[Stoves] TLUD as a lighting technique for coal stoves

Wed Dec 15 10:11:03 CST 2010

Dear Friends of Coal

There may still be some
.

I have been testing different ignition techniques for coal stoves to see
where the best overall performance or shall I say effectiveness lies.

There are a few new kids on the block, meaning stoves, and TLUD may be the
most effective way to get a low smoke ignition while retaining the ability
to be refuelled continuously as needed.

A new form of the GTZ 7 stove (July 2010) currently titled the 7.4 was
ignited using the following technique:

On the grate, place 15 mm square (approx)  wood to cover all the holes (long
grooves actually). 

Then place 12mm coal on top about 15 mm deep. 

Drop 8 to 10mm square wood about 75mm long on top by dropping about a large
double handful randomly into the vertical combustion chamber which lands on
the layer of small coal.

Fill in the gap leading to the coal hopper with similar wood longitudinal
like the grate bars. 

Stack more 15mm square wood across, about 50 g. Fill behind with coal, about
25-30mm.

Cover the wood and coal with small to ordinary coal about 50mm deep. Leave
the hopper open (it is normally closed and has a pretty good seal).

Drop 2 capfulls of diesel (there is no paraffin locally available) onto the
small wood sitting at the bottom combustion chamber. Light a piece of paper,
drop it in and close the top of the stove.

The result is a diesel ignited fire burning small wood rapidly. This ignites
the wood leading to the hopper. The larger wood in the hopper provides a hot
fast fire blowing across the top of the small coal pieces on the grate. The
effect is to melt the coal (it is frozen!) then roast out some volatiles
which join the fire.

For 10 minutes the hopper is left open, supplying as much air as it wants to
draw. Then close it. After 5 more minutes, shake the grate, open the hopper
and load with fuel to the top (about 5 kg).

The effect of this is to create a hot fire immediately with very low PM
levels. The way the coal is initially lit creates a form of TLUD to get the
system hot. The rapid rise in temperature in the combustion chamber provides
immediate draft and drives the whole system.

Attached are three photos:

A picture of the GTZ 7.4 stove

A picture looking into the combustion chamber just before ignition

A picture of the temperature readings. They are in order from top to bottom:

Red – flame temperature measured 50mm below the cooking surface. It rises to
700 C within 2 minutes, a lot of that merely the time taken to heat up the
tip of the thermocouple. The dip is the point at which significant moisture
from the coal (26% moisture content) starts to evaporate in significant
amounts.

Yellow – temperature of the top of the cooking surface.

Brown – temperature in the heat exchanger exit, roughly the bottom of the
chimney.

Purple – temperature at the point where the chimney leaves the building (all
heat passing this point is lost). It settles on 200 C

Green – ambient temperature

Blue – Outside temperature

It turns out that the speed of the rise in the flame temperature is very
important to limit the PM emissions. In this case the PM2.5 was negligible
throughout the test which ran for 200 minutes (the picture does not include
the whole time period).

The combination of TLUD ignition and crossdraft operation – given sufficient
starter material – has proven to give the cleanest overall operation of a
space heating and cooking stove we have seen.

The PM2.5 emitted was 1.0 mg/MJ during a burn that generated 59 MJ of heat.

It is interesting to note that as the air in the lab was carrying 240
microgrammes of PM 2.5 per cubic metre, and as the stack contained
substantially less than that, the stove was net-negative for PM2.5 emissions
for more than ¾ of the total burn. It was not negative overall as there was
a little smoke during ignition (after the TLUD wood was about ½ done). 

Perhaps there is a real chance that we will find a way to have net negative
PM emissions while burning a supposedly ‘smoky fuel’ – in this case lignite
containing 26% moisture and 50% volatiles in what is left!

I thanks for the Asian Development Bank for supporting the laboratory, World
Bank for coordinating the UB-Clean Air Project and GTZ for supplying the
stove to spec.

Regards

Crispin

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