，其中財務指標分析項目包含淨現值(NPV)、內部報酬率(IRR)、回收年期(PB)、自償率(SLR)以及敏感性分析。 研究結果顯示，採傳統往復式內燃機機CHP系統，且沼氣優先使用於CHP系統進行發電，並將其產生之電力躉售予再生能源售電業，CHP系統產生之熱能再應用於污泥厭氧槽持溫以及作為其他設施之熱源方案(S2A-RE方案)為最優財務效益方案，計畫NPV為283,966仟元，計畫內部報酬率(IRR)為23.28%，計畫回收年期(PB)為5.1年，自償率(SLR)為232.09 %；惟同樣將沼氣優先使用於CHP系統進行發電，但產生之電力做為廠內用電使用時，無論採往復式內燃機機或微渦輪發動機CHP系統(S2B-RE/MT方案)皆不具財務可行性(計畫NPV小於0)。 將沼氣優先使用於厭氧消化系統持溫以及污泥乾燥機，其餘沼氣再送入CHP系統發電方案下(S1方案)，無論採何種發動機形式，以及CHP系統產生之電力為躉售或廠內自用，皆具備財務可行性，且採往復式內燃機機CHP系統、產生電力採躉售方案(S1A-RE)下具有最佳財務效益，計畫NPV為145,084仟元，計畫內部報酬率(IRR)為34.44%，計畫回收年期(PB)為3.86，自償率(SLR)321.41 %。 此外在敏感性分析成果部分，營運收益為影響整體沼氣發電系統財務效益之最大因素，營運成本則為最低；納入進流污泥溫度變化、發動機負載率對於發電效率的影響因子後，S1B-RE方案之計畫NPV下降32.25 %，S1B-MT方案計畫NPV下降7.52 %，RE方案的影響大於MT方案。

The majority of public wastewater treatment plants (WWTPs) in Taiwan adopt the secondary treatment process, while some larger plants use the primary treatment process. In either primary or secondary treatment process, the sewage sludge will be generated from primary clarifiers or secondary clarifiers by the separation of the solid from liquid. Typically, WWTPs are equipped with sludge thickener and sludge dewatering machine to decrease the moisture content of the sludge to around 80%. Then the sludge is sent to the incinerator for disposal after the sludge is dehydrated. The treatment costs of sludge often account for more than 40% of the overall operating costs of WWTPs. However, the sludge is rich in organic matter so that can be utilized through anaerobic digestion process to produce biogas, which can be further utilized for power generation. This could potentially benefit the economics of WWTPs. This research is the economic evaluation of a biogas power generation system based on a WWTP (referred to as Plant A) in the northern part of Taiwan. It aimed to explore the financial benefits of implementing a biogas power generation system under the existing treatment capacity and processes. The research primarily focused on two types of biogas power generation systems: reciprocating engines and microturbines as combined heat and power (CHP) systems respectively. Financial analysis was conducted based on its distinct parameters, installation costs, and operating expenses. The financial evaluation period consisted of a 1-year construction phase and a 20-year operational phase, totally 21 years. The financial performance indicators analyzed included net present value (NPV), internal rate of return (IRR), payback period (PB), self-liquidating ratio (SLR), and sensitivity analysis. The results of this study revealed that the most financially beneficial option was the implementation of a traditional reciprocating engine CHP system, where biogas was prioritized for electricity generation in the CHP system and the generated power was sold to the renewable energy grid. The heat energy produced by the CHP system was utilized for maintaining the anaerobic digestion tank temperature and as a heat source for other facilities (referred to as the S2A-RE scheme). This scheme yielded a projected NPV of 283,996 thousand TWD, an IRR of 23.28%, a PB of 5.1 years, and an SLR of 232.09%. However, when the biogas was used for internal electricity consumption, both reciprocating engines and microturbine CHP systems (referred to as the S2B-RE/MT scheme) were financially unfeasible (NPV less than 0). With the scheme where biogas was prioritized for maintaining anaerobic digestion system temperature and sludge dryer, and with the excess biogas channeled into the CHP system for power generation (referred to as the S1 scheme), both reciprocating engines and microturbine CHP systems demonstrated financial viability, whether the generated electricity was sold or consumed internally. Among these options, the reciprocating engine CHP system with power sold (S1A-RE) exhibited the best financial performance, with a projected NPV of 145,084 thousand TWD, an IRR of 34.44%, a PB of 3.86 years, and an SLR of 321.41 %. Furthermore, with the sensitivity analysis, project revenue was identified as the most influential factor affecting the overall financial performance of the CHP system, while operating costs were found to have the lowest impact. Considering the factors of temperature variation in the influent sludge and Partial-Load on electricity efficiency, the S1B-RE scheme shows a decrease of 32.25% in the NPV, while the S1B-MT scheme shows a decrease of 7.52% in the NPV. This indicates that the impact of the CHP system adopt RE is greater than the MT.