A NGCC power plant, consisting of a single gas turbine and generator (topping cycle), is equipped with a highly efficient Heat Recovery Steam Generator (HRSG) unit designed to capture waste heat from the gas turbine exhaust. Steam produced by the HRSG unit powers a steam turbine and generator (bottoming cycle) to produce additional quantities of electric power.
Use of the otherwise wasted heat in the gas turbine exhaust results in high thermal efficiency compared to other combustion based technologies. With advancements in combined-cycle technology, natural gas plants today are capable of achieving greater than 60% efficiency, compared to standard steam turbine efficiency at around 40%.
According to Environment Canada, approximately 5% (31 million megawatt hours) of all electricity generated in Canada is attributed to the combustion of natural gas, accounting for about 9,500 megawatts of capacity. Boilers account for less than 20% of this capacity whereas gas turbines account for slightly more than 75%. The remaining capacity results from the operation of internal combustion engines. Gas turbine plants fueled by natural gas are considered one of the cleanest fossil fuel-fired options available for electric power generation. Since natural gas contains no ash and practically no sulphur or metals, emissions of these substances are virtually zero. According to the US EPA, the average emissions rates in the United States from natural gas-fired generation are: 1135 lbs per MWh (megawatt hour) of carbon dioxide, 0.1 lbs/MWh of sulfur dioxide, and 1.7 lbs/MWh of nitrogen oxides. Compared to the average air emissions from coal-fired generation, natural gas produces half as much carbon dioxide, less than a third as much nitrogen oxides, and one percent as much sulphur oxides at the power plant.
EXHAUST GASES composition by volume (% v/v)
|Exhaust Gas||Air (Wet)||Coal Firing||Biomass
|Nitrogen N2 (mw 28)||77.3||60 to 73||c.66||72||66||74 to 78||77 to 78|
|Oxygen O2 (mw 32)||20.7||5 to 6||c.8||0||10||14 to 14||15|
|Carbon Dioxide CO2 (mw 44)||0.039||11 to 12||c.10||14||12||3 to 5||4|
|Water Vapor H2O (mw 18)||1.0||10 to 12||c.15||13||11||5 to 6||2 to 3|
|Argon Ar (mw 40)||0.93||1||1||1||1||1||1|
|Carbon Monoxide (mw 28)||0||0||0||1||0||0||0|
Methane, a primary component of natural gas and a harmful greenhouse gas (GHG), can also be emitted into the air when natural gas is not burned completely.
Similarly, methane can be emitted as the result of leaks and losses during transportation. Emissions of sulfur dioxide and mercury compounds from burning natural gas are negligible. The CO2 and NOX emissions from a NGCC plant, are significantly reduced compared to those produced by coal power plants with relatively the same power output. This is because of the higher heat content of natural gas, the lower carbon intensity of gas relative to coal, and the higher overall efficiency of an NGCC plant relative to a coal-fired plant. For these reasons and for economic reasons, an NGCC power plant is the best option for a standalone, utility scale, natural gas plant.
Although relatively new since 2009, there are several companies which have developed utility-scale CO2 scrubber technologies, such as Siemens, RWE and Fluor. (GE is also working on developing its own CO2 scrubber technology involving the use of amino silicones.) About the same size as the power plant, the cost of a CO2 scrubber system to date can nearly double the cost of a standard NGCC plant. This has tended to impede the wide-scale adoption of CO2 scrubbers in the highly competitive power industry. Because NGCC plants are known to be the cleanest of the thermal power plant technologies burning fossil fuel, there has been little incentive for NGCC plant operators to spend the extra money to add the CO2 scrubbers.
Other projects now under development by 4 SKIES, utilizing a large-scale NGCC plant for utility scale power generation with CO2 scrubbing technology, have so far indicated that there is sufficient scale within the vertical farm operation to absorb the 10% of CO2, which most scrubbing technologies cannot remove economically, and generate more than sufficient revenues from power sales and organic produce sales to justify the additional cost of the scrubbers. In each project now under development by 4 SKIES, revenues from organic produce exceeds revenues from power sales by almost 50%, and more than double when comparing their contribution to EBITDA, making Tri-Gen-Zero™ an extremely viable option.