The NY Times has a surprisingly enlightened piece on the fairly esoteric topic of co-firing:
Power Plants Try Burning Wood With Coal to Cut Carbon Emissions
In the article, the reporter points out some of the opportunities and challenges facing coal-fired power plants. Keep in mind that until recently, fully 50% of domestic power was generated from coal combustion, and even now, it's something like 45%. (Coal has come into disfavor with utility companies due to both low cost of "natural" gas, and increased costs of emissions compliance.) The potential is huge; the challenges are equally huge.
This is a topic I'm pretty familiar with, having done research into various alternative energy schemes over the years. I'll summarize and provide some nuance and opinion below the orange flame.
As you probably know, coal has been mined from the ground for about 200 years. In general, the coal is first crushed to a small size, and fed to a large furnace. Here, the coal is burned (oxidized) and converted to CO2 and steam, mostly. (And smaller amounts of various pollutants. And ash.) While burning, an enormous amount of heat is released, which is used to create steam. Water passes through tubes in the furnace, and absorbs the heat. As the water is heated, it boils and becomes high-pressure steam. The steam is pushed through a turbine, causing it to spin, generating electricity. This process has undergone many changes over the years, providing increased efficiency and better capture of pollutants, but the overall scheme is more-or-less unchanged.
Co-firing is a process of burning two fuels simultaneously in a furnace. As a species, we've been burning biomass (aka wood) for heat since well before recorded history. Biomass combustion is not a new idea. But doing so with coal is a relatively new challenge. As pointed out in the article:
Coal plants are finely engineered, designed to burn one particular kind of coal, and adding wood can be tricky.
There are several technical / engineering issues that are pretty challenging at the scale of a large power plant. Feeding the fuel to the boiler is a big one; coal has distinct handling characteristics, and wood handles differently. Some thought, and probably a new method, is required for feeding both simultaneously. And the new method should be cheap, since any additional cost will be born by the rate payers. Pollutants created in combustion of biomass are not the same as those generated in combustion of coal/biomass, and under some conditions, can be a great problem.
Ash handling, it turns out, is a really big deal, one that isn't addressed in this article. The inorganic elements present in coal (things that don't burn, mostly metals) have distinct characteristics in a boiler; for example, the ash can build up on tube surfaces, the ash can come out as fly ash, ash requires disposal, etc. Over years, the power industry has established methods for addressing these challenges, but the use of biomass completely upturns those solutions. Woody biomass has quite a bit less ash than coal, but the ash it does have is quite different from that in coal. The solutions for coal ash handling will not work for the ash of a mixture of biomass and coal.
Supply chain is a serious headache for utilities contemplating biomass co-firing. Obtaining long-term contracts for large quantities of wood chips is not straightforward. The Times cites a utility company in Minnesota that had such problems, and I've worked with a utility in Nevada that has had similar challenges. (I assume this problem is less so near the forests that for years were used to provide timber for the pulp and paper industry, but which now stand fallow, or at least, underutilized. This is the case in the Northwest, and in the Southeast. I assume this, but I don't know it as a fact.) Until a utility company secures long-term contracts for fuel supply, it would be very risky to proceed with investing millions or billions to retrofit a coal-fired power plant for co-firing. The US DoE has done a pretty rigorous study on biomass availability in the US, (Billion Tons Update caution- this is a mighty large PDF) and determined that large amounts are available now, but economics is a dominating issue. One assumes that these markets will develop eventually, but getting the right policy drivers in place is essential.
Carbon neutral? Burning biomass can be considered to be carbon neutral, so the thinking goes, by considering the full carbon cycle. (1) Harvest, chip, and transport wood. (2) Burn it to produce power. (3) Grow more biomass. Repeat.
Cycles 1 and 2 release CO2, while cycle 3 consumes CO2. Wood is grown by photosynthesis (and is therefore stored solar power), which uses CO2 and H2O, so you can think of it as combustion in reverse. Analysis of the entire cycle is really complicated, and I'm not up to speed on it. But you can be sure the "carbon footprint" of wood usage or co-firing is a whole lot less than the footprint of coal firing alone. It does require that your analysis take a longer-term perspective, to allow for incorporation of the Carbon back into your fuel, but if we're discussing global climate change, then I think we can agree that a long-term perspective is required.
Now, for some opinion. Many of the problems associated with coal power can be described as externalized environmental costs. Use of coal causes significant environmental problems to society, both during mining and in utilization during combustion (or gasification). The costs of these environmental hazards are born by society as a whole, and the beneficiaries of this usage are not required to pay those costs, any more than any member of society. Everyone who breathes pays for it, not just the owners of the mines, the power company, or electric utility customers.
These externalized costs should be internalized using a market mechanism: A carbon tax could be used to directly address the more significant impacts of CO2 emissions related to global warming. I favor this approach, rather then imposing specific targets for power plant efficiency, or even requiring a specific renewable power portfolio. Yes, those policy tools can work in the short run, but power companies are constantly trying to do an end run around these regulations.
Instead, let's force the beneficiaries to pay the costs, by requiring that any power derived from use of coal be taxed directly to the rate payer. I'm not an economist, but I know there are qualified economists who can work through the assumptions and calculations to derive a reasonable cost associated with emission of every pound of CO2. Add that to the rate payer's bill. This will motivate companies to compete and find inexpensive alternatives, and will certainly result in increased investments in renewable power! Australia has recently imposed such a tax, with an average increase to rate payers of something like 18%.
This would certainly cause several deleterious economic effects, especially to the poor. There are two strategies to mitigate the effects. First, the tax should be phased in over a very predictable period, perhaps five years. This would allow customers to become more efficient slowly and stably, and would allow industry to make investments wisely and for the long term. The second mitigation strategy is to return the tax revenues to the economy. 75% of the revenues should be returned to rate payers, in direct proportion to their energy usage; if you bought coal power, this will help alleviate the increased cost, and if you bought "green" power, then it's a windfall. The remaining 25% should be used to develop and deploy new renewable technologies.