By Steve Blankinship,
A New Hampshire technology company has developed a new combined-cycle power plant designed to produce ultra-low emissions compared to conventional combined-cycle gas plants (see table). NovelEdge Technologies, in conjunction with heat recovery steam generator (HRSG) manufacturer Nooter/Eriksen and duct burner manufacturer Coen Company Inc., are collaborating to bring the new design to market.
The cycle differs from the conventional cycle in which gas turbines produce most of the power. In the new design, for a 2×1 arrangement, a large steam turbine, rated nominally in the 450 MW range, produces most of the power. The key is a significant amount of continuous duct firing.
“A typical combined cycle usually has three pressures of steam — high pressure (HP), intermediate pressure (IP) and low pressure (LP),” says William Rollins of NovelEdge. “What our new design does is use a single pressure HRSG and the large water heating section in lieu of IP and LP steam. To optimize the cycle you need to use a generous amount of supplemental firing. We operate the duct burners on a continuous basis, whether the gas turbines are at 50 percent load or 100 percent load. In addition, you need to control the rate of supplemental firing to achieve the exact amount of water flow, meaning feed water, through the back end of the HRSG.” A paper on the low-emissions potential of this technology will be presented at POWER-GEN International.
The conventional HRSGs act in a heat recovery mode only, making steam from the exhaust gas energy they are supplied. Once the gas turbines achieve full power, and more power is still needed, the duct burners, if supplied, are fired. “So the duct burners are only operating in this narrow band of operation, such as with a conventional 2×1 configuration, from perhaps 525 MW to 600 MW,” says Rollins. “In the new configuration, the duct burners run all the time, for example, from 375 MW to 750 MW on a new technology 2×1 arrangement”.
Coen has recent field data and CFD analysis showing that under the new design, the duct burners will actually incinerate emissions from the gas turbine. That means that each HRSG stack will be very clean, producing very few emissions in a correct setup. Considering that the technology allows a 2×1 plant to produce 50 percent more power than a conventional 2×1 with half the level of emissions in the stack, the design has the potential to reduce criteria pollutants such as CO, volatile organic compounds and carbon particulates by a factor of three, on a tons/year basis. Similar results are anticipated for other emissions such as unburned hydrocarbons (UHC) and formaldehyde.
“In order to accomplish these levels of emissions reduction,” says Rollins, “the duct burner exit temperature — the bulk exit temperature of the exhaust gasses out of the duct burners — can’t be below 1250 F. That’s because the ignition temperature of carbon monoxide and methane is about 1200 F. You want to stay above lower temperatures where you can quench out the combustion process.”