[Gasification] Calculations for a new gasifier

[Daniel Chisholm]

On Sat, Jan 7, 2012 at 07:49, Robert Kana <sinan at biomassindo.com> wrote:

> We need some help calculating the amount of fuel needed to generate enough
> power for 160 Kwe every hour. Suggestions are welcomed.
>

Are you looking for an approximate quantity (so you know how big to build
your gasifier)?  That's pretty straightforward (to get an estimate within
say +/-15%).

If all you need is a rough estimate, mine is that you'll need 160-240
kg/hour of fuel input, with 190kg/hour being my best first guess.


***** some detail on how to do the calculations yourself, better.

You need to know your overall system efficiency (electricity output per
unit of fuel energy input), then you multiply that by 160kW (your
electrical power output).

Your overall system efficiency is the product of three subsystems:
- gasifier
- engine
- generator

A conventional gasifier might have a 70% "cold gas efficiency" (which means
the fuel energy of its cooled, ready-to-use output gas, divided by the fuel
energy of its input feed).

A typical small spark ignition engine might give 20% efficiency (shaft
power out divided by fel energy in).  But there could be quite a big range
here, this figure could easily be 15%-30% so the best idea is to lookup the
data of that engine and find out the manufacturer's ratings (if they give a
specific fuel consumption "sfc" for natural gas or gasoline/petrol, you can
convert that).

Electrical generators are pretty efficient.  For this calculation let's use
90% (but for better results, look up the specs on the generator you are
using).

Now we can calculate your overall system efficiency:

OverallEfficiency = GasifierColdGassEfficiency * EngineEfficiency *
GeneratorEfficieny
OverallEfficiency = 70% * 20% * 90%
OverallEfficiency = 12.6%

The final thing you need to know is then energy content of your input fuel.
 You know your fuel better than I do but I will assume that it is "standard
wood":

InputFuelEnergyContent = 20,000,000 Joules / kg (in Imperial units 8612
BTU/lb)

You want your generator to produce 160kWe.  This is 160,000 Watts of
electrical power output, which is 160,000 Joules / second.

Because your overall efficiency is 12.6%, to get 160,000 Watts out you will
need to feed:

FuelInputRate(power) = OutputPower / OverallEfficiency
FuelInputRate(power) = 160000 Watts / 0.126
FuelInputRate(power) = 160000 Watts / 0.126
FuelInputRate(power) = 1,269,841 Watts

So you need 1269841 Watts (which is Joules/ sec) of fuel input.  How much
is that?

FuelInputRate(massflow) = FuelInputRate(power) /InputFuelEnergyContent
FuelInputRate(massflow) =1,269,841 Joules/sec   /  20,000,000 Joules / kg
FuelInputRate(massflow) =1,269,841 Joules/sec   /  20,000,000 Joules / kg
FuelInputRate(massflow) = 0.0634921 kg/sec

If you want an hourly rate, multiply it by one hour (which is 3600
seconds/hour):

FuelInputRate(massflow) = 0.0634921 kg/sec * 3600 sec/hr
FuelInputRate(massflow) = 230 kg/hour

So my estimate is that you will need 230 kg/hour fuel input in order to
produce 160 kWe

(this is a system end-to-end Specific Fuel Consumption of 1.4 kg wood per
kWh of electricity output)

The accuracy of this calculated result depends directly on the accuracy of
the input data.  If your engine actually has 28% efficiency rather than the
20% I assumed/used in my calculation, your fuel consumption will be 20/28=
71% of what I've calced (so that would be 160kg/hour).


-- 
- Daniel
Fredericton, NB  Canada
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