Forests & Energy
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?
Open fires convert only 5% of the wood’s potential
Credit: Roberto Faidutti
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.
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
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.
– 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
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.
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
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
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
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
high-value agriculture lands
used for food production.
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
liquid biofuel closest to
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
increases, agricultural residues may become more important
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.