[Gasification] CO2 recycling

[Daniel Chisholm]

Hi George,

My comments below are based on my understanding that your process will
use microwave energy as the principal energy source to create a plasma.
If I've misunderstood this, then some of my comments may well need
fixing or retraction...!

On Sat, 2007-13-10 at 19:52 -0400, gfwhell at aol.com wrote:
>  Dan,
> Raising the temperature in an endothermic reaction with the use of
> micro wave energy is a comparatively new process.
> If this reaction subtracts from the overall performance of the
> "gasification process" big deal.
> The creation of clean tar free gas for use in powering an IC engine
> which provides the power for this process will be nominal.

I think you are saying "big deal" as in, "this wouldn't be an unbearable
penalty".

I am saying that it really is a big deal - as in, you will fundamentally
end up consuming more electrical energy in this process than you will be
producing in the product's usable thermal energy (i.e. it is a
deal-breaker!)

The key point of my argument, if it is correct, is that the amount of
electrical energy required for this process will be very large (greater
than the thermal energy in the process's product gas).

Unless you can design a process in which the amount of electricity
consumed is quite small in relation to the amount of product gas, it
won't work in the economic sense.  Of course it will work and make
producer gas - but what I am saying is that the energy of this producer
gas will cost you more than an equivalent amount of electricity, which
is one of the most expensive and valuable forms of energy).



> Especially if exhaust gas from the engine is utilized for this
> process.
> GF

But it is important to see that you can only get a comparatively small
amount of your process's heat requirements from the engine's exhaust
gas.

Let's say your plasma reactor requires that the reaction zone be
maintained at 6000C, and that your inputs (fuel, water, air) are at 25C,
and that your engine's exhaust gas is at 600C.

You can use your engine's exhaust gas to heat your incoming feed
materials, absolutely!  But, you absolutely cannot heat them any hotter
than 600C using your 600C exhaust gas stream.

A very large amount of your endothermic reaction's energy (i.e. to raise
it from <600C to 6000C, and then to feed it energy at 6000C) must come
from one of two sources:

- partial oxidation of your fuel (this will work up to 1200-1500C)
- microwaves, derived from electricity (this will get you up to your
plasma temperatures).

The cost of these energy sources are:

- engine exhaust: the energy is free, though it adds to your equipment's
capital cost (you have to build heat exchangers)

- partial oxidation: the energy costs as much as your input fuel's
costs, which will probably be in the range of $2-$6 per MMBTU or GJ
(that would be the price range from coal, the cheapest, to medium
grade/cost biomass)

- microwaves: this heat energy costs you the cost of electricity,
divided by the efficiency of your conversion from electricity to
microwaves.  "Cheap" (10c/kWh) electricity is about $30/MMBTU or GJ,
your microwave power is going to cost more than this.

Your output will be an engine-grade producer gas.  What is it worth per
unit of energy?  That certainly depends a lot on local circumstances,
but it is probably pretty safe to say that it will not be worth more
than the market value of the electricity you can make from it, plus the
market value of recoverable waste heat from your genset.  The overall
efficiency of your engine plus generator will be in the range of
20%-35%, so call it somewhere between one-fifth and one-third the cost
of electricity.  The value of waste heat is necessarily lower - probably
a fair price would be the same as the energy cost of your input feed
material.

So the upper bound on the price of producer gas would be:

Price of produce gas  = 0.35 * Price of electricity  + 0.65 * Price of
biomass

Using plausible values of Pel=$30/GJ and Pbm=$4/GJ, then Ppg would be at
most $13/GJ.


And that is where the feasibility of using microwaves as a *primary*
gasification energy source breaks down.  Please note that I am not
saying that microwave-fired plasmas have no place in the scheme of
things - if they are a small-energy portion of the cycle, used to do
something quite valuable (e.g. "polish" out the last bit of tar), they
can make eminent sense.

But, if microwaves (i.e. ultimately electricity) are the source of
_most_ of the energy in the output gas, you end up with a process that
takes high value energy (electricity) and produces a smaller amount of
lower-value energy (thermal energy in a combustible gas).




-- 
- Daniel
Fredericton, NB  Canada