Wald & Energie

3. How is bioenergy produced?

• 3.1 How is solid wood used to produce bioenergy other than biofuels?
• 3.2 How are liquid biofuels produced?

Bioenergy can be produced from wood materials by various processes, ranging from burning sticks and branches for cooking and heating to gasification of wood chips to produce transport fuel. These systems differ in terms of energy efficiency, installation cost, carbon emissions and labour intensiveness. National and local circumstances will largely determine whether each system is economically, environmentally and socially appropriate. To achieve the maximum climate benefits of bioenergy in terms of greenhouse gas emissions avoided, the amount of carbon dioxide released during biomass production, processing, transportation and use should be equal to or smaller than the amount that was absorbed by the harvested biomass. More...

3.1 How is solid wood used to produce bioenergy other than biofuels?

Solid wood has long been used for cooking and heating. However, the efficiency of wood as an energy source varies depending on how it is processed. For instance, an open fire only converts about 5% of the wood’s potential energy. But this figure reaches from 36% for traditional wood stoves to about 80% for modern wood pellet stoves for residential use. Larger systems designed for industrial use – e.g. power boilers, combined heat and power systems (CHPs) and gasification systems – also vary widely with regards to cost- and energy-efficiency.

Steam-turbine power boilers can be used in sawmills to generate steam by burning bark and other waste products. The steam, in turn, can be used to generate the energy necessary to operate the mill. Similar recovery boilers can be used in pulp and paper mills. Power boilers generally harness about 40% of the available energy in wood. Financial incentives for installing such electrical generating capacity in mills have not been sufficient due to the historically low cost of fossil fuels and have often been passed over in favor of systems powered by historically inexpensive fossil fuels.

Combined heat and power systems (CHPs) are a highly efficient means of producing heat and electricity at the same time for large-scale industrial and residential use. Several technologies were recently developed to boost the efficiency of CHPs, resulting in systems that can harness as much as 80% of the total available energy from wood. A recent study found that a CHP power plant operating on wood chips released 7 times less CO₂ by unit of energy produced than a similar plant using natural gas.

Gasification – the process of heating wood to a very high temperature to produce gas – is a wood-based system of energy generation that is appropriate for smaller-scale industrial facilities and villages. This system is considered more energy-efficient and cost-effective than power boilers and, when integrated into a CHP system, can boost the efficiency of heat and energy production even further. However, gasification may prove challenging for smaller facilities, due to the difficulty of maintaining an adequate supply of wood. Thus, this technology appears to be a more economical option for medium-sized facilities.

Wood pellet furnaces, using the most advanced technologies for conserving and recovering energy, have become an attractive technology option, especially for home or small-scale use. The wood material that is burnt is formed from sawdust and other waste products of wood processing that are dried, ground and pressed into pellets. These furnaces serve the dual purpose of removing waste and generating energy. More...

3.2 How are liquid biofuels produced?

Recently, high oil prices, concerns over greenhouse gas emissions and reliance on foreign oil have boosted an interest in biofuels. ‘First generation’ biofuels, which are derived from food crops, have attracted the most attention because of their relatively lower prices and advanced state of development. However, technological developments are expected to increase future interest in ‘second generation’ biofuels, which are derived from non-food plant matter. More...

3.2.1 First-generation biofuels include bioethanol (made from sugars and starch) and biodiesel (made from oilseeds). The food crops used to generate them vary by geographical location. A variety of cereals are used in temperate regions, with about 90% of the world’s biodiesel generated from rapeseed in Europe. In contrast, cane sugar and palm oil are predominately used in tropical regions, with the highest production occurring in Asian and Pacific countries.

The technologies for production of bioethanol from sugars and starch have been refined and developed over the years, notably in Brazil and the US. Sugar cane has the advantage that, besides the sugar which is fermented to produce bioethanol the cellulosic component of the plant’s stalk – known as bagasse – can also be used to provide energy for the production of bioethanol, thus increasing overall energy efficiency.

Oilseed crops are used in the production of biodiesel, particularly in Europe. However, growing oilseeds requires optimal soil conditions, which may lead to forest clearance to make room for suitable agricultural fields.

The production of food crops for biofuel production can significantly contribute to global greenhouse gas emissions if they induce deforestation and land degradation. In Southeast Asia, one fourth of all oil-palm plantations are located on drained peat lands.

Recently, the use of other, more resistant oilseed plants for biodiesel production has been explored. Jatropha curcas, for instance, is a plant that grows well on marginal lands and can also be used to restore degraded lands, suggesting that Jatropha production, if carefully managed, may be expanded without directly competing with natural forests or high-value agriculture lands used for food production. More...

3.2.2 Within an estimated 10 to 15 years, wood and low-cost agricultural residues derived from the production of various cereals will likely be used to produce economically competitive second-generation liquid biofuels. At present, bioethanol is the second-generation liquid biofuel closest to commercialization.

Agricultural residues are likely to be among the lowest-cost liquid biofuel feedstocks. Currently, only a small share of such residues is available for energy generation but, as bioenergy production increases, agricultural residues may become more important biofuel feedstocks.

Forestry residues and wood from forest plantations are other potential feedstocks. Today, only a small proportion of liquid biofuels are forest-based, but forest biomass to produce cellulosic liquid biofuels for the transport sector could become widespread.

Currently, two basic technologies are being developed to convert wood to liquid fuels. In biochemical conversion, wood is treated using enzymes to release sugars that can then be further converted to ethanol. In thermochemical conversion, liquid fuels are produced through a process that involves heating wood and bark in the absence or minimum presence of oxygen.

The processes used in second-generation biofuel production will likely be more profitable when integrated into existing manufacturing facilities, such as paper mills. More...