[Stoves] Vietnamese brick kilns

[Marc Pare]

Sure thing, Crispin.

Crispin help me set up an interesting efficiency calculation the last few
weeks, and the results may be of general interest to the list.

I have been trying to quantify the efficiency of the direct combustion of
rice hull for heating applications. This should have some interest in stove
design since sometimes stoves are used for such applications e.g. space
heating.

The big question was how efficient is direct combustion of rice hull when
it is powered by natural draft?

The tricky part of answering this question is that the amount of excess air
(the amount of air pulled into the system that isn't used for combustion)
has a great effect on the overall efficiency. It's too hard to measure the
outlet velocity of chimney. Plus, you would need an accurate measurement of
fuel consumption. Finally, it's not really feasible to measure the
temperature of the products of combustion.

Luckily, you don't need any of these things. By leaning on the chemistry of
the combustion reaction, you can calculate the overall "efficiency as a
heater" with just a gas concentration reading (O2 or CO2) and an outlet
temperature.

In other words, with just a combustion analyzer, you can see how much heat
energy was available for your system. This is exactly the number you need
to compare between options for energy sources.

The details of the calculation with this method are in the attached
document -- BS845, a British boiler efficiency standard.

The results from direct combustion of rice hull are really fascinating. I
attached a plot of the measured efficiency.

To get most of the energy out of the fuel in the traditional process, the
combustion is done in phases.
Phase 1 (5-10 minutes): A pile of raw hull is dumped on an inclined grate.
It roars up into flame. Reaches > 1000 C
Phase 2 (10-20 minutes): The rice husk turns to char. It's still red hot
and still burning. This proceeds slowly and the temperature gradually
decreases to < 500 C

The char is scraped off of the grate. It hasn't combusted all the way to
ash.

What's interesting about this is that the efficiency of "Phase 2" drops
considerably. This isn't the case with other fuels I have seen reports of.
I plugged in some of the data from an assessment in India of charcoal
kilns, and even with similar excess air values, the efficiency doesn't drop
as low as rice hull.

What I think is responsible is that the particular chemical structure of
rice hull (high ash, a silica skeleton) acts as a brake on the "char
reactions" (I'm not sure of the technical term for this). You have to
strike a balance between getting the energy out of the rice hull char and
running at low efficiencies in your process.
I'm going to take a deeper look into this area some time soon.

Any others have thoughts to weigh in?

Marc Paré
B.S. Mechanical Engineering
Georgia Institute of Technology | Université de Technologie de Compiègne

my cv, etc. | http://notwandering.com


On Sun, Oct 14, 2012 at 11:19 PM, Crispin Pemberton-Pigott <
crispinpigott at gmail.com> wrote:

> Dear Marc
>
> Would you like to describe some of your recent work on using rice hull to
> fire bricks in Vietnam?
>
> It is an interesting application and he methods you used are of general
> interest.
>
> Thanks
> Crispin stuck in Beijing
>
>
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