Running a power plant on corn

Well, after reading this link on the overproduction of corn in the US, it got me thinking that maybe all this corn could be used for power instead of coal, oil or natural gas. How cheap would corn have to get in order to do this? Are we even close?

Would this result in a lot of pollution?

Well, you can buy a corn stove today: http://www.harmanstoves.com/harman_corn_stove.htm

A table on this page: http://www.clarksdalewebinfo.com/jsp/user_page.jsp?pageid=4918 indicates that 1,000,000BTU generation with corn costs $3.61 vs. $8.62 with fuel oil.

Quite a lot of gasoline (here in the Midwest anyway) at the pump is 10% ethanol. Ethanol is made from corn.

The problem as I understand it, is that at higher concentrations it is not very good for some parts in your car.

Some power plants at grain millers and grain processors use corn, corn byproducts, and corn husks/stalks in their boilers. This is also done with a wide range of other consumer biomass products, including wheat, straw, sugar cane (bagasse), and even olive pits.

Some people convert the corn to ethanol and utilize that as a fuel as well.

Generally, these options are only economical in the US for those who have a huge amount of the product already shipped to them and ready to burn (such as an ADM-type company). As far as pollution goes, corn is generally a very low-sulfur fuel, and due to its low combustion temperature produces low NOx as well. Its ash/residue is not especially harmful, and in any event is safer than coal ash. It will of course emit CO2, but hopefully it is nearly a “carbon neutral” fuel, and thus that is not a large concern.

I can’t quote an exact number right now, but one study I just finished ended up with the overall cost, including O&M and emissions costs, being about 2 to 3 times that of a good-quality coal. However, this situation only applies to the particular site I was studying, and there will be a range of economic results depending upon the location, transportation, processing, local environmental credits and renewables portfolio objectives, and so forth.

GaryM, I have some disgreement with the natural gas and fuel oil costs shown on that table you linked, but then I’m also thinking large, power-plant scale, not small-scale like they are, so that may be the difference.

Una,

I don’t claim that their figures are correct, although I guess they do, but as you point out scale could easily make a big difference. The fellow buying 200 gal. of fuel oil is going to pay considerably more than the power plant using several million gallons a month. OK, how much oil per whatever would a medium sized plant burn?

I looked at corn stoves several years back. IIRC the suger in the corn caused a “cake” to form in the bottom of the burn basket. This cake had to be removed on a regular basis. So the stove couldn’t be used continuously.

Biomass boilers are fairly common and are technically simple. They’re used a lot in processing plants were the waste would normally be thrown away. The only problem comes with variation in your feedstock (eg, changes in composition or water content) that can lead to some minor issues.

A reason why biomass isn’t used for mainstream generation is that the land footprint to grow the biomass would very high. Some classmates did a study to run a ferry on hemp oil, and they came up with a footprint of a few hundred hectares.

Using Google on the terms “biomass energy economics” will get you some more detailed info. I’m backing Una Persson here, and I think the economics of using biomass usually relies on the fuel being free. Once you include the cost of drying it, processing and the cost of importing fertiliser (the corn husk can be tilled back into the farm’s soil), the overall cost isn’t that much better than fossil fuels.

Don’t confuse burning biomass with converting corn into ethanol.

Burning biomass is essentially the same as burning sawdust and scraps from your workshop.

Converting corn to liquid ethanol as a replacement for gasoline requires a number of steps and is only economically practical (without subsidies) when the cost of crude oil becomes very high.

Sorry if it seemed like I was picking on you, Gary, I was only commenting that I think their numbers are off, but don’t know the reason.

As to how much oil could a medium-sized power plant burn…well, let’s take a very efficient non-CT plant with a net plant heat rate of 10,000 Btu/kWh. If this plant puts out 500MW gross (about 450MW net, or roughly enough to serve half a million Midwestern US homes), then it needs 450,000kW*10,000Btu/kWh = 4500MBtu/hr (note that this is “net” plant heat rate, thus I multiplied by the net power, not gross).

If fuel oil has a higher heating value of about 19,000 Btu/lbm, then we will need 4,500,000,000 Btu/hr / 19,000 Btu/lbm = 236,842 lbm/hr of heating oil. At roughly 7 lbm/gal (US), we need 33,834 gal/hr.

Over a year, assuming a 70% capacity factor, we will need 87600.733,834 = 207,473,684 gallons/year. Or about 5 million barrels a year, roughly.

If we assume a CT that has perhaps a 7500 Btu/kWh efficiency, we only need about 155,605,263 gallons/year, or about 3.7 million barrels a year, roughly.

I believe another problem would be the energy density of the fuel itself, ie burning one gallon of gasoline liberates about one power of 10 more energy than ethanol. For that reason it would be extremely impractical to run a car off of 100% ethanol, because even a very fuel efficent car, say 40 miles per galon of gasoline, would get about 4 miles per galon of ethanol.

I’m sorry, but that’s not correct at all.

Gasoline has an HHV of 47.3 MJ/kg, LHV of 44.0 MJ/kg, whereas ethanol has an HHV and LHV of 29.7 MJ/kg and 26.9 MJ/kg, respectively. Gasoline density is 0.72-0.78 kg/dm3, whereas ethanol density is about 0.785 kg/dm3. Assuming a median value for gasoline density and using the HHVs, the energy difference liberated between gasoline and ethanol is 1.59:1 on a mass basis, 1.52:1 on a volumetric basis.

Would it even be worthwhile from an energy expenditure viewpoint?

Let’s say I have some corn, ready to be burned. How much energy was expended to grow, process, and transport the corn?

Energy must be expended to grow the seed. (Diesel fuel burned by a tractor.)
Energy must be expended to harvest the seed. (Diesel fuel burned by a tractor.)
Energy must be expended to ship the seed to the farm. (Diesel fuel burned by a truck.)
Energy must be expended to plant the seed. (Diesel fuel burned by a tractor.)
Energy must be expended to harvest the corn. (Diesel fuel burned by a tractor.)
Energy must be expended to ship the corn to the generator. (Diesel fuel burned by a train.)

So here’s my question: when you add up all the energies listed above (and I’m sure I have missed a few), do you come out ahead? In other words, do you expend more energy producing the corn that what you get when burning it?

Or, put it another way - can you make it a closed system? Use all ethanol for the growing, harvesting, transportation and processing. Anything left over at the end?

No, you don’t expend more energy producing corn (or any other feedstock) than is available from the fuel. I haven’t looked for updated sources in the last year or two, but the general cost of alcohol is a little over $1.00 per gallon when produced from any type of biomass. There are significant differences depending on whether the feedstock is high quality, high cost (corn) or low quality, low cost (wood waste) but this is a ball park number. Add the 0.7 factor for energy value and it’s not competitive with wholesale gasoline except with tax incentives. It also wasn’t competitive with methonal produced from natural gas, but that may be changing.

Note that Brazil runs autos on either straight alcohol or a mix that is over 20% alcohol and it’s economical for them because of cheap feedstocks (sugar cane, I think).

Extract and ferment the corn’s carbohydrates>>>ethanol
Extract and trans-esterize the corn’s oil>>>biodiesel
Protien left over for human and animal feed
Cob husks and other dried chaff to fuel boilers or other external combustion technology.

I’m a bit too tired to go through the bioenergetics and the biochemical stoichiometry of photosynthesis as it relates to burnable mass, but I think I can draw a few mental pictures that may clarify this.

  1. Corn (or any photosynthetic plant) is basically solidified CO2 and H2O. Simple sugars (C6-H12-06) and water make up 95-98% of a corn plant’s mass. Cellulose (woody plant fibers) is a branching chain of glucose (a simple sugar,also called ‘blood sugar’) Starch, the other single major solid component of a plant is also a polymer of simple sugars. These two solids create cylindrical chambers (cell walls) filled mostly with water. There are few truly solid structures in a plant that aren’t mostly water, like the kernels. Proteins and fats really make up remarkably little of the plant’s structure, though it does play critical biochemical roles. The fats are mostly created from CO2, as are the proteins (with some added fixed nitrogen and other nutrients from the soil)

This is why some plants can grow so fast (some bamboos can grow 1-2 ft per day). They only synthesize thin-walled hollow structures (requiring only a realtively small mass of material be synthesized) that are filled with water. Young green bamboo derives much of its strength from the surface tension and other properties of he water, unlike mature dried bamboo

  1. The CO2 (from the air) and H2O (from irrigation and rain) are combined chemically into sugars with energy from sunlight, just as you were taught in grade school. The solar irradiance at the Earth’s surface is roughly 1 calorie/cm^2 (8.4 J/cm^2), which really adds up over, say, 12 hours a day (after allowing for reduced illumination in the morning and evening) over a whole growing season. Most of that energy is not used. Most that is used, drives the evaporation of water, which siphons nutrients up from the soil. The remainder, used to power chemical syntheses is still pretty substantial. 90 days x 12 hrs/day x 3600 sec/hr x 1 cal/cm^2 x 10,000 cm^2/m^2 is a LOT of energy, so even a tiny fraction is a sizable amount.

  2. Take enough tractor fuel to plow a 40 acre field, and spread it over 40 acres, and you’ll barely have enough to ignite. Quintuple it to allow for plowing, planting, harvesting, etc. – and you’ll still get a brief low flame, at best. An acre is 43,000 sq feet, and you can plow many acres on a tank of gas. it works out to a bare misting of gasoline or diesel - exactly what you’d expect because the engine is fueled by a mist fron the engine’s carbeurator or fuel injectors, as the tractor passes over the field

Now ignite a 40 acre field of dry 8-ft corn stalks - you’ll have a major conflagration. The tractor spends bare seconds over the site of each plant, but if you’ve ever seen a mature domestic corn stalk up close, you know it’s an impressive organism.

I hope that this helps people visualize the issue of whether burning corn yields a sizeable net fuel gain over the tractor fuel used (It certainly does) and clarifies the source and nature of that energy (think of the full summer’s output of a solar cell, released in hours or minutes)

er - I just spotted an error: the solar irradiance at the surface of the earth (roughly 1 cal/cm^2) is 4.2 J/cm^2. I said 8.4 out of habit, but that’s the solar irradiance of the planet Earth as an overall system, not the surface, which is shielded by the atmosphere.

I’d still like to see some numbers. The analysis should be straightforward. For a 100 acres plot of land,

  1. Estimate how much diesel fuel and electricity is required to grow, harvest, process, package, and distribute the seed. Convert this fuel into kWh.
  2. Estimate how much diesel fuel is required to prep the land for planting. Convert this into kWh.
  3. Estimate how much diesel fuel is required to plant the seed. Convert this fuel into kWh.
  4. Estimate how much diesel fuel is required to harvest and process the corn. Convert this into kWh.
  5. Estimate how much diesel fuel is required to transport the corn to the generator. Convert this to kWh.

Add up 1 through 5. This is the energy expended to produce the final corn product. Note that this would not be a conservative estimate.

Then estimate how much energy you could get (in kWh) from burning 100 acres of corn.

I have not run the numbers, but I have a hunch more energy is expended making the final corn product than the amount of electrical energy you would get from burning it.

Again, it’s just a hunch. Can someone run the numbers and prove whether or not I’m on the right track?

Wouldn’t it be much simpler to just look at the price of corn/ton than to re-invent the wheel (making inevitable numerous errors) by trying to figure out how many kWh it takes to package corn seeds, plow dirt, and all that other jazz? Those things should all be factored into the final price; leaving you only to figure out how to adjust that number for subsidies/profits. I’d think that to be far more accurate than starting from scratch; especially since you’ll end up converting it to $ anyways.

Yes, you could look at the price. But there’s a problem: Assuming the demand for other uses of corn doesn’t change (livestock feed, alcohol, food products, etc.), and assuming the demand for competing land uses doesn’t change (soybeans, etc.), using corn as a fuel for widespread energy production would create a huge surge in demand for corn. This would certainly increase the cost of corn, though I’m not sure how much.