Energy Saver: Understand Cogeneration, Part 1

Aug. 11, 2015
Consider installing duct burners to produce incremental steam.

All process plants need electrical power and thermal energy and almost always there is a concurrent demand for both of these energy streams. The ability to use one form of primary fuel (typically, natural gas) to simultaneously provide two forms of energy (thermal and power) is known as combined heat and power (CHP) or cogeneration. One of the most common forms of CHP is use of a natural-gas-fired gas turbine generator (GTG) operating in conjunction with a heat recovery steam generator (HRSG). The GTG provides electrical power and the HRSG provides steam at the required pressure.

Plants also have natural- or fuel-gas-fired boilers that generate steam at required pressure. The energy efficiency of boilers depends on several factors but primarily on the heat recovery equipment, excess-air controls and the boiler load. Most of these boilers have an operating energy efficiency of ~80–85%.

Now let’s take a look at the cogeneration system. The GTG exhausts at ~1,050°F (isotherm) and then heat from that stream is recovered in the HRSG to produce steam. Generally, electrical demand drives the GTG and in most cases the generator operates at full load conditions. This implies that the steam produced in the HRSG is relatively constant and can’t be controlled. There are ways to reduce this steam production rate but they come at a significant penalty and so the CHP system operates base-loaded all the time.

In some CHP systems, upstream of the HRSG, additional burners, known as duct burners, consume incremental fuel (natural gas) to add more thermal energy to the exhaust gases from the GTG. This raises the exhaust gas temperatures significantly and allows the HRSG to produce additional steam at extremely high energy efficiencies — 97–99%, compared to traditional boiler efficiency ranges stated above. This super-efficiency occurs because exhaust gases from the GTG are at a very high temperature already. This is akin to having an air-preheater in a regular boiler that takes the combustion air to a very high temperature; hence, only a small amount of fuel energy is used to heat the combustion air and most of the energy goes into producing steam.

With duct burners, the CHP system’s overall thermal energy efficiency goes up as duct-firing increases. Additionally, fuel and energy cost savings are realized at the boilers because they have to produce a reduced amount of steam. The net result is a win-win situation that decreases the overall operating cost and the amount of on-site emissions, while still providing the same amount of power and steam.

Most manufacturers of CHP systems offer duct burners as a standard option; the units may make the initial cost of the system marginally higher. Many of these duct burners may not have significant modulating capabilities, but I have seen some duct burner systems (with modern state-of-the-art controls) that adjust to meet steam demand in the plant. There also have been occasions where I have seen duct burners retrofitted in a CHP system because the system loads changed and, instead of adding a new boiler, the plant opted to add duct burners to its CHP system. These are design-level changes and will require a thorough investigation with feasibility studies and detailed engineering. Remember, the HRSG is a heat exchanger; so you must check if its existing heat-exchange surface area will suffice or requires augmenting to provide the incremental steam generation.

Note, regular maintenance and upkeep of the HRSG will play a cardinal role in steam generation. Water treatment also is very important and should not be neglected. Perform periodic thermographic analysis on the HRSG to identify any hot spots and insulation breakdown in the HRSG. (For more on thermographic analysis, see: “Use Thermal Imagery for Process Problems.”)

So, as you optimize operation of your cogeneration systems, investigate installation of duct burners. Make an effort to understand the operation of duct burners and use them effectively for incremental steam production at the highest efficiency.

Riyaz Papar, PE, CEM, is director, Global Energy Services, at Hudson Technologies Company, Pearl River, N.Y. He has more than 20 years of experience in industrial energy systems and with best practices. He also is a U.S. Department of Energy (DOE) Steam Best Practices senior instructor and a DOE steam energy expert. He has provided energy consulting services in 100+ industrial plants in the U.S. and internationally. You can email him at [email protected].

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