Carbonization of wood

Charcoal is produced by controlled combustion by supplying very limited available oxygen by which the volatile vapours and gases are driven off from biomass particularly wood.
Charcoal
Charcoal has a higher energy density than wood and is smokeless. Traditional charcoal kilns are used to make Charcoal by simply heaping mounds of wood covered with earth, or pits in the ground. But the process of carbonisation is very slow and inefficient in these kilns. Traditional earthen kilns have yields closer to 10% as there is less control. In general during the process of carbonization, considerable amount of carbon is emitted to the air and approximately 50% of the carbon in the biomass can be obtained in the form of charcoal. Carbon yield obtained using various types of kilns is less than 50 % on the average, which means that half of the total carbon contained in the raw material will be emitted to the air or stored in the form of half-carbonized matter which can easily get decomposed, but there is a need to increase the carbon yield. Field research has improved the efficiency of charcoal furnaces.Improved charcoal furnaces operating at about 600°C produce 25-35% of the dry wood as charcoal, and even the gases produced can be used for kiln drying. The charcoal produced is 75 - 85% carbon and is useful as a compact, controllable fuel. It can be burnt to provide heat on a large and small scale.
Composition of charcoal
Charcoal is composed of (1) moisture, (2) ash content, (3) volatile matter and (4)fixed carbon. These parameters are calculated as given below using standard methods.
Methods for calculation
(1) Moisture content: Moisture content of charcoal immediately after finishing carbonization in the kiln will be nil, but increases subsequently due to absorption from the air. Moisture content of charcoal is calculated as :
Moisture content (%) = (Weight of charcoal – Oven dried weight of charcoal)/Weight of charcoal x 100
(2) Ash content: It comes from few minerals contained in material (CaO, K2O,MgO, etc.)Ash content of charcoal is calculated as:
Ash content (%) = Weight of ash in charcoal/Oven dried weight of charcoal x 100
(3) Volatile matter: It contains all liquid and tarry matter not fully driven off in the process of carbonization. If the carbonization is prolonged and if the temperature is high , then the volatiles content of is low. Volatile matter content of charcoal is calculated as:
Volatile matter content (%) = Weight of volatile matter in charcoal/Oven dried weight of charcoal x 100
(4) Fixed carbon:Fixed carbon content of charcoal is calculated as:
Fixed carbon content (%) = 100 – Moisture content - Ash content – Volatile matter content.
Pure carbon in charcoal is difficult to be decomposed as it shows no chemical affinity to the oxygen of the air and it does not rot, neither in an aerobic nor in an anaerobic way. This component mainly contributes to carbon storage.
(5) Charcoal yield and carbon yield: Usually, efficiency of charcoal production is indicated by charcoal yield. Charcoal yield is calculated using formula as:
Charcoal yield (%) = Weight of charcoal (kg)/Oven dry weight of wood material (kg) x 100
It expresses the percentage of pure carbon derived from wood material which is preserved in the produced charcoal. This indicator, however, cannot explain the efficiency of carbon fixation potential.

Co-firing of biomass and wastes

Co-firing means firing materials along with coal or other fuels in the existing or modified boiler firing systems. Materials such as biomass and waste are solid fuels in their own right, but of low calorific value, of variable composition, and requiring special low-intensity combustion conditions. If mixed with coal, the average calorific value is increased, enabling it to be burned in conventional combustors and to have more average fuel properties. If coal is mixed with material such as a typical biomass, then the combustion process can be considered to be increasingly CO 2 neutral. If coal is mixed with refuse or other waste materials, its combustion provides a route to the disposal of unwanted materials with the advantageous production of energy.

Biomass co-firing

There have been extensive research to increase the use of biomass, and one of the easiest ways of introducing this technology is by co-firing with coal in existing coal installations. A similar approach can also be taken with the disposal of waste, especially domestic refuse. The combustion of biomass therefore also parallels that of coal in many respects. The first step involves devolatilization similar to that of coal.

Namely: Biomass volatiles + char

In this reaction volatiles have a composition controlled by the biomass composition, the heating rate, and the final temperature. A typical biomass will have cellulose linked via oxygen bands; hence the high oxygen content in biomass fuels. Many biomass materials contain mainly carbon and some hydrocarbons; consequently, the composition of the volatiles is dominated by the production of CO, some hydrocarbons, and a small amount of tar. The char itself has reactivity very similar to coal char and has enhanced catalytic reactivity at low temperatures due to the metallic content of Na and K. For the mixtures of biomass and coal, the initial components devolailize and their chars burn independently. The volatiles mix, however, and the calorific value of this mixture and resultant flame temperature is very much a function of the composition of the components; the combined flame temperature is fed back to the initial chemical steps and consequently there is a very significant synergistic interaction. The subsequent rates of burn out of the chars are determined by their individual reactivity (and combined temperature) and generally biomass chars burn out more rapidly than coal chars.

Waste co-firing

The types of wastes suitable for co-firing include biomass waste, municipal solid waste, and automotive tires. The comparison is variable but has a calorific value between 10-20 MJ/kg. When the waste is taken from a controlled source, e.g., waste paper, with a constant calorific value, then the process of co-firing is simplified. In the other extreme, relatively small amounts of certain toxic wastes can be co-fired with coal in cement manufacture. The cement contains the trace elements in a generally satisfactory way although great care has to be used to ensure that it meets the required environmental emission standards.