Gas Engine Cogeneration System with Steam from Hot Wastewater

Tokyo Gas Co., Ltd. Mitsubishi Heavy Industries, Ltd. Miura Co. Ltd.  and Kobe Steel, Ltd. have developed a cogeneration system that recovers hot wastewater with high efficiency from a gas engine in the form of steam. Mitsubishi Heavy Industries and the Tokyo Gas Group plan to begin selling this system in July 2015.

A gas engine cogeneration system uses a gas engine to drive a power generator to generate electricity, while the system's waste heat found in the exhaust gas and the coolant of the gas engine are used as steam and hot water. Such systems have been widely introduced in office buildings, hotels, factories, and the like.

Of such locations where these systems have been introduced, medium- to large-sized gas engine cogeneration systems are most often introduced to factories where the steam can be widely used in heating, sterilization, and other production processes. However, in some cases where the use of hot water is limited, not all of the hot water can be used, and the utilization ratio of waste heat is thus decreased.

This system combines products marketed by the three manufacturers and can use the waste heat of a gas engine to efficiently convert hot water into steam.

More specifically, the new system combines 1) a "high-temperature specification gas engine" (manufactured by Mitsubishi Heavy Industries) that heats wastewater to a higher temperature to recover more steam; 2) a "steam generator that uses the heat of wastewater" (manufactured by Miura) to efficiently convert hot wastewater into low-pressure steam; and 3) a "compact screw-type steam compressor" (manufactured by Kobe Steel) that further pressurizes the converted low-pressure steam to make it suitable for use in factory production processes. By combining these technologies, improvements were made to the high-temperature specification gas engine, and controls were developed for the efficient operation of the system.

As a result of this new design, the steam recovery efficiency of this system has improved by approximately 10% compared with that of the system with only an exhaust gas boiler, and a combined efficiency of approximately 71% for power generation and steam use has been realized. Consideration was also given to convenience so that the settings can be changed in accordance with the usage conditions. For example, when the required amount of steam decreases, the user can switch to the operation of a conventional gas engine cogeneration system.

Tokyo Gas, Mitsubishi Heavy Industries, Miura, and Kobe Steel are endeavoring to achieve further energy savings and CO2 reductions through the utilization of cogeneration technology in the future as well.

[ System configuration 1] 1 Typically, two sets of steam generators that use heat from hot wastewater and compact screw-type steam compressors are installed.

[Key points in the system development]
(1) "18KU30GSI" high-temperature specification gas engine
The specifications of a gas engine released in February 2009 by Mitsubishi Heavy Industries with a power generation output of 5,750 kW were partially modified through joint developments with Tokyo Gas. Although the temperature of coolant in a gas engine is generally approximately 90°C, the temperature setting was changed to 120°C to convert more of the waste heat into steam for use. In conjunction with this setting change, some of the gas engine parts, as well as the structure and configuration of the auxiliary equipment, were changed to support the higher temperature specification, and performance and endurance tests were then conducted.

(2) "VS-400M" steam generator that uses heat from hot wastewater
Developed jointly by Miura and Tokyo Gas and released in April 2013, this equipment efficiently converts unused wastewater discharged from factory production processes and the like into steam. When this was completely integrated with this system, it was equipped with a proportional control function to enable steam generator operation in accordance with the gas engine load, as well as quantity control to efficiently control multiple units, thereby enabling the steam generator to track changes in the gas engine load or changes in the load of the used steam.

(3) "Steam Star MSRC160L" compact screw-type steam compressor
A compact screw-type steam compressor was launched by Kobe Steel in 2011. This compressor can pressurize low-pressure steam, which was often disposed in the past because of difficulties with re-use, to approximately 0.8 MPa. Kobe Steel and Tokyo Gas jointly developed this steam compressor for integration with the entire system and made necessary programming changes to ensure smooth linkage with other devices configuring the system and changes to support universal air pressure. Durability testing for the mitigation of water quality criteria with respect to chlorine concentration was also conducted, and some specifications were adopted to support various water qualities.