January 20, 2007

Geoplasma Answers Trash Vaporization Questions

On September 10th I drafted an article about a plan to use plasma-arc technology to gasify waste in St. Lucie County, Florida titled FLORIDA: County to Vaporize Trash. In addition to writing my concerns about the emissions from the process, I listed five questions that I wanted to have clarified by the developer, Geoplasma of Atlanta, Georgia. I sent the list to the president of Geoplasma, Hilburn Hillestad, who very graciously sent me the following reply:

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Response to BioConversion Blog Questions about Trash Vaporization
From Hilburn Hillestad, President of Geoplasma, LLC.

Below you will find our answers to your questions. Thank you for your interest in our proposed facility.

1. Question: How much energy does the plasma-arc use?
Answer:
The plasma-arc facility uses approximately 40 megawatts of energy per hour. This is approximately one-quarter of the total output of hourly energy received from MSW.

2. Question: What will be the source of the plasma-arc energy?
Answer:
The facility will receive its energy from its total output. For St. Lucie, it is expected that the 3,000 tons of MSW processed per day will create 160 megawatts of energy per hour. As stated previously, 40 megawatts will be used to power the facility and the remaining 120 megawatts will be sold to an Electric Utility.

3. Question: What does the energy source emit?
Answer:
See question 5.

4. Question: Is the high heat of the plasma-arc being captured and utilized?
Answer:
Because of the nature of a closed-loop system the heat will be captured and utilized both in the plasma gasification process and later in the production of steam.

5. Question: How are they going to combust the syngas to keep the emissions low?
Answer:
There is no combustion during the gasification process. The Plasma-arc gasification process is a chemical reduction process that converts MSW from its original state to a glass-like aggregate solid at the bottom, and a synthetic fuel gas, also known as syngas, at the top.

Once gasification is over, the syngas is cleaned in a multi-step process, bringing it to levels near natural gas cleanliness. It is then compressed before being used as fuel for a gas turbine.

The gas turbine for this process is a modified natural gas turbine that mixes the cleaned syngas with air from the atmosphere, combusts the mixture and sends the hot gases through a turbine. The turbine spins an electric generator to produce electricity. The discharged hot gases are then passed through a heat recovery steam generator to produce more steam and to cool the hot gases. The cooler exhaust gases are then discharged into the atmosphere via a stack.

Emissions from this process are very similar to natural gas combined cycle plants which are considered to be ‘clean’ and are located and permitted all over the U.S., and for that matter the whole world.


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3 comments:

Anonymous said...

First, Mr Hilburn Hillestad got his units wrong. It is either "megawatts" (power) or "megawatt hours" (energy). "Megawatt hours per hour" would also make sense as average power delivered. That does not raise my confidence that he knows what he is talking about. But let's assume for a moment that he is talking about "megawatt hours per hour".
Then 3,000 tons of MSW per day have to be vaporized with an input power of 40 MW. That makes 34.72 kg of material per second, with an input energy of 40 MJ per second. Or 1.15 kJ per g of material. Now let's look at some specific heat capacities (taken from Wikipedia) and how much you can heat the material with that.
Water (in wet waste): 4.184 J/(gK), plus heat of vaporization of 2.26kJ/(gK) => you'd boil the water and vaporize some of it at 1 bar pressure. No plasma.
Paraffin wax (to represent hydrocarbons): 2.5 J/(gK). => not counting heat of vaporization you'd heat that up by 460 degrees centigrade. Sufficient for the syngas creating Fischer-Tropsch process (again according to Wikipedia), but still no plasma.

Unfortunately I couldn't find numbers for cellulose (paper) on short notice, but I think the picture becomes clear: 40 MW is not nearly enough.

Anonymous said...

What are the precise gases discharged into the atnosphere?
CO2?

contemplative said...

I think that in this case, doing something with solid waste which shows some return is better than having it take up space in land fills where nothing is returned. Even if this was a zero sum process, it is better than simply doing nothing. I don't understand the debate...?