Report: Combined Heat and Power can Significantly Alter Energy Consumption and Carbon Emissions for Corn Ethanol Production
|Total net fuel consumption for dry mill ethanol plants, Btu/Gallon. Click to enlarge.|
The adoption of combined heat and power (CHP) in dry mill ethanol plants can reduce total energy use by up to almost 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, according to an updated report by the US Environmental Protection Agency’s (EPA) CHP Partnership.
The analysis only considers the energy consumed in the plant itself; it does not consider the energy consumed in growing, harvesting, and transporting the feedstock corn, or in transporting the ethanol product.
|Total net CO2 emissions for dry mill ethanol plants, lbs/gallon. Click to enlarge.|
The revised report, Impact of Combined Heat and Power on Energy Use and Carbon Emissions in the Dry Mill Ethanol Process, 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).
Natural Gas CHP
Case 1: 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: Gas turbine with power export—Thermal output sized to meet plant steam load without supplemental firing; excess power generated for export.
Case 3: 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.
Coal CHP, Case 4: 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.
Biomass CHP, Case 5: 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. 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.