Saturday, October 30, 2010

Waste to Energy -- Many different routes, most are economic

I'm shocked that we still rely on landfills for 55% of the roughly 4.4 lbs of garbage we generate on average here in the US. Roughly 33% is recycled and only 12% is combusted. Of the 12% combusted, only some of the heat released in the combusted is captured at the 87 waste-to-energy plants in the US. Only 2.7 GigaWatts of electricity is generated at these power-plants. And this is only 0.4% of the total average power generation rate in the US.

So what's keeping us back from generating more electricity from municipal solid wastes?
We need electricity, and we will be needing even more of it the future (to drive cars, to power robots, and to stream HD everything.)

There is understandably some concern with incinerating trash. Whenever you have partial combustion of hydrocarbons with chlorine species present. The chlorine comes from PVC wastes, such as some clothing, pipes, and portable electronics.

When the waste is combusted at too low of a temperature, there is a chance of forming dioxins. Dioxins can cause significant harm to the body, effecting both the physical appearance and neurological system. Dioxons can bio-accumulate, so unfortunately we possibly can injest dioxins via the food we eat.

So, how can we generate electricity from waste without generating dioxins (or other forms of air pollution, such as particulates, heavy metal vapors, NOx, and SOx)

First, we could combust the waste and then inject the gases underground, as is proposed for coal power plants. Or we could figure out how to remove all of the pollution from the gas stream. Luckily, most of the pollutant gases are acidic, and so they can be captured by flowing the flue gas through a basic solution, such as a mixture of water and limestone/sodium hydroxide.

There are also ways of minimizing the generation of certain of the pollutants. For example, if the waste is combusted in an environment of pure oxygen (rather than air), then the production of NOx can be eliminated. Also, if the combustion occurs in a pure oxygen stream, then it is easier to obtain temperatures that are high enough such that dioxins will not form. At temperatures above 1200 C and in the presence of significant amounts of oxygen, the production of dioxin is negligible.

Another option is to add limestone (or sodium hydroxide) directly into the combustion environment, in which case the calcium or or sodium can capture the chlorine, keeping it from entering the flue gas stream in the first place.

So, why is waste incineration not taking off in the US?
Right now, I think that there is a fear of building power plants that could cause any environmental damage. It's not completely a question of economics.
For example, NYC is paying on the order of $90 per ton of waste generated in the big apple. At this price, and at an electricity price over $0.15/kWhr, any of the processes described above make sense economically, even sequestering the flue gas miles underground.

We need to get over our fear of everything just because we have a history of environmental damage in the past. We have to compare the possibility of environmental damage due to incineration with the very real environmental damage due to landfilling, such as contamination of groundwater and/or aquifers by leakage or the harbouring of diseases.

We need to overcome this fear really quickly because there are some major advantages to incinerating waste, such recovering precious metals leftover in the ash. The price of precious metals is increasing, and will continue to increase in the future as demand increases and supply decreases (especially if China decreases the amount of precious metals exports.)

So, I'll continue more in another blog to go other many of the different routes, and the estimates of the economic viability of the processes.

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