Designing Cogeneration Engines to Maximize Energy Efficiency: Focusing on Heat Recovery

Biogas fueled combined heat and power (CHP) has been a cornerstone of resource recovery and renewable energy generation at wastewater plants for over 50 years. However, In the past decade, however, making the case for new CHP projects within the US has become increasingly difficult due to historically low energy prices and a scarcity of O&M labor. CHP remains in the interest of many utilities as an excellent way to achieve sustainability goals, but thoughtful and creative designs are needed to make projects work economically. By focusing on heat recovery aspects of CHP, a project to utilize biogas can also serve as an opportunity for a fundamental heating system upgrade at a WRRF while also providing positive economics. This paper provides insight into CHP design strategies that can be used to drive projects forward in a financially viable way, with a focus on the heat recovery portion instead of the more typical focus on electric power generation. Waste heat recovery is often an afterthought in most cogeneration discussions, but in reality it can be the tipping point for positive economics. It also is one of the most site specific and engineering intensive aspects of CHP design. Understanding the types, quality, and amount of waste heat to be generated and matching these sources to existing plant heat demands, hot water loops, boiler feeds, condensate return systems, and controls is a crucial part of the design process. The supporting case study for this paper is a recent CHP design by the Columbus Division of Sewerage and Drainage (DOSD) in Columbus, Ohio. DOSD is currently installing two 1.4 MW reciprocating CHP engines at its Jackson Pike WWTP which treats an average influent flow of 68 MGD. The plant's existing heating systems use steam for heating the water loops that ultimately heat their digesters as well as for space heating for buildings (Figure 1). There are three sets of boilers installed in different locations at the plant, all installed under different contracts and in different eras. This resulted in various steam and condensate systems that are not highly integrated and require the plant staff to operate multiple heating systems at once. The desire to implement CHP was driven largely by city wide sustainability goals, but DOSD staff also recognized the opportunity that a CHP project presented to unify and optimize their core heating systems. Arcadis was engaged to design the CHP system while focusing on developing heat recovery solutions that integrated into the existing plant systems, optimized energy recovery, and offset future capital projects by upgrading heating infrastructure. The design included the following elements: - Jacket water heat exchangers to deliver medium-grade heat to existing digester heating water loops. - Reduction of flow and temperatures of existing digester heating loops to maximize heat transfer from CHP. - Heat Recovery Steam Generators (HRSGs) to recover high-grade exhaust heat and produce steam for space heating. - Repurposing oversized digester heating boilers to service the entire plant steam system as a backup to CHP. - New cross-connection between two existing steam headers to unify systems. -Reconfiguration of condensate returns and centralization of deaeration equipment. - Retirement of aging building heating boilers while eliminating a capital project to replace them. The amount of CHP engine heat rejected into the jacket water loop was sufficient to heat the digesters even in the most demanding winter scenario (Table 1). However, in order to maintain the similar hot water supply temperatures into the sludge heat exchangers, a flow reduction in the primary hot water supply circuit was required. The plant staff experimented during the colder months of 2019/2020, changing pumps speeds and hot water circulation rates in their supply loop while monitoring digester temperatures. It was found that the primary hot water supply flow could be reduced by 25% while maintaining hot water supply temperatures at each sludge heat exchanger. The new reduced flow operation allowed a greater temperature rise across CHP heat exchangers and ultimately more effective heat recovery while also reducing pumping energy. Existing steam to hot water converters in the digester heating loop will also be maintained for backup when the CHP system is offline. HRSGs installed on the engine exhaust will be used to provide additional heat recovery and inject steam into existing building heating systems. These HRSGs needed to fit into an abandoned venturi scrubber building (Figure 2) which was being repurposed to house the new CHP system and eliminate the need for constructing new buildings or containers. An additional layer of complexity for this site is that Jackson Pike WWTP is a site with historically significant and preserved building architecture, so any building modifications needed to minimal and/or in line with historical standards. The HRSGs were designed in a vertical configuration to make better use of the tall but narrow building orientation. Existing building mezzanine grating, preserved to the largest extent possible, provide access to the exhaust system components reaching upwards towards the building roof. Figures 3 and 4 illustrate the final exhaust piping design work that delivered an intricate layout solution in the existing space while adhering to all back-pressure, acoustical, and accessibility constraints. Analysis of existing digester boiler capacity and actual digester heating loads revealed that the digester boiler system was oversized by approximately 5X. In contrast, the existing building space heating boilers were being operated a maximum capacity every winter with no redundancy at all. These boilers were also of 1970s vintage and in need of replacement with a capital project. A new steam cross connection was designed between the digester heating boilers with excess capacity and the aging and overtaxed building heating boilers. This will allow the building heating boilers to be retired without replacement, eliminating the need for a boiler replacement capital project which was counted as a capital credit to the cogeneration project economics. The existing digester heating boilers will now serve as a single backup system to heat the entire plant when the CHP engines are offline. A key finding during design was that unifying heating systems did not require large amount of capital spending, but it did require a large amount of engineering effort and brainpower. This is perhaps best exemplified in the condensate collection and return systems that needed to be reconfigured and reconceived. Currently, three different condensate return systems are dispersed throughout the plant and only return water to their dedicated boilers. Under the CHP project, all condensate returns will be rerouted to a centralized deaerator which will be equipped with sophisticated boiler feedwater pumping controls that deliver the feedwater to the various boiler systems as needed including the HRSGs installed with CHP (Figure 5). Table 2 shows the economic analysis of the CHP project with the above mentioned advanced heat recovery design approaches incorporated. The economic benefits from maximizing the amount of useable heat were significant. The CHP system could heat the entire digester systems while also providing 61% of the building heating in winter (on top of generating ~3MW in green power). This led to a minimized about of natural gas consumption needed to heat the balance of the plant and significantly brought down O&M spending. Also, by avoiding a capital project to replace existing building boilers, a capital credit was applied to the CHP project which played a considerable role in the financial success of the project. These heat recovery benefits served as the tipping point for the CHP project become financially viable while also providing a pathway for the plant to upgrade its core heating infrastructure. The bidding was executed in December 2020 with construction award expected in early 2021. By using heat recovery to help make CHP viable, DOSD and the City of Columbus now have a financially and operationally sensible project to make significant achievements towards city wide GHG reduction goals.