Report: Combined Heat and Power can Significantly Alter Energy Consumption and Carbon Emissions for Corn Ethanol Production
20 December 2007
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| 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.
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| 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.
Resources
Similarly, the heat required and produced during gasification of biomass and coal into hydrogen production can also be recycled into electrical generation, hence greatly improve the efficiency of biomass and coal gasification into hydrogen.
At least one gasification plant for each community can address the local demand for hydrogen in the future hydrogen economy without requiring long-distance transportation of hydrogen that would be too bulky and hence inefficient.
Posted by: Roger Pham | 20 December 2007 at 09:26 AM
You can do the same with a nuke plant, though that would be more controversial and expensive.
__A central (though it can come from multiple independent sources while having centralized distribution ala electric utilities) steam and heat system could also help reduce GHGs from heating/hot water/low temp steam usage in medium and high density areas. Granted, it is expensive to emplace a network of insulated pipes throughout a city, but large portions of infrastructure in many cities are at, or are beyond the century mark. It is time to dig up the streets and replace utilities anyway. Hence, it might make sense for citizens, businesses, planners and municipal officials to consider building and/or expanding/upgrading a waste heat generated low temp steam distribution system.
Posted by: allen_xl_z | 20 December 2007 at 09:30 AM
It would be interesting to see one more chart which shows the relative costs of the various options. The coal-using options look very bad for CO2, but I'll bet they are cheap, at least in the short run.
Posted by: Nick | 20 December 2007 at 09:58 AM
Gulf Ethanol (GFET) A completly new way of production is what will take place as the variable in saving costs.
Posted by: NickW | 20 December 2007 at 10:39 AM
Gulf Ethanol (GFET) A completly new way of production is what will take place as the variable in saving costs.
Posted by: NickW | 20 December 2007 at 10:39 AM
why not use solar heat?
Posted by: itsme | 20 December 2007 at 11:04 AM
Solar heat can be used in a vaccuum assistated distallation column. But it's intermittent.
Posted by: Mark | 20 December 2007 at 11:46 AM
This is a standard practise in the sugarcane ethanol sector. Cane residues (bagasse) are used to generate steam and electricity, which powers the plant. Excess power is sent to the grid.
This makes the energy and GHG balance of sugarcane ethanol very strong.
Posted by: Jonas | 20 December 2007 at 12:17 PM
I've seen these vacuum solar columns aimed for use on domestic rooftops and they're a pretty slick, elegant technology. I wonder if one could set up a hybrid system, using solar as the auxiliary assist, sort of the way the SkySails uses wind to assist the engine. With electronic thermometers and control systems I'd think this might be fairly easy to design.
Posted by: Jim G. | 20 December 2007 at 12:28 PM
I follow your thinking Jim. The vaccum pump needs some form of shaft horsepower and if it were in ICE, the wast heat could be recovered for the distillation process.
Posted by: Mark | 20 December 2007 at 12:45 PM
Why would anyone need unreliable solar when you can use a highly efficient (+80%), renewable (biomass), reliable and clean (biomass) CHP system?
Posted by: Jonas | 20 December 2007 at 01:52 PM
Are they talking about recovering the waste heat to send to steam tunnels to keep the local buildings warm? My university had central heat like that. The thing is, depending on climate, you only need the heat 4-6 months of the year, so this may not be of value for a high fraction of the operating time. It's good for those times. It's good for a campus or downtown environment...though I wonder if that makes it inconveniently far from the ethanol refinery?
Posted by: HealthyBreeze | 20 December 2007 at 02:04 PM
Waste heat can be used in large absorbtion type air conditioning systems common in large buildings.
Posted by: tom deplume | 20 December 2007 at 02:55 PM
Jonas,
I'm still a believer in CHP. Part of my point was that one could supplement the power using solar with some smarts (electronic management) to handle the intermittency (or "unreliability" as you put it) by kicking up the burner when the sun isn't shining; these arrays can't be very expensive to stick these things on the roof of a CHP plant, and, in a future world in which carbon credits are traded, this would decrease the emissions further and enhance the plant's revenue. This is a summary of the technology:
http://en.wikipedia.org/wiki/Solar_thermal_collector#Evacuated_Tube
and a vendor I just googled, which shows a photo:
http://solrheatsystems.com/evacuated_tube_solar_collectors.htm
Posted by: Jim G. | 20 December 2007 at 02:58 PM
Nick. Would the coal option still be cheap if the total cost of getting rid of the pollution created was factored in?
Posted by: Harvey D | 21 December 2007 at 08:26 AM
We may see more energy plants in the future and not just power plants. CHP has made sense for a long time, but putting it all together to get the most out of everything has not been a priority until now.
Posted by: sjc | 24 December 2007 at 09:12 PM