[PDF]solid biomass

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BIOMASS

AS A SOLID FUEL

Introduction



PRACTICAL ACTION

Technology ctiallertgjng poverty





Figure 1 : Domestic biomass use in
Sri Lanka. Photo: Jean Long / Practical
Action.



What is biomass?

Biomass is the term used to describe all the organic
matter, produced by photosynthesis that exists on the
earth's surface. The source of all energy in biomass
is the sun, the biomass acting as a kind of energy
store. To make use of biomass for our own energy
needs we can simply burn it in an open fire to provide
heat for cooking, warming water or warming the air in
our home. More sophisticated technologies have
been developed for extracting this energy and
converting it into useful power and heat in more
efficient and convenient ways.

Until relatively recently it was the only form of energy
which was used by humans and is still the main
source of energy for more than half the world's
population for their domestic energy needs.

The extraction of energy from biomass is split into 3
distinct categories:



• Solid biomass - the use of trees, crop
residues, animal and human waste (although
not strictly a solid biomass source, it is often
included in this category), household or
industrial residues for direct combustion to

provide heat. Often the solid biomass will undergo physical processing such as cutting,
chipping, briquetting, etc. but retains its solid form.

• Biogas - biogas is obtained by anaerobically (in an air free environment) digesting
organic material to produce a combustible gas known as methane. Animal waste and
municipal waste are two common feedstocks for anaerobic digestion. See the Biogas
Technical Brief for more details.

• Liquid Biofuels - these are obtained by subjecting organic materials to one of various
chemical or physical processes to produce a usable, combustible, liquid fuel. Biofuels
such as vegetable oils or ethanol are often processed from industrial or commercial
residues such as bagasse (sugarcane residue remaining after the sugar is extracted) or
from energy crops grown specifically for this purpose. Biofuels are often used in place
of petroleum derived liquid fuels. See the Liquid Biofuels and Sustainable Development
Technical Brief.

This technical brief looks at the use of solid biomass fuels, and their associated technologies.
Biomass use

Solid biomass is widely used in developing countries, mainly for cooking, heating water and
domestic space heating.

Biomass is available in varying quantities throughout the developing world - from densely
forested areas in the temperate and tropical regions of the world, to sparsely vegetated arid
regions where collecting wood fuel for household needs is a time consuming and arduous task.







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Biomass



Practical Action



In past decades the threat of global deforestation, provided a focus for the efficient use of
biomass (as well as introducing alternative fuels) in areas where woodfuel was in particular
shortage. Although domestic fuelwood users can suffer greatly from the effects of deforestation,
it often arises because of land clearing for agricultural use or for commercial timber.

There have been many programmes aimed at developing and disseminating improved stove
technologies to reduce the burden, primarily borne by women, of fuelwood collection as well as
reducing health risks associated with smoke from burning fuelwood. Technologies have also
been introduced to help with the processing of biomass to improve efficiency, allow for easy
transportation or to make it more useable.

Crop and industrial biomass residues are now widely used in many countries to provide
centralised, medium and large-scale production of process heat for electricity production or other
commercial end uses. There are several examples in Indonesia of timber processing plants
using wood waste-fired boilers to provide heat and electricity for their own needs, and
occasionally for sale to other consumers. There are also small scale options to utilising crop
residues.

Combustion theory

For solid biomass to be converted into useful heat energy it has to undergo combustion.
Although there are many different combustion technologies available, the principle of biomass
combustion is essentially the same for each. There are three main stages to the combustion
process:

Drying - all biomass contains moisture, and this moisture has to be driven off before combustion
proper can take place. The heat for drying is supplied by radiation from flames and from the
stored heat in the body of the stove or furnace.

Pyrolysis - the dry biomass is heated and when the temperature reaches between 200 S C and
350 a C the volatile gases are released. These gases mix with oxygen and burn producing a
yellow flame. This process is self-sustaining as the heat from the burning gases is used to dry
the fresh fuel and release further volatile gases. Oxygen has to be provided to sustain this part
of the combustion process. When all the volatiles have been burnt off, charcoal remains.
Oxidation - at about 800 S C the charcoal oxidises or burns. Again oxygen is required, both at the
fire bed for the oxidation of the carbon and, secondly, above the fire bed where it mixes with
carbon monoxide to form carbon dioxide which is given off to the atmosphere.

It is worth bearing in mind that all the above stages can occur within a fire at the same time,
although at low temperatures the first stage only will be underway and later, when all the
volatiles have been burned off and no fresh fuel added, only the final stage will be taking place.

Combustion efficiency varies depending on many factors; fuel, moisture content and calorific
value of fuel, etc. The design of the stove or combustion system also affects overall thermal
efficiency and table 1 below gives an indication of the efficiencies of some typical systems
(including non-biomass systems for comparison).



Type of combustion technology


Percentage efficiency


Three-stone fire


10-15


Improved wood-burning stove


20-25


Charcoal stove with ceramic liner


30-35


Sophisticated charcoal-burning stove


up to 40


Kerosene pressure stove


53


LPG gas stove


57


Steam engine


10-20



Table 1 : efficiencies of some biomass energy conversion systems

Source: Adapted from Kristoferson, 1991

Improved stoves

Much of the research and development work carried out on biomass technologies for rural areas
of developing countries has been based on the improvement of traditional stoves. This was
initially in response to the threat of deforestation but has also been focused on the needs of
women to reduce fuel collection times and improve the kitchen environment by smoke removal.



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Biomass Practical Action

There have been many approaches to stove improvement, some carried out locally and others
as part of a wider programmes run by international organisations. Figure 2 below shows a
variety of successful improved stove types, some small, portable stoves and others designed for
permanent fixture in a household.




Figure 2: A variety of Improved Cookstoves




Some of the features of these improved stoves include:

• a chimney to remove smoke from the kitchen

• an enclosed fire to retain the heat

• careful design of pot holder to maximise the heat transfer from fire to pot

• baffles to create turbulence and hence improve heat transfer

• dampers to control and optimise the air flow

• a ceramic insert to minimise the rate of heat loss

• a grate to allow for a variety of fuel to be used and ash to be removed

• metal casing to give strength and durability

• multi pot systems to maximise heat use and allow several pots to be heated
simultaneously

Improving a stove design is a complex procedure which needs a broad understanding of many
issues. Involvement of users in the design process is essential to gain a thorough
understanding of the user's needs and requirements for the stove. The stove is not merely an
appliance for heating food (as it has become in Western society), but is often acts as a social
focus, a means of lighting and space heating. Tar from the fire can help to protect a thatched
roof, and the smoke can keep out insects and other pests. Cooking habits need to be
considered, as well as the lifestyle of the users. Light charcoal stoves used for cooking meat
and vegetables are of little use to people who have staple diets such as Ugali (Cornmeal
commonly made from maize flour), which require large pots and vigorous stirring. Fuel type can
differ greatly; in some countries cow dung is used as a common fuel source, particularly where
wood is scarce. Cost is also a major factor among low-income groups. Failing to identify these

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Practical Action



key socio-economic issues will ensure that a stove programme will fail. The function of an
improved stove is not merely to save fuel.

Local manufacture of stoves



Since 1982, the Kenya Ceramic Jiko (KCJ), an improved charcoal-burning stove aimed at the
urban market has been developed and manufactured by large numbers of small producers. The
KCJ has two main components; metal and fired clay. Both these parts are made by
entrepreneurs; the metal part (cladding) being made by small-scale enterprises or individual
artisans, while the clay part (liner) is manufactured by slightly larger and more organised
enterprises or women's groups. The KCJ is sold by the artisans directly to their customers or
through commercial outlets such as retail shops and supermarkets. The stove was initially
promoted heavily to develop the market, by the NGO KENGO and by the Kenyan Ministry of
Energy, through the mass media, market demonstrations and trade fairs.

As a result of this substantial promotion, there are now more than 200 artisans and micro-
enterprises manufacturing some 13,600 improved stoves every month. To date, it is estimated
that there are some 700,000 such stoves in use in Kenyan households. This represents a
penetration of 16.8% of all households in Kenya, and 56% of all urban households in the
country.

Source: Dominic Walubengo, Stove Images, 1995



Charcoal production

Charcoal production is the most
common methods for processing wood
to make them cleaner and easier to use
as well as easier to transport but
charcoal does not increase the total
energy content of the fuel - in fact the
energy content is decreased. Charcoal
is often produced in rural areas and
transported for use in urban areas.

The process can be described by
considering the combustion process
discussed above. The wood is heated in
the absence of sufficient oxygen which
means that full combustion does not
occur. This allows pyrolysis to take Fl 9 ure 3: Charcoal Kiln, Kenya. Photo: Heinz Muller/

i _i ■ ■ « *u I . m , Practical Action.

place, driving off the volatile gases and
leaving charcoal (carbon). The removal

of the moisture means that the charcoal has a much higher specific energy content than wood.

Other biomass residues such as millet stems or corncobs can also be converted to charcoal.

Charcoal is produced in a kiln or pit. A typical traditional earth kiln will comprise of the fuel to be
carbonised, which is stacked in a pile and covered with a layer of leaves and earth. Once the
combustion process is underway the kiln is sealed, and then only once process is complete and
cooling has taken place can the charcoal be removed.

A simple improvement to the traditional kiln is also shown in Figure 5. A chimney and air ducts
have been introduced which allow for a sophisticated gas and heat circulation system and with
very little capital investment a significant increase in yield is achieved.




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Figure 4: Improved Charcoal Kiln found in Brazil, Sudan and Malawi

Pro-Natura has developed a process based on the continuous carbonisation of renewable
biomass, savannah weeds, reeds, straw of wheat or rice, cotton and corn stems, rice or coffee
husk and bamboo to produce green charcoal.

Briquetting

Briquetting is carried out on many materials to make them more suitable to be used as an
energy source. Nearly all biomass has the potential to be briquetted into a hard stable fuel that
has a high energy density and provide more consistent combustion and improved storage and
transportation.

The important factures in making briquette are the ash content or non combustible components
and the moisture content. The raw materials that are commonly made into briquettes and pellets
include:

• Wood & Sawdust

• Biomass waste such as rice husk, cotton stalks etc.

• Bagasse fro sugar cane

Although briquetting is often a large scale commercial activity most waste biomass can be used
as a fuel source either by directly briquetting or through the production of charcoal that is then
briquetted on a small scale. Binders used for direct briquetting include starch paste, cellulose
from woody material, cowdung and clay, which can be extruded of formed by hand into balls.

One example of briquetting sawdust with a binding agent in Malaysia first carbonised the
sawdust then uses starch as a binder. The starch paste is made in a separate cooking tank.
Charcoal = 73%
Starch = 5%

Calcium carbonate = 2%
Water = 20%.

These charcoal briquettes can be made with a low-pressure mould.

Research by Chardust Ltd. into making charcoal briquettes from various crop wastes including
sisal waste. One report concluded that carbonising sisal was technically quite difficult in respect
to regulating the temperature resulting in non-homogenous carbonisation but once the sisal
waste had been carbonised it was relatively easy to produce briquettes. These were made by
producing a paste of carbon dust and water which is then combined with 15% clay.

Briquettes made without a binder are partially carbonised or not carbonised at all. The drawback
is that the pressure has to be increased and the equipment used is more complicated.

Most waste materials can be burnt directly without being briquetted beforehand. Sawdust stoves
and rice husk stoves are relatively common; see the Stoves for Rice Husk and Other Fine
Residues technical brief.



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Dung collection

Many poor families in rural and
urban areas use animal dung as
a fuel source or collect dung as
their source of income. There is a
group of women in Bangladesh,
who traditionally collect dung,
make cakes and sell them to
commercial markets. The
traditional collectors of dung are
teenage girls from poor families.
They bring back dung to their
homes and convert it into round
cakes and cone-like sticks for
drying in the open air.




Figure 5: A woman putting cow dung onto sticks prior to
drying. These will be used as fuel, Bangladesh. Photo: Zui
Mukhida / Practical Action.



Dung is considered to be one of the best fuels for the traditional mud stove for the
following reasons

• it burns slowly

• cooks fast

• generates powerful heat compared to other sources of fuel found locally

• easy to store

• Less toxidity

Problems related to dung as a fuel are;

• there is a scarcity of dung

• cattle owners do not permit collection form their fields

• as dung is being dried there is a risk that it could be stolen

• It burns faster than wood when it is not properly compressed

Source: Mohammed Aslam, Practical Action Bangladesh



The alternative approach to using cow dung and other animal waste is the biogas digester which
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