Starting the cycle

(The compulsion leapfrogged this impromptu piece past everything else.  Surprise.)

In the news, a specialist in Fischer-Tropsch synthesis (Rentech) has signed a deal to purchase the outstanding shares of Royster-Clark Nitrogen, Inc (h/t:  GCC).  The major asset of RCI appears to be a Danville, Illinois fertilizer plant.  Nitrate production in N. America has been mostly shut down due to high natural gas prices (natural gas is the standard feedstock used to make hydrogen for Haber synthesis of ammonia).  Rentech intends to build a set of coal gasifiers on the site to supply hydrogen to the ammonia plant (boosting its capacity from 830 tons/day to over 900 tons/day), as well as creating 87 million gallons per year of motor fuel by Fischer-Tropsch synthesis and generating electricity.  The exact breakdown of the plant's output is not specified, but it is stated that it will consume 5200 tons/day of coal which is "the commercial equivalent capacity of a 650 megawatt Integrated Gasification Combined Cycle (IGCC) power plant."

This may be a step backward for CO2 emissions, but it's a step forward for reliability of US energy supplies.  But it's not the end.  A plant which burns coal in a wet-slagging gasifier has the potential to burn many other things besides.  The possibilities include charcoal and raw biomass.

Anyone who has travelled through southern Illinois and the neighboring area of Indiana has seen that farming is very big.  Corn is king.  Corn byproducts, such as leaves and stalks (stover) and cobs are certainly available in abundance.  If these farms are typical, much of this material goes to waste.  It therefore represents several unrealized potentials:

• closing the carbon loop,
• reducing or eliminating heavy-metal emissions from the plant, and
• creating another revenue stream for local farmers.

Feedstocks

Fischer-Tropsch synthesis requires a synthesis gas consisting of mostly carbon monoxide and hydrogen.  Methane is undesirable (too stable), and larger molecules ditto.  The gasifiers used to make F-T synthesis gas are almost always high-temperature, entrained-flow units burning finely powdered coal; the gasifiers are engineered to break down their inputs into the simplest molecules possible.  It stands to reason that a machine for burning finely-powdered carbonaceous material may not be overly fussy about its exact diet.

The Wabash River IGCC plant (using E-gas gasifiers) burns coal or petroleum coke with equal facility, and it appears likely at first glance that similar gasifiers could also process charcoal with relative ease.  Raw biomass would be more difficult; fibrous materials cannot be handled as easily as powders, but advances in processing may make this feasible.  I'll speculate on both possibilities.

Getting back to Danville, the re-engineered fertilizer plant will consume 5,200 tons/day of coal, or 1.90 million tons/year.  Assuming 25 million BTU/ton of coal, this is 47.4 trillion BTU/year.  (The article states this as the equivalent of a 650 MW IGCC plant; this appears to be based on an assumption of roughly 41% efficiency.)  From this it will produce 87 million gallons/year of F-T motor fuel (roughly 12.8 trillion BTU worth), 330,000 or more tons of fixed nitrogen, and an unspecified amount of electricity.

The energy supply

At a typical yield of 150 bu/ac, corn yields approximately 2.5 dry tons of excess stover (not needed for erosion control) per acre.  If it contains 15.8 million BTU per dry ton, the yield is 39.5 million BTU/ac; if it can be processed into charcoal at 28% yield and 15,000 BTU/lb (30 million BTU/ton), each acre could produce 0.70 tons of charcoal per year yielding 21 million BTU/ac.

There are three different possibilities for supplying the energy requirements of such a plant using biomass:

1. Feeding it with charcoal produced off-site.
2. Producing charcoal on-site, with the carbonizer off-gas used either for (a) a co-fuel in the gasifier or (b) fed to the power-generation block as gas-turbine fuel.
3. Direct feed of biomass (either raw or lightly processed) to the gasifier.

Case 1:  charcoal produced off-site.  This case comes closest to a feed of coal, with the difference that charcoal will have less intrinsic moisture (almost none) than coal.  Supplying 47.4 trillion BTU of energy with charcoal at 30 million BTU/ton requires 1.58 million tons/year of charcoal.  At a charcoal yield of .70 tons/ac, the production from 2.26 million acres would be required.  This is about 3530 square miles, or a circle about 34 miles in radius.  Allowing for non-cropland in the area, the plant could probably take the stover-derived charcoal from all the cornfields within roughly 40 miles.

Case 3:  biomass fed directly.  Supplying 47.4 trillion BTU of energy from biomass at 15.8 million BTU/ton requires 3.00 million tons.  At a dry biomass yield of 2.5 tons/ac, the production from 1.2 million acres would be required.  This is 1,880 square miles, or a circle about 24 miles in radius.  Allowing for non-cropland in the area, the plant could probably take the stover from all the cornfields within roughly 30 miles.

Energy sufficiency

A big question for sustainability is if a process can yield enough energy to run itself and still produce a surplus.  If the planting, cultivation and harvest of an acre of corn requires 6 gallons of diesel, the 87 million gallons of F-T fuel produced by the plant would suffice for 14.5 million acres of crops.  This is roughly 6.5 times the crop area required for case 1, and 12 times the crop area required for case 3.  This is an 540% to 1100% excess, which is clearly sustainable.

The other question is the nitrogen balance.  Corn is fertilized with an average of 77 pounds of nitrogen per acre.  The plant's production of 330,000 tons/year of nitrogen would suffice for 8.6 million acres of corn.  This is a 280% to 580% excess, which is also clearly sustainable.

Co-products

The press reports do not specify the electric production expected from the repowered fertilizer (to become polygeneration) plant.  On the other hand, the production of charcoal would produce heat and off-gas with an energy content which can be estimated.  Turning 39.5 million BTU/ac of stover into 21 million BTU/ac of charcoal releases 18.5 million BTU as heat and gas.  If this energy can be turned into electricity at 50% efficiency, the processing of the stover from 2.26 million acres would yield 24.6 trillion BTU (7.21 billion kWh) of electricity.  This is an average of 823 megawatts.  A single stover-to-charcoal plant handling the product from 2.26 million acres could co-produce 0.18% of the nation's electric demand by itself.  The 80.7 million acres planted to corn in 2004 might fuel 36 such plants; these could produce 260 billion kWh/year, enough electricity to almost replace hydropower or displace 37% of the power produced from natural gas (data).

Carbon balance

The natural gas input to the original plant is not specified, but that never stopped me from guesstimating.  Production of 330,000 tons/year of fixed nitrogen would require 70,700 tons/year of hydrogen; produced from methane via partial oxidation of CH₄ to CO + 2 H₂ followed by shift conversion of CO + H₂O to CO₂ + H₂, the process would consume 189,000 tons/year of methane and produce 520,000 tons/year of CO₂.

The coal-fired polygeneration plant would produce quite a bit more.  The 1.9 million tons/year of coal would contain 1.24 million tons of carbon.  All of this would wind up as CO₂, adding 4.54 million tons per year to the atmosphere.  The production of motor fuel and electricity would offset this somewhat.  The contribution from electricity is not quantified, but 87 million gallons/year of F/T diesel at 7.67 lbm/gallon would offset roughly 1.05 million tons of CO₂ from petroleum.  The net CO₂ contribution of the plant is roughly 2.97 million (4,540,000 - 520,000 - 1,050,000) tons per year, minus offsets from the unspecified electric production.

The carbon production of the biomass-fuelled plant would be a big fat zero.  To the extent that its F-T fuel production would displace 1.05 million tons/year of CO₂ from petroleum, its net contribution would be negative.  Electric production would drive the total further into the negative.  If CO₂ emissions credits were worth even $20/ton, the avoided cost would be about $59 million/year.

Farm income

The question of what biomass is worth is a good one.  Is it to be rated by its BTU value compared to a particular fuel, by the avoided carbon emissions, by avoided environmental contaminants?  It's hard to tell what farmers could or should be paid for.

Selling by BTU's (avoided cost) is relatively direct and simple.  If coal costs $30/ton at the plant, the plant is paying $1.20 per million BTU.  A farmer reaping 2.5 dry tons/ac of stover at 15.8 million BTU/ton could gross another $47.40/acre (minus harvest and transport costs), equivalent to about an extra 30¢/bu in the price of corn.  Compared to current prices of ~$2.50/bu, this is significant.

If other fuels are being displaced, this figure could go considerably higher.  Natural gas is currently running over $12/million BTU wholesale.  If carbonizer heat and off-gas is worth $8/million BTU as input to a gas-turbine generator, the 18.5 million BTU/ac from the carbonizer would be worth a whopping $148/ac.  If the farmer could get 50% of that, it would pay another $74/ac, or roughly another 50¢/bu; this might make farming highly profitable.  Sales of the .7 ton/ac of charcoal (worth about $21/acre at coal prices) to coal consumers might pay for the carbonization process, or it could be returned to farmers as fuel to heat homes and barns.  The potassium and phosphorus in the ash would be right where it needs to be to close the cycle.

What it can't do

We're clearly not going to fuel the nation from crop wastes.  87 million gallons per plant times 36 plants is only 3.1 billion gallons per year, a minuscule fraction of our 139 billion gallon/year gasoline appetite.  Even if yields were sextupled through e.g. the growth of switchgrass or Miscanthus at 15 tons/acre we would only get to about 30% of distillate fuel consumption or 9.3% of total motor fuel consumption.  The outlook for electricity would be rosier, but it would still not come close to replacing coal.

But that's not so bad; it would lay the groundwork for more efficient systems to follow, and by itself it would be a very promising start.

¶ 11/09/2005 07:38:00 PM