Thursday, 6 September 2012

Chemistry of peat fires

Proceedings of the Combustion Institute
NOTE: This paper received the Distinguished Paper Award on Fire Research at the 34th International Symposium on Combustion by The Combustion Institute.

We have just published a paper in the Proceedings of the Combustion Institute on the chemistry of smouldering peat:

 "Study of the competing chemical reactions in the initiation and spread of smouldering combustion in peat"

Smouldering is the slow, low-temperature, flameless burning that represent the most persistent type of combustion phenomena and which leads to the largest and longest burning fires on Earth. Smouldering megafires in peat and coal deposits occur with some frequency during the dry season or eventual droughts in, for example, North America, Siberia, the British Isles, the subartic and South-East Asia.

In this work, we use an experimental methodology to study the smouldering combustion of samples of peat under a wide range burning conditions. By varying the oxygen concentration and the ignition conditions we investigate the competing pyrolysis and oxidation reactions.

We focused on the three main solid species involved in smouldering fires: peat, char and ash . It shows clearly how pyrolysis concentrates carbon in the char while a large fraction of the hydrogen is released, while the oxidation releases most of the carbon and concentrated the minerals in the ash which H, C and N contents are negligible. The fraction of carbon in char is ~1.5 times higher than in peat, and ~35 times higher than in ash. The change is even greater in terms of carbon density, it increases from 77 kg-C/m^3 in the peat to 133 kg-C/m^3 in the char, to then sharply drop to 0.7 kg-C/m^3 in ash.

The experiments clearly show that there are pyrolysis and oxidation reactions. Char is formed by pyrolysis and consumed by oxidation. So at the beginning of a test there is no char, and at the end only a small amount of char remains, but in between substantial amounts of char (~50% of initial weight) were momentarily formed. Smouldering produces and consumes its own char: it initially produces char through pyrolysis before being consumed by char oxidation reactions. The competing nature of the production and consumption char reactions has been experimentally shown (see figure below).

 Evolution of peat and char fractions through an experiment.

The virgin peat reacts during the first 15 min to produce char and ash. Thereafter, only char reactions take place producing ash. Tracking the amounts of peat and char at any given time shows that first char is formed. It reaches a maximum fraction (~50% of the initial mass) in 20 min and then slowly the char is consumed down to ash (10% mass). At the end of the experiment, 90% of the initial mass has been released as gases, leaving a void and a thin layer of ash.

By varying the oxygen concentration and the thermal conditions we investigate the competing pyrolysis and oxidation reactions at a fundamental combustion level. The figure below shows infrared images of the surface of samples at different oxygen levels (21% is normal air) during the early burning stages of ignition (5 min after first heat exposure). As the oxygen level is increased, the temperature of the sample surface increases (indicated by brighter colour) showing that although pyrolysis dominates in this early stages of spread, oxidation reactions also play a role.

 Infrared images of the sample at [O2] of 17%, 21%, 25% and 35%just 5 min after first heat exposure.

The results presented here can be used to advance our fundamental knowledge of large-scale smouldering wildfires which are currently not well understood.

Title: "Study of the competing chemical reactions in the initiation and spread of smouldering combustion in peat"
By: Hadden, Rein and Belcher In: Proceedings of the Combustion Institute (in press), 2012.