Tuesday, July 22, 2008

Dumping Quicklime into the Oceans

Tim Kruger at Cquestrate has an idea for sequestering large amounts of CO2: dump quicklime (CaO) in the ocean.

The basic idea is to convert limestone (CaCO3) and CO2 into calcium bicarbonate (Ca(HCO3)2).

CaCO3 + energy -> CaO + CO2 Burn limestone into quicklime
CaO + H2O -> Ca(OH)2 Dissolve quicklime in ocean to make calcium hydroxide
Ca(OH)2 + 2CO2 -> Ca(HCO3)2 Calcium hydroxide absorbs CO2 to make calcium bicarbonate

CaCO3 + H2O + CO2 + 178 kJ/mol -> Ca(HCO3)2

The problem is the amount of energy required. Let's say it comes from coal. Typically, you can get 30 MJ/kg out of coal. To get your 178 kJ above, you'll produce a half mol of CO2 just burning coal, assuming perfect efficiency. That's half your benefit gone right there.

But, it's a high temperature reaction (840 C). That means you have to get the reactants (calcium carbonate, coal and coal oxidizer, e.g. air) up to that temperature, react them, then drop the reaction products back down to normal temperature. To get perfect efficiency, all of the heat from the cooling products has to be transferred to the reactants. There is going to be some loss.

Let's say you lose 25% of the coal heat, and 75% goes to making quicklime. Then, for every 2 kg of coal burned, you will eventually absorb the CO2 that was produced by burning another kg of coal somewhere else.

Bottom line: we'd have to triple the rate at which we burn coal to get carbon neutral with this scheme. That's not practical. It'll get better if we use natural gas or oil, but it won't change the basic calculation that we'd have to multiply our existing consumption of fossil fuels to get carbon neutral.

Now, if someone wants to tell me about a scheme in which limestone is burned in a solar furnace to make cement, I'm all ears. CO2 sequestration from such cement manufacture makes more sense than it does from coal-fired powerplants, because limestone burning (no air) releases pure CO2, whereas coal burning releases CO2 mixed with lots of nitrogen from the air. However, there are lots of other problems.

Sigh. We're not getting out of this mess easily.

Tuesday, July 15, 2008

The Pickens Plan

Check out the guy's website, if you haven't already. There is not a lot of meat there. Basically, the idea is that if we build enough wind turbines to provide 20% of our electricity, we can reduce the amount of natural gas that we burn to make electricity. This natural gas can be used to power special new cars, which will reduce our imports of petroleum.

Mr. Pickens' chief aim is to reduce U.S. petroleum imports. That's great, because that's the energy policy issue I care about most, too. However, I see two problems with his plan:
  1. As things stand now, large fast changes in wind turbine output will have to be accomodated by throttling natural gas turbines. Gas turbines cannot throttle down to zero power efficiently. So, even when the wind is blowing a large amount of power will have to come from gas turbines running at partial throttle ready to take over if the wind cuts out. If wind is supplying 20% of our domestic power, these partial-load gas turbines will have to supply some similarly large amount, and as a result there may not be a large amount of gas actually saved.
  2. I don't forsee a switch to compressed natural gas burning cars. I suspect it would be cheaper and have a larger, more immediate impact to convert the natural gas (and some coal) into gasoline in a refinery, and then feed that into the existing transportation system.
I have two humble suggestions for Mr. Pickens, or energy policymakers.

1. Switch home heating to electric heat pumps.

In 2006, 5 billion gallons of distillate fuel oil was sold to residential users, almost all of it used to heat their homes. Ignoring refinery gain, this is 160.8 million barrels, or about 3.6% of the 4.5 billion barrels of oil imported that year.

Nearly all the houses heated by distillate fuel oil have grid electricity. These houses can be upgraded to air-source heat pumps for a few thousand dollars each. Electricity can come from coal or natural gas, either one of which is better than petroleum. The economics are probably already there for the switch, so some public education and low-cost financing should push homeowners to embrace heat pumps en masse. This can happen a lot sooner than moving the U.S. car fleet to compressed natural gas.

This switch can reduce our oil import bill without requiring the first step of lots of wind turbines. Maybe I'm just nitpicking, but $21.4 billion dollars per year (for the 160.8 million barrels imported) seems like an interesting amount of money.

2. Make air conditioners work on intermittent electricity

This is also known as "Direct Load Control" or "Demand-side Management".

One of the problems with wind energy is that it's intermittent. Increasing the amount of wind generation in the national grid will increase the variation in load that the other generators must accomodate. This will cost money. It will cost less money if the other generators have 10 or 15 minutes to accomodate variation.

Air conditioners and heat pumps naturally store energy. It takes time to cool or heat a building. Usually, the pump cycles on or off every few minutes. If the utility has a fast way to shut down large numbers of compressors for a few minutes, it can filter out much of the short-term variation in load and supply. Instead of throttling gas turbines from 50% to 100%, a few minutes' notice gives the utility time to turn on gas turbines -- from 0% to 100%. That means that the 50% rated capacity that was otherwise being produced by a gas turbine can be produced by a coal-fired turbine instead, which is much cheaper.

This change is a good idea regardless of whether a massive wind turbine build happens, because it will allow utilities to use less natural gas and more coal. That may alarm some folks. Some may see a hidden agenda here. I think if the same bill in Congress mandates Direct Load Control on HVAC devices, and guarantees a production tax credit for all non-carbon domestic sources for a decade, that should assure doubters and put some real fire in the market.

Right now, hydroelectric turbines are the cheapest load-following generation around. They produce just 7.1% of the electricity in the United States (2006). Unfortunately, all of this load following capacity is already used.

For comparison, HVAC uses more than 29% (page 44 here, plus this, both from the EIA) of our generated electricity. Instantaneous control over this much load would be sufficient to accomodate any amount of wind power that we care to build. Of course, the utilities (really the system operators) can't control HVAC, yet. I don't think this is a problem, because we don't have 450 gigawatts of wind turbines yet either.

I suspect the average lifetime of HVAC equipment is around 20 years. If the government mandated that all HVAC equipment sold after, say, 2009 had Direct Load Control features, then we'd see about 15 new gigawatts of Direct Load Control every year. There is little danger of us building wind turbines faster than that in the near future.

Wednesday, July 09, 2008

Burning coal is burning oil

I found some numbers for the oil cost of burning coal.

Freight trains in the United States burn 1 gallon of diesel to move a ton of frieght 436 miles.

Average distance coal travels in US: 628 miles from mine mouth to powerplant. At $4.03/gallon, that's $5.80 for the diesel to move a ton of coal from the mine mouth to the powerplant, on average. Wyoming coal costs $9 at the mine mouth. So, electric producers pay almost as much for the diesel to move the coal as for the coal itself. Since marginal petroleum is imported, it's fair to say that coal is not entirely a domestic fuel.

The average powerplant cost for coal in the U.S. in 2006 was $34.26/ton. That's because coal mined outside of the Powder River basin in Wyoming costs a lot more to dig out -- the average mine-mouth price across the U.S. in 2006 was $25.16/ton. The difference is $9.10/ton, which is the cost of transport. The cost of diesel was a bit lower in 2006, but it looks like around half the transport cost is the diesel.

If the coal is 22 MJ/kg, and the plant is 35% efficient, then for each kWh at the powerplant you spend on average 1.8 cents for the coal. Just the fuel cost of the coal plant is more than the total operating cost of the Palo Verde nuclear powerplant, per kWh. This result is entirely independent of subsidies or clean coal. The black stuff is apparently just really expensive.

A while back, I snarkily suggested that mine mouth coal powerplants were a way to keep the pollution away from rich people. Looks like I was wrong:
  • Transporting a kWh of electricity 1000 miles increases the cost by 19%.
  • Transporting the coal necessary to make that electricity 1000 miles costs $14.49/ton, assuming cost is linear with distance. That's a 58% increase in the cost of the fuel. Assuming the fuel cost is 70% of the cost of producing electricity, that's a 40% increase in the cost of the electricity.
  • 4000 miles (across the continent) by electricity: increase cost by 107%.
  • 4000 miles by coal train: 160% increase.
What about the extra carbon? Transporting 1000 miles as electricity means you must make an extra 8.7% more electricity which gets lost in the wires, which produces 8.7% more CO2. Transporting 1000 miles by coal train burns 6.3 kg of carbon in the diesel to deliver perhaps 800 kg of carbon, which increases the total carbon released by 0.8%. Clearly the diesel locomotive is the lower carbon, if much more expensive, alternative.

Average distance coal travels in China: 230 miles. They're burning a lot less diesel to take advantage of their domestic coal.