Reading the article “Wood Pellet Combustible Dust Incidents” of John Astad, I remembered the following.
All biological products are subject to deterioration.
This deterioration is caused by micro organisms (bacteria and micro flora)
To prevent bacterial deterioration, it is necessary to condition the circumstances in such a way that micro organisms cannot grow.
1) By killing the micro organisms through sterilization, pasteurization or conservation. In transport also the gassing with methyl bromide is common but not without danger.
2) Creating an environment that micro organisms cannot develop by f.i. adding acids, salt, sweet or drying and cooling.
In storing cereals, grains, seeds, and derivatives, drying is the mostly used method to prevent bacterial heating.
To prevent bacterial deterioration those materials need to be DRY before storing.
Bacterial deterioration is an action of micro organisms or other chemical reactions, whereby the generated heat is more than the heat that can drain off.
As the stored material normally has very low heat conductivity, this situation is easy reached.
Micro organisms can develop above a certain moisture content.
The micro organisms break down a part of the material and convert that to CO2 + H2O + heat. (Carbon dioxide + water + heat).
Carbon dioxide is a gas that can escape.
Depending on the kind of organism, other chemicals can be formed s.a. CO, acid, alcohol and other chemical compounds. (Sometimes those compounds can be smelled).
There are 4 stages to recognize:
1) up to 40 degrC during a few days and production of CO2 and H2O. CO2 concentration from 0 to 2%
2) between 40 degrC to 70 degrC (duration approx 8 days) and an increased production of CO2, some CO and fermentation smell. CO2 concentration from 2 to 9%.
3) between 70 degrC to 100 degrC. The temperature of 100 degrC will remain until the water content of the material is almost disappeared. (steam production). CO2 concentration 10%
4) After that, the temperature rises fast under strong production of CO2, CO, and H2. (Measured in cases up to 500 degrC). Oxygen can be attracted from the environment or from the material itself (f.i in silos) – under the forming of carbon. Progressive forming of smoke can be observed and specially in open systems (or when a silo is opened) a fire can develop rapidly.
The stages up to 70 degrC are caused by bacteria.
Above 70 degC are caused by chemical reactions, which can be so strong that self ignition can occur.
Apart from the moisture content, other parameters s.a. fat content and (residual) oil content influence the occurrence of bacterial heating.
How progressive a bacterial heating will develop depends on the initial bacteria concentration and the kind of bacteria, moisture content (optimum RH 40% to 80%), oxygen content for aerobe bacteria (anaerobe bacterial heating is also possible), and temperature.(Optimum starting temperature 30 degrC)
The detection of heating can be done by temperature measurements, although not very accurate, because a hot spot is often very local.
Detection of the formed CO2 above the stored material is good indication of the progress of a possible hot spot. The space above the material must not be ventilated during monitoring.
To stop already started heating, the corrective measures are:
1) in the first stage the material should be taken out of the silo and filled back again. (turn over)
2) Remove colorized material and turn the material over
3) Empty the silo immediately
4) Call the fire brigade.
Because of the fact that a part of the material is converted into gasses and evaporable fluids, which can escape, there will be loss of material mass.
Also because of the higher temperatures, the water, which is present in the material and formed by the chemical reactions, will evaporate and escape, causing an additional loss of mass.
The loss of mass is related to the intensity and time period of the heating process.
I made this note in 1989, when I had to figure out how much mass of a biological product could be lost during a certain time of silo storage.
As you can notice, I am not only focused on pneumatic conveying.