part 1: overview of cogeneration and its status in asia - Fire

24 Part I: Overview of cogeneration and its status in Asia
2.9 Innovation in Exhaust Gas Heat Recovery
Sites requiring more thermal energy than that is available at the exhaust of reciprocating
engine or gas turbine have the option of adopting post-combustion of oxygen-rich exhaust
gases. For this, either the fuel required by the prime mover or an alternate cheaper fuel may
be employed. New types of burners have been designed in the recent years that can be
operated efficiently to provide the varying thermal energy demand of the site.
The “GRC Induct” type of burners has been specially designed by EGCI Pillard for
combustion of either liquid or gaseous fuels, by making use of the gas turbine exhaust gas
(leaving at around 500°C and 13 per cent of O2 content) as the oxidizing air. Located at the
inlet of the heat recovery boiler, it helps to increase the temperature of the gas turbine
exhaust gas, and thus the overall efficiency of the cogeneration installation. In case the gas
turbine is out of operation, these burners can assure steam generation by making use of cold
inlet air from the surrounding. The heat output per burner can range from 4 to 50 MW.
These burners function equally well on natural gas as well as liquid fuels (light or heavy fuel
oil, residual fuel) or in simultaneous mixed mode. Steam or compressed air assures
pulverization of the liquid fuel. The design based on the GRC LONOxFLAM technology,
assures perfect flame stability, a low-pressure drop and an excellent combustion with low
emissions of unburnts and NOx, thus well within the environmental pollution thresholds set by
the regulation. When there is a combustion zone in the boiler, it is possible to reduce the
oxygen level in the exhaust gas to around three to four per cent for further increasing the
efficiency, while still maintaining the emission of pollutants lower than the norms. 3
For its operation with cold ambient air, the control flaps close a part of the recovery section.
While using heavy fuel oil, a suitable adaptation is necessary for limiting emissions. One of
the main features of the system is the mechanism for quick dismantling which allows to
change the burners during operation by opening the whole frame laterally within 15 minutes.
2.10 Research and Development on Cogeneration Technologies
There has been a steady rise in the efficiency of gas turbines and diesel engines. The inlet
temperature of a large size gas turbine has risen to 1,350ºC and can be expected to reach
1,500ºC in the near future. The thermal efficiency of gas engines has been increasing thanks
to an increase in compression ratio, and the application of pre-chamber lean burn
technologies. These improvements have been made possible mainly due to the progresses
made in cooling, heat-resist materials, turbo machinery and combustion technologies.
Various projects are ongoing to achieve rapid efficiency improvements by the year 2000. 4
These include development of ceramic gas engine and gas turbine that require advanced
technology related to ceramic science. To prove the concept, the Miller cycle gas engine
system is being developed which has a unique intake and exhaust timing mechanism that
allows to power generation efficiency exceeding 35 per cent.
In a ceramic gas engine, ceramic is used as the materials of the combustion chamber to
allow an advanced combustion. Similar to a thermos structure, air gap is provided and
gaskets with low thermal conductivity are placed between the ceramic and metallic parts to
3
Energie Plus, “Lumières et ombres sur la cogénération, No.197, pp. 6, 15 December 1997.
4 M. Motokawa, “R&D efforts for cogeneration technologies with high efficiency”, Proceedings of the
Conference on Natural Gas Technologies: A Driving Force for Market Development, International Energy
Agency, pp. 627-636, Berlin, 1-4 September 1996.