3.1 Firewood

3.1.1 General scope

Firewood is a classic energy source, and is still important household energy source in many developing countries. In the latter half of the 20th century, firewood was deprived of many uses by petroleum, but firewood production occupies more than half of the harvested wood, and firewood covers 14% of the world energy consumption, and 36% of the energy consumption in the developing countries.

However, in some regions, the amount of wood is decreasing with the increasing population, and they have to travel far to get firewood. They have troubles even for getting firewood for cooking. In Asian countries, most of the forestry wood has difficulties in use, due to the troubles encountered for transporting wood from the forest of high slope area.

For the left side of Fig.1, which is the supply side of firewood from raw wood to the furnace, what matters now is not the amount of the resource, but the energy and cost for the transporting the wood from the forest. When external energy supply for this transportation *e and energy available from the product firewood E have the relationship I*e > E,

this system fails to be a net energy producing system. This aspect is also very important for the case where chipping or palletizing is made so that the fuel is easily handled at the furnace.

*e transportation, etc. p^[E] ^cooker or heating

Raw wood————————————- > stove————— > [flue gas]

*e drying, cutting, etc. *—- dash]

Fig.3.1.1. Material — and energy-flow around stove in firewood system
*e ^energy supply from outside, E ^useful energy

For the right side of Fig. 3.1.1, which is the user of the firewood, what matters is the low energy efficiency of the old heating devices similar to traditional kitchen stoves.

In addition, hygiene of the indoor air is to be considered when small stoves are used that often accompanies with incomplete combustion. The problems of soot, carbon monoxide (CO), tar, Non-methane volatile organic matter (NMVOC), and polyaromatic hydrocarbons (PAH, carcinogen) are pointed out.

Ash content of the firewood is lower than that of coal by one order, but ash removal is important from the view point of mass balance, although it usually does not cause a serious problem. Ash of wood has high content of potassium, which is an important fertilizer, and return of ash to the forest is essential for the sustainability of the system.

Herbaceous plants have higher ash content than wood by 5-20 times, and ash treatment is a large problem for production of artificial firewood from straw, husk, and bagasse.

Heating values of plant is about 20GJ/t-dry for various woody biomass (half of heating value of oil), and mostly decided by its water content. Woody biomass is not suitable for transportation of long distance due to its bulkiness. This is why utilization of firewood near the forest is insisted.