Report: CHP to power corn ethanol production boosts energy balance of fuel, reduces emissions
One of the reasons why Brazil's sugarcane ethanol has such a strong energy balance and low carbon emissions profile is to be found in the fact that biomass (bagasse) is used as the energy source to power the production of the fuel. The renewable biomass cogenerates both heat and power to the biofuel plant, with excess electricity sold to the grid. Now a report by the US Environmental Protection Agency’s (EPA) CHP Partnership shows that the adoption of combined heat and power (CHP) can achieve similar benefits in dry mill corn ethanol plants. CHP can reduce total energy use by up to 55% over state-of-the-art dry mill ethanol plants that purchase central station power, and can result in negative net CO2 emissions depending upon the fuel type used and CHP configuration.
The revised report, 'Impact of Combined Heat and Power on Energy Use and Carbon Emissions in the Dry Mill Ethanol Process' [*.pdf], includes updated data on energy consumption and carbon dioxide emissions for state-of-the-art dry mill ethanol plants fueled by natural gas, coal, and biomass with and without CHP systems.
Dry milling has become the primary production process for corn ethanol. In the process, whole dry kernels are milled and sent to fermenters where the starch portion is fermented into ethanol. The remaining, unfermentable portions are produced as distilled grains and solubles (DGS) and used for animal feed.
Most dry mill ethanol plants use natural gas as the process fuel for raising steam for mash cooking, distillation, and evaporation. It is also used directly in DGS dryers and in thermal oxidizers that destroy the volatile organic compounds (VOCs) present in the dryer exhaust.
Although new plants use only about half of the energy used by the earliest ethanol plants, the rising price of natural gas is pushing the industry to explore other means to cut energy consumption, or to switch from natural gas to other fuels such as coal, wood chips, or even the use of DGS and other process byproducts.
The report evaluated five CHP system configurations and compared them to three base-case non-CHP ethanol plants (powered by natural-gas, coal and biomass).
In all cases, fuel consumption at the plant increases with the use of CHP. However, total net fuel consumption is reduced, as electricity generated by the CHP systems displaces less efficient central station power (graph 1, click to enlarge). In the two natural gas CHP cases with excess power available for export (Cases 2 and 3), the displaced central station fuel represents a significant credit against increased fuel use at the plant. The total fuel savings for Cases 2 and 3 are 44 percent and 55 percent, respectively, over the natural gas base case.
Total CO2 emissions are reduced for all CHP cases compared to their respective base case plants. Total net CO2 emissions in Case 2 represent an 87% reduction compared to the natural gas base case. Total plant CO2 emissions for Case 3 are actually less than the displaced central station emissions, resulting in a negative (-0.71 pounds per gallon) net CO2 emissions rate compared to the base case. The lowest net CO2 emissions at the plant are obtained when biomass is used as the fuel (graph 2, click to enlarge).
The report is set to undermine many of the critiques leveled against corn ethanol. Some have argued that the biofuel - as it is currently produced - requires more energy to produce than consumers get out of it and that it doesn't contribute much to lowering carbon emissions. But when efficient cogeneration is used as the power and heat source to drive processes at the plant, both the energy and GHG balance of the fuel improves considerably.
Add the fact that in the future CO2 from the fermentation of ethanol will be captured and geosequestered, and the carbon balance of the fuel improves still further. Just recently, the US Department of Energy announced it is funding such a project.
References:
EPA CHP Partnership: Impact of Combined Heat and Power on Energy Use and Carbon Emissions in the Dry Mill Ethanol Process [*.pdf] - November 2007.
Biopact: Towards carbon-negative biofuels: US DOE awards $66.7 million for large-scale CO2 capture and storage from ethanol plant - December 19, 2007
The revised report, 'Impact of Combined Heat and Power on Energy Use and Carbon Emissions in the Dry Mill Ethanol Process' [*.pdf], includes updated data on energy consumption and carbon dioxide emissions for state-of-the-art dry mill ethanol plants fueled by natural gas, coal, and biomass with and without CHP systems.
Dry milling has become the primary production process for corn ethanol. In the process, whole dry kernels are milled and sent to fermenters where the starch portion is fermented into ethanol. The remaining, unfermentable portions are produced as distilled grains and solubles (DGS) and used for animal feed.
Most dry mill ethanol plants use natural gas as the process fuel for raising steam for mash cooking, distillation, and evaporation. It is also used directly in DGS dryers and in thermal oxidizers that destroy the volatile organic compounds (VOCs) present in the dryer exhaust.
Although new plants use only about half of the energy used by the earliest ethanol plants, the rising price of natural gas is pushing the industry to explore other means to cut energy consumption, or to switch from natural gas to other fuels such as coal, wood chips, or even the use of DGS and other process byproducts.
The report evaluated five CHP system configurations and compared them to three base-case non-CHP ethanol plants (powered by natural-gas, coal and biomass).
Case 1: Natural gas in a gas turbine/supplemental-fired heat recovery steam generator (HRSG)—Electric output sized to meet plant demand; supplemental firing needed in the HRSG to augment steam recovered from the gas turbine exhaust.
Case 2: Natural gas in gas turbine with power export—Thermal output sized to meet plant steam load without supplemental firing; excess power generated for export.energy :: sustainability :: biomass :: bioenergy :: biofuels :: ethanol :: cogeneration :: combined heat and power :: efficiency :: energy balance :: emissions ::
Case 3: Natural gas in a gas turbine/steam turbine with power export (combined cycle)—Thermal output sized to meet plant steam load without supplemental firing; steam turbine added to generate additional power from high-pressure steam before going to process; maximum power generated for export.
Case 4: Coal CHP, High-pressure fluidized bed coal boiler with steam turbine generator—Exhaust from steam-heated DDGS dryer integrated into the boiler intake for combustion air and VOC destruction.
Case 5: Biomass CHP, High-pressure fluidized bed biomass boiler with steam turbine generator—Exhaust from steam-heated DDGS dryer integrated into the boiler intake for combustion air and VOC destruction:
In all cases, fuel consumption at the plant increases with the use of CHP. However, total net fuel consumption is reduced, as electricity generated by the CHP systems displaces less efficient central station power (graph 1, click to enlarge). In the two natural gas CHP cases with excess power available for export (Cases 2 and 3), the displaced central station fuel represents a significant credit against increased fuel use at the plant. The total fuel savings for Cases 2 and 3 are 44 percent and 55 percent, respectively, over the natural gas base case.
Total CO2 emissions are reduced for all CHP cases compared to their respective base case plants. Total net CO2 emissions in Case 2 represent an 87% reduction compared to the natural gas base case. Total plant CO2 emissions for Case 3 are actually less than the displaced central station emissions, resulting in a negative (-0.71 pounds per gallon) net CO2 emissions rate compared to the base case. The lowest net CO2 emissions at the plant are obtained when biomass is used as the fuel (graph 2, click to enlarge).
The report is set to undermine many of the critiques leveled against corn ethanol. Some have argued that the biofuel - as it is currently produced - requires more energy to produce than consumers get out of it and that it doesn't contribute much to lowering carbon emissions. But when efficient cogeneration is used as the power and heat source to drive processes at the plant, both the energy and GHG balance of the fuel improves considerably.
Add the fact that in the future CO2 from the fermentation of ethanol will be captured and geosequestered, and the carbon balance of the fuel improves still further. Just recently, the US Department of Energy announced it is funding such a project.
References:
EPA CHP Partnership: Impact of Combined Heat and Power on Energy Use and Carbon Emissions in the Dry Mill Ethanol Process [*.pdf] - November 2007.
Biopact: Towards carbon-negative biofuels: US DOE awards $66.7 million for large-scale CO2 capture and storage from ethanol plant - December 19, 2007
1 Comments:
The future of U.S. Corn ethanol production is the Poet Voyager ethanol refinery expansion. It will produce "cellulosic" ethanol from corn cobs, and corn fiber, and utilize both biomass, and bio-gas. It will, also, produce corn oil from the germ, and, of course, distillers grains from the corn endosperm.
It will have a "Very Strong" EROEI, and will give us a domestic base to build from.
Poet has a webinar on the process at it's website: Rhapsodyingreen. It's worth the visit. (No, I'm NOT connected with Poet in any way - just an interested observer.)
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