I heard about an idea that you could use small trees (as biomass) in a biomass electric plant.
I looked around, but could not find much info. It sounds like they boil the sawdust in a boiler, and use the steam to turn a turbine?
Why not just boil water? Does the wood add heat?
And how so they heat the water? Do they burn the wood? If so, doesn’t that create a lot of air pollution?
Biomass, very generally defined, is organic matter. Since at it’s most basic structure it consists of carbon molecules bonded to “other stuff”, most of it can be burned or otherwise processed to release energy.
In some types of power plants, organic or fossilized organic fuels are burned in order to boil water and create steam that drives a turbine, generating electricity. basically, in the situation you describe the wood would be the “fuel for the fire” that boils the water that drives the presssurized steam into the steam turbine and spins it.
This is from a DOE site (http://www.eere.energy.gov/biopower/basics/index.htm):
Biomass can be converted into electricity (or heat) in one of several processes. Today, the majority of biomass electricity is generated using a steam cycle: biomass material is converted to steam in a boiler; the steam then turns a turbine which is connected to a generator. Biomass can also be used with coal to produce power an existing power plant. Cofiring is t he most economical near-term option for introducing new biomass power generation, and lowers the air emissions from coal-fired plants.
They say ‘biomass is converted to steam in a boiler’. I guess that means they burn it? Kind of misleading huh?
Any idea how clean these plants are?
That Dept of Energy site is incredibly poorly written. The majority of biomass plants just burn the material. From a quantity standpoint, most are byproducts of other industries like sawmills or food processing.
It is possible to use two alternatives. You can gasify which involves heating the material in an environment deficient in oxygen so the hydrocarbon material is converted to a gas like natural gas. (This can also be done with coal.) The second way is to ferment the material to create fuels like alcohol and off gases.
Unsophisticated plants are pretty dirty from a particulate emission standpoint. Modern plants are like any other hydrocarbon fueled plant. Since biomass is a bit hydrogen rich compared to coal, some CO2 reduction is possible.
In theory, this is a non-CO2 process since all the material has been grown and has taken CO2 out of the environment to balance the release in the burning process. Of course, if you look at the big picture, burning coal and oil is non-CO2 also since (once upon a time) it all took CO2 out of the atmosphere.
To minimize emissions (non-CO2) fluidized bed combustion can reduce emissions. That’s a whole other topic however. I’ll let the interested parties Google that one.
Biomass is an idea that’s catching on, at least to some degree. There’s discussion about possibly putting one in Albuquerque to help get rid of all the salt cedar and Russian olive and other non-native plants that are clogging up the bosque.
However, if what I remember reading is right, burning this takes more work and can cause more pollution because of the chemical composition than the burning of fossil fuels, especially natural gas. The good part is that it’d work well with a program of cleaning out underbrush and trees in choked forests out here in the West, as not only do you get rid of the material (most of which probably wouldn’t be all that commerically viable for other things) and burn it, producing power and saving some fossil fuels.
AS far as biomass pollution goes, there are really two concerns:
one is standard power-plant stuff like acid rain, ozone (smog), and particulates. This depends a lot on the particular design of the plant, though food and wood-products waste are inherently a little more polluting than say, natural gas, it’s not an insurmountable hurdle.
The second issue is carbon dioxide emissions. There’s nothing you can do to stop them if you’re burning something. The deal, as KenGr points out is that growing the next batch of biomass fuel takes all that carbon dioxide back out of the atmosphere, so there’s no net gain. This is very different from fossil fuels, where the carbon dioxide just stays in the atmosphere.
However, the twist is that some methods of biomass production use a lot of energy from fossil fuels (to run the tractors, fertilize the crops, run the chemical plant that processes the biomass, etc.) , in fact sometimes more energy than you end up with in the biomass fuel. Obviously this is a net loss as far as global warming is concerned.
First off, one needs to make a difference between “biomass” and “biological-dervied refuse”, which is essentially wood and other waste that has a majority biological content (such as 2x4’s, sawdust, and particle board).
True “biomass” combustion typically involves feeding the solid biomass into a furnace to be burned, which then heats water to produce superheated steam. From that point on, the power plant is essentially like most other steam power plants.
This biomass can be fed in “raw”, hogged, chopped and processed finely, or can even be pulverized. It can be fed in separately from a co-fire fuel (such as coal, which is most typical), and can even be sent to the pulverizers and burners right along with the coal. In a CFB plant, it can be burned in a variety of manners.
Biomass in general is a clean fuel which in general has lower SO2 emissions, and lower NOx emissions due to a lower combustion temperatures (particularly depending on the ratio of volatile-bound nitrogen to fixed carbon-bound nitrogen) and higher moisture levels. In fact, drying of biomass can be a big energy sink that reduces the overall efficiency of the process. It does typically tend to produce an ash which is high in potassium and calcium, but this is not typically a problem outside of operations and maintenance concerns (slagging, high-temperature tube deposition, and corrosion risks).
Biomass can also be conververted into bio-diesels and bio oils, and burned in an oil or gas plant, or co-fired along with coal again. It can also be gasified in a fluid bed under starvation conditions, and injected into the furnace as a low-heating value gas.
The other type of biomass, waste-based, does often have problems with the heavy metals (arsenic, chromium, mercury, vanadium, etc) that are often used to treat the wood or paper. These heavy metal emissions are some concern in my line of work (say “hexavalent chromium”, everyone).
Comparing biomass pollution to natural gas is invalid, as aside from trace levels of hydrogen sulfide natural gas contains little sulfur to form SO2. It’s better to compare it with coal combustion.
The rule of thumb for carbon neutrality for agricultural biomass is assumed to 96% (EPRI figures).
I need to limit myself on what I say about specifics of biomass application in the US right now due to potential conflict of interest, as I am managing two biomass combustion projects at this time. I will say that interest in biomass combustion is momentarily high in the Southwest, but otherwise is at about the same level as always. It’s just getting more press and more attention than before, IMO.
Una