[Stoves] How to make smokeless coal?

[Crispin Pemberton-Pigott]

Dear Darpan

“I beg to differ on that. Commercial customers like power plants have control devices like electrostatic precipitator, fabric filters etc to control pollution, whereas emissions from coal cook stoves are uncontrolled.”

Power plants do not burn coal in anything like the manner in which domestic users do it. They blow the entire mass of coal through the heat exchanger and the ash is captured on the far side. Domestic combustors burn the coal in situ and when properly constructed, have lower CO and PM emissions per unit of delivered heat than even a $1bn power plant.

>It is thus a major contributor to the overall degrading air quality.’ [referring to domestic scale combustion of coal]

This is only true in places that do not use modern combustion devices. I too am mystified by what you have referred to as ‘uncontrolled combustion’. I know of no such device. Even a South Africa $5 mbaula has a degree of control over the cooking power and the air flow is regulated by the hole pattern.

The result of a series of investigations into how to burn coal on a small scale for domestic use, both at the SeTAR Centre in Johannesburg and by other members of the South-Sough Sustainable Stoves Group (S4G) is that a number of products are hitting the market which are cleaner than anything yet witnessed in the small stove scene.

The pollution you speak of is caused by the improper and incomplete burning of coal (or wood or anything else). Smoke is not a property of raw coal, it is a result of incomplete combustion. Smoke is not a property of sugar cane leaves or dung or wood or biomass pellets. It is a by-product of cooking but we don’t devote much time to discussion cooking pollution (from the food).

A paper was presented at the Domestic Use of Energy conference 2017 by a group from North-West University, Potchefstroom on the subject of emissions from a standard, worn, old fashioned cast iron cooking and heat stove that was variously fueled with coal that had been roasted to several different temperatures starting at 450 C. They produced coal at different temperatures then measured the performance in terms of duration of the fire, maximum heating power, CO, PM2.5 and time needed to achieve ‘ignition’. They concluded that the ‘best performance’ was achieved with coal heated to 550 in an airless container.

The caveat for the study is that the device was not designed to burn coal well – it is just a cast iron box surrounding a grate, with a chimney on one end and cooking plates on top. The same fuels placed into a variety of other stoves would produce a variety of other ratings.

One of the curious things you will notice about the combustion of solid fuels in general is the oft-repeated claim that ‘fuels’ have a ‘performance’ (good, bad or otherwise). It is like saying gasoline has ‘good performance’ without referring to the device which burns the fuel.  If I put 85 octane gasoline in a ’69 Super Stock Dodge Hemi will it give ‘good performance’?  What does ‘good’ mean? Nikhil goes farther, asking “What does ‘performance’ mean?” To a little old lady from Pasadena, that is a critical question.

If you want to reduce the indoor and ambient emissions from coal stoves in India without changing the stove, then semi-coking the fuel will reduce the level of smoke. It will also create a toxic hydrocarbonaceous by-product for which there may not be a local market. It will also triple the cost of fuel. Will the fuel be subsidised? Would it be better to subsidise a better stove than fuel forever?

Attached is a video of a clean burning coal fire. It is a domestic stove. It does not have uncontrolled combustion. The flame is distorted (made worse) by the removal of the pot hole cover plate, necessitated to make the video. When the plate is in place, the flame is even better and doesn’t move around as much.

The fuel is raw bituminous coal. The PM2.5 emissions are not zero, because there is a little fly ash that gets into the heat exchanger. But there is very little BC or other unburned carbon forms. The processes taking place inside the stove are identical to that which takes place (separately) when coal is coked and then the coke is burned. The difference is that in this stove it happens continuously, provided there is fuel added from time to time (about per 12 hours). So, it is making a pyrolysed, semi-coked fuel, then coking it immediately, the burning the coke immediately, all in one continuous operation. There is no increase in fuel cost and no toxic liquid by-product.

This method does not remove inherent emissions that are compositional properties of the fuel. Burning a coal containing fluorine would in this same stove not produce non-toxic emissions. Same for Laburnam of Yew in a wood stove.

Tell me, are you interested in introducing very low smoke stoves in India or just an altered fuel? Both work for reducing smoke.

Thanks
Crispin
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