Tuesday, June 06, 2006

More on Plasma

So municipal waste disposal is in the news today, with the Ottawa Citizen running a story on the number of GTA municipalities that are looking at incineration for waste disposal. (Unfortunately, their story is behind a pay wall.) The Swedes are busy selling their well-tested combined heat and power (CHP) systems for garbage disposal, and even the Ontario Environment Commissioner is cautiously on board.

The main selling point with these CHP systems is that they are relatively low emissions, and they can deliver energy on to the grid in the form of steam for heating and electricity.

Looking at a plasma waste disposal system - which could feed an ethanol plant, as we've seen - has some interesting advantages over traditional incineration. The first is that the temperatures are much higher, leading to a greater destruction of harmful organic compounds like dioxin. At a certain temperature, the all hydrocarbons dissociate in to carbon monoxide and hyrdogen. That's syngas, and it's a useful feedstock for all sorts of chemistry, not just ethanol.

Another advantage is that with a high enough temperature, there's little need to sort trash before you burn it - even aluminum and zinc can be in the mix, and they'll just be sorted out of the dross afterwards. This actuall leads to another important point with gasification (as opposed to incineration): in gasification, things like sulphr and nitrogen can be filtered out of the waste stream before being vented - the sulphur can be sold commercially, as I imagine could the nitrogen (fertilizer anyone?)

Whatever's left in the solids can then be treated with silicates and covered in an obsidian-like matrix, leaving it essentially bound up and contained for millions of years. This technique has been proposed for low-level nuclear waste disposal, so it should be fine for things like mercury. This is a clear advantage over incineration or regular gasification.

The big drawback is that, while CHP incinerators are net producers of energy, a plasma reactor (on its own) would be a net consumer of electricity. That's not to say it couldn't sill produce some of it's own electricity, and there's liable to be plenty of heat to use for district heating. But using the numbers for a sea-based plasma system here, I'm calculating roughly 4.2kw of power to dispose of one kg per hour.

That means that to dispose of the one billion kilograms of waste Toronto sends to landfill every year, we'd need to have 500 megawatts of power running 24/7, though I'm not assuming any kind of electrical or heat exports. That's a lot of electricity for a city that already has supply issues.

The reason the electricity demand is so high is that a lot of that energy would be contained in the syngas, and would be exported as ethanol (or whatever fuel is best.) So while the facility would be a net electricity consumer, it would hopefully be a net energy producer.

Of course, I wouldn't want plasma disposal to be seen as an alternative to recycling or the successful green bin program, but the fact remains that Toronto is always going to produce a lot of landfill wate, even with optimistic projections for recycling and composting. Given plasma's other advantages over incineration or conventional gasification, I'd say it's worth exploring.

1 comment:

Mark Francis said...

The issue remains is that that which feeds these generators best are the very things which which we can -- and should -- divert from the waste stream in the first place.

Catching nitrogen from organics is a nifty trick, but is still a poor substitute for real compost. And compost, done correctly, is cheap to do: make a pile, layer it with earth as it grows, and voila! Great soil! Energy use: nothing but minor muscle power.