[Gasification] Conserve -- Run an Arc reformer now!!

[Daniel Chisholm]

On Fri, 2007-22-06 at 16:06 +1000, rkurt at tadaust.org.au wrote:
> Could we look at this energy in/energy out situation from a slightly 
> different perspective, and could someone do the maths please, as I'm no 
> qualified engineer/chemist and would like to understand a bit more than 
> I do at the moment.

Hey, what fun, sure! ;-)

I'll run some of the numbers for you, but the really good news is that
there is some free software available that you can use for this purpose.

I've sung praises before of the work done by the Biomass Gasification
Group ("BGG") at the Danish Technical University, in particular their
50kWe "Viking Two Stage Gasifier", a low-tar wood gasifier that everyone
interested in engine-grade gas should read all about (Google is your
friend...).

DTU also have made available on the web site a program called
"gasifier.exe", which I have found to be extremely useful to use in
modelling the gasification process in order to learn and understand it.
The only disadvantage is that is is a Windows ".exe" program (so bad
news for Linux, Mac or UNIX people, but probably not an issue for most).

You can fetch it here:
http://www.bgg.mek.dtu.dk/Research/Modelling.aspx
http://www.bgg.mek.dtu.dk/upload/subsites/mek-bgg/gasifier.exe

Probably the best thing to do is to go out and download the program,
then follow along with me and run the same numbers on your copy as I do
here...

> Take 1 kg of biomass, assume a suitable moisture content. Ie CxHyOz.nH2O

The program already assumes a decent formula for biomass, and you can
adjust it at will.  See the upper right hand corner blue box "Fuel
data", the default values are C,H-1.586,O-0.7089, plus 30% moisture
content wet basis.  Tom Reed often suggests another figure that uses
rounder numbers for the H and O figures, but I can never remember what
that is.

They model the quantity of biomass as a flow rate per hour.  The pink
box at the upper left hand corner has a default value of 200 kg/hr of
biomass on a dry basis (if you change your fuel moisture content, this
figure is unchanged, but the greyed-out "Moisture" box is re-calc'd
accordingly).

I think they choose 200kg/hr because this is almost exactly one MW per
hour of fuel thermal value input.  I'll leave these nice round numbers
as the are.

In addition to the "Moisture" parameter, which is H2O that comes along
with your fuel (and which you have full control of), they include a
separate input labelled "Steam".  This allows you to model a separate
addition of additional H2O mass, and more importantly, it provides a way
of modelling an energy addition from an external heat source.  The
default value they use is 36 kg/hr, we'll leave it as-is for now.

In the green "Gas composition" box, this has the calculated values of
the different gas species, however it does not model the methane
production, you have to input this as a parameter.  You have to do so
intelligently too, based on your process parameters, or else you will
get unrealistic results.  At this point I will SWAG the CH4 parameter to
0.0%, and perhaps revisit it later (my reason for so SWAGing: most of
the arc process happens at very high temperatures, and methane formation
is favoured by lower temperature reactions)

In the center of the screen is a flow diagram of the gasifier that they
are modelling, with a number of the process variables given reasonable
default values, and with the ability for you to change them.  You can
experiment with all sorts of interesting "what-if" scenarios.  Truly,
one's imagination is the most limiting factor.  Here are some that I
have thought of running, and have found very interesting:

- the effects of preheating the air, and/or preheating the fuel feed
stream
- the effects of adding steam, possibly extremely energetic steam (this
is what we will be doing to model the arc reformer).
- the effects of running the gasifier at different output temperatures
(basically, how impatient are you, and how off-equilibrium can you
afford to go)
- the effect of fuel moisture content on the gas quality and the
system's energy balance.  If you ever wonder why Doug seems so
absolutely, unreasonably (! ;-) fixated on having no more than 15%m.c.
for an Imbert-style gasifier, you should do some runs here with 60%,
40%, 30%, 15%, 10% and 5% moisture contents.  I did...
- modelling a straight charcoal gasifier (change your biomass formula to
C-1, H-0, O-O, with 0% m.c.).  Get a straight CO producer, or add some
steam or fuel moisture and get blue water gas


If anyone has run other interesting gasification scenarios, I would love
to hear about how you did it (oftentimes I just don't have the
imagination needed to fully take advantage of something).

Or, if you have an interesting scenario and want me to run it (and I
find it interesting too ;-), perhaps I can take a crack at modelling
it...


> Reform it with arc generated heat. the result will be H2 and CO and H2O.

OK, the way we will model this is that we will enter unrealistically
high values for the temperature of the input steam, and also of fuel and
air preheat.

The first thing I did was to set the steam temperature to 4500C, and the
program warned me that this was outside its range of competence (3500K
max), so I will use 3200C as the steam temp.

Clicking on the "New Calculation" button in the lower center of the
screen gives the results at this stage.  Looking at the pink "Mass
flows" box I see 373 kg/hr air flow calculated by the model, which means
that a lot of the heat is coming from some of the fuel being oxidized
(burnt).  Now is as good a time as any to explain what happens when you
ask the model to calculate...

The model asks for enough parameters that it is able to calculate how
much air flow is required to balance the reaction.  The more air that is
added, the more heat-supplying partial combustion takes place.

The model has these mass inputs:
- fuel
- water (fuel moisture, plus steam injection)
- air

And these energy inputs:
- air preheat
- preheat of the fuel (which is the dry fraction plus the moisture
fraction)
- heating of the steam feed
- ***the partial combustion reaction***

These are the mass outputs:
- gas produced
- char loss

The energy consumption goes to:
- the internal endothermic load of the gasification process
- a heat loss parameter
- sensible heat of the output gas
- chemical (combustion) energy of the output gas

Notice that you control (as a parameter) every input except for the
amount of partial combustion that occurs.  The model calculates the
amount of air that has to be supplied, so that just the right amount of
partial combustion takes place, so that the chemical composition of the
gas is appropriate to the system's mass and energy flows.

By looking at the amount of air flow the model asks for, we can see how
much of the fuel has had to be consumed in order to produce the heat
required to run the reaction.

In a traditional Imbert gasifier, all of the energy comes from the
partial combustion from the feed air.

In a steam-only gasifier, or an arc gasifier, none of the energy comes
from partial combustion, all of it comes from externally supplied heat
inputs.

The model helpfully gives an energy balance (the orange box in the lower
left corner), for *most* of the interesting parts of the system.
(Sometimes I've had to puzzle out some things I've wanted, but I can't
at the moment remember what they were...)

The "Thermal input" is 1014kW - this is the thermal fuel energy of the
wood chips.

Since we're not preheating the fuel feed, we are putting 0kW into the
"preheating dry wood chips" and "preheating moisture" accounts for now.

The air preheat from 25C->450C is an energy input to the system of
46.1kW.

The preheating of the 36kg/hr of steam from 25C->3200C is 109.4kW.


OK, I will now set the wood chips preheat to 3200C, and also the air
preheat to 3200C.  Running this gives an air mass flow rate of
-104.7kg.hr, so obviously we have given too much input heat, and the
model is balancing the energy account by calculating a negative amount
of combustion - clearly unrealistic! ;-)

I'll now set the air preheat to zero (air enters the gasifier at 25C),
and I will keep guessing at wood chip preheat values until the air flow
rate in the upper left pink box is approximately zero.  This will mean
that none of the system's heat is being supplied by partial combustion,
but rather it will all be coming from my preheat sources (i.e. I am
modelling the arc's energy as being equivalent to very generous amounts
of heat applied to the fuel and steam inputs).

Using a wood chips preheat temperature of 1920C roughly balances things
out - the model figures it needs to use 0.4kg/hr of combustion air,
which is close enough to zero (i.e. balanced) for our purposes.

Let's now look at what the model indicates:

Mass flows: 200kg/hr dry wood chips, 85kg/hr of fuel moisture, 36kg/hr
of steam, 0.4kg/hr of air.

Gasifier input parameters: 25C air preheat, 1920C wood preheat, 3200C
steam, 800C gasifier reaction zone/outlet temperature, 1% char loss.

Gas composition, dry basis: 33.7% CO, 9.6% CO2, 0.0% CH4, 56.6% H2.  (On
a wet basis, there is 13.6% H2O in the gas).

Now what Kurt was asking for, the energy balance:

1014kW of fuel energy in (that's the 200kg/hr of chips)
157.4kW to preheat the dry wood chips
167.3kW to preheat the chips' moisture
109.4kW to preheat the steam.

Adding the last three items together gives 434.1kW energy input.  That
would be your arc energy.

The summary of the output energy is:
1169kW internal energy in gas (this is the gas's heating value)
182.2kW cooling of gas (that the thermal energy of the gas at 800C as it
is produced vs the 25C that it is used at).

Overall, here's what we have:

1014kW of fuel energy in
434kW of electrical arc energy in

1169kW of combustion energy available in the cooled output gas.  Note
that is about 115% of the 1014kW of wood chip energy that we fed in.
(Don't worry, we haven't invented a perpetual motion machine - I will
leave the reason why unanswered for now, in case anyone who has read
this cares to point out what is happening

> How much electrical energy will be required. How much syngas will be 
> needed to produce that amount of electrical energy, using a reasonably 
> efficient generator setup.
> How much syngas will be left over and how much energy will it contain.

As the process stands, we need 434kW of electrical power in, which
produces 1169kW of syngas.  If you want we could refine the model a bit
(credit some of the steam and fuel preheating to energy extracted from
the sensible heat of the output gas, but the most you could possibly get
there is 182kW.  We can talk about this later, if you want...)

1169kW of syngas run through a genset operating at 20% efficiency will
produce 234kW of electricity.  This is not enough to power the arc.

You will have no syngas left over, in fact you'll have to supply about
1000kW of syngas to the system just to keep it running.

Or, if you produce electricity from your syngas at nearly 40% efficiency
(which is unrealistically high!!!), your plant could break even - i.e.
it wouldn't need syngas, nor produce any syngas, but it would sustain
itself - it would consume 1014kW of biomass, and produce (on a steady
state basis) a total of 1014kW of heat from the various points of the
process (heat loss, gas cooling, engine heat loss, generator heat loss,
etc).


> How much energy would be in producer gas made with a representative 
> gasifier from that same amount of biomass.
> 
> Ignore questions of scale and losses. Also the influence of potassium, 
> phosphorus and other nutrients in the biomass. This should give an idea 
> of a theoretical maximum output from such a system.
> 
> please :)
> Kurt

Well as I have run the numbers, the theoretical maximum is somewhere
between -100% efficiency and 0% efficiency.  (My "-100% efficiency"
figure is a bit fast and loose - by that I mean that your net syngas
production is about -1000kW, for a fuel feed rate of 1014kW).

Which isn't particularly exciting, I would think.

BTW this is the first time I've "run the numbers" on the arc reformation
process.  My earlier comments that it was infeasible were based on a
qualitative estimate of the relative values of the energy sources (i.e.
using high (heat) grade high ($$) value electrical energy to produce low
(heat) grade, low ($$) value syngas.

FWIW, I took a screenshot of "gasifier.exe" software with the final
values filled in, and placed them on my server:

http://strangejava.wort.ca/woodgas/Screenshot-gasifier-arc-reformation.png

If that format doesn't display for you, here are two others that might:

http://strangejava.wort.ca/woodgas/Screenshot-gasifier-arc-reformation.bmp - this is sharp,
but it is about 3MB in size, so beware if you're on dialup...

http://strangejava.wort.ca/woodgas/Screenshot-gasifier-arc-reformation.jpg - use
this as a last resort.  Everyone should be able to read this,
unfortunately JPEG is a poor format to use for text images, the edges of
the letters are pretty fuzzy


I hope at least some people on the list have found this interesting or
useful.  I find it really is easy using this software to "run the
numbers" on all sorts of interesting scenarios that make it past your
qualitative ideas triage stage, and sketch out what is and is not
feasible, what process factors are usefully tuneable or perhaps not
justified, e.g.:
- good values of fuel moisture?
- steam addition?
- How much air preheat is useful, and is it economically recoverable
from the gas stream with a reasonably sized and priced heat exchanger?
- how much do heat losses or char losses hurt?  What amount of
insulation is justifiable?
- What about hot-gas operation for close-coupled combustion?
- what about different reaction zone temperatures?
- what about gasifying charcoal?


Let me say a big thanks to Felicia Fock and Kirstine Thomsen of DTU MEK
(Danish Technical University mechanical engineering department) for this
software!


-- 
- Daniel
Fredericton, NB  Canada