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GENERAL ARTICLES
CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010
627
H. S. Mukunda, P. J. Paul and N. K. S. Rajan are in the Department o
f
Aerospace Engineering, and S. Dasappa is in the Centre for Sustainable
Technologies, Indian Institute of Science, Bangalore 560 012, India;
Mahesh Yagnaraman and Mukund Deogaonkar are in First Energy Pri-
vate Limited, Baner Road, Baner, Pune 411 045, India; D. Ravi Kumar
is in Energy Division, GE Office, New Delhi 110 015, India.
*For correspondence. (e-mail: mukunda@cgpl.iisc.ernet.in)
Gasifier stoves – science, technology and field
outreach
H. S. Mukunda*, S. Dasappa, P. J. Paul, N. K. S. Rajan, Mahesh Yagnaraman, D. Ravi Kumar and
Mukund Deogaonkar
Development of a new class of single pan high efficiency, low emission stoves, named gasifier
stoves, that promise constant power that can be controlled using any solid biomass fuel in the form
of pellets is reported here. These stoves use battery-run fan-based air supply for gasification (pri-
mary air) and for combustion (secondary air). Design with the correct secondary air flow ensures
near-stoichiometric combustion that allows attainment of peak combustion temperatures with ac-
companying high water boiling efficiencies (up to 50% for vessels of practical relevance) and very
low emissions (of carbon monoxide, particulate matter and oxides of nitrogen). The use of high
density agro-residue based pellets or coconut shell pieces ensures operational duration of about an
hour or more at power levels of 3 kWth (~12 g/min). The principles involved and the optimization
aspects of the design are outlined. The dependence of efficiency and emissions on the design
parameters are described. The field imperatives that drive the choice of the rechargeable battery
source and the fan are brought out. The implications of developments of Oorja-Plus and Oorja-
Super stoves to the domestic cooking scenario of India are briefly discussed.
The process development, testing and internal qualification tasks were undertaken by Indian
Institute of Science. Product development and the fuel pellet production were dealt with by First
Energy Private Ltd. Close interaction at several times during this period has helped progress the
project from the laboratory to large scale commercial operation. At this time, over four hundred
thousand stoves and 30 kilotonnes fuel have been sold in four states in India.
Keywords: Biomass stove, domestic stove, gasifier stove.
RESEARCH, development and dissemination on biomass-
based domestic combustion devices otherwise termed
cook stoves have a long history of over five decades.
Many developments have occurred due to intuitive appro-
aches to examine heat transfer aspects relegating the
combustion issues to a peripheral state. A broad summary
of the scientific work performed till mid-80s has been re-
ported
1
. More than a hundred designs from all parts of the
world have been documented and these are discussed in a
website devoted to cook stoves (www.bioenergylists.org).
Stoves are made of metal, mud, refractories of various
qualities with and without chimney. Chimney-based
stoves are considered superior to chimney-less stoves be-
cause any emission is taken out of the kitchen thus pre-
serving the indoor air quality. However, these emissions
create a load on the environment and has become a matter
of serious concern in the last several years. Further, even
the single pan stoves without chimney have air supply by
free convection since electric supply was not to be found
in most rural dwellings which is where these stoves were
largely expected to be used. Most of these stoves had
utilization efficiencies assessed by water boiling tests
between 10% and 20% (see for instance, Bhattacharya et
al.
2,3
) while one slightly complex design shows an effi-
ciency of 40% at small power levels (see Mukunda et al.
4
for details). It has been known that both kerosene and
LPG stoves have efficiencies as high as 65% and 70%.
The question as to what limits the efficiencies in biomass
stoves in comparison to kerosene and LPG stoves had
remained un-examined till Mukunda et al.
4
studied this
aspect experimentally. They specifically showed that the
efficiency is directly related to the operation of the stove
at near-stoichiometric conditions that lead to peak com-
bustion temperature of the product gases; it is this
temperature that influences the heat transferred to the
vessel within the limited bottom area available. Hence,
the stove operation can be expected to be the best when
GENERAL ARTICLES
CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010
628
the air-to-fuel ratio is maintained near stoichiometry,
albiet on the lean side to ensure minimum CO emissions;
limiting the emissions of NO
x
is taken care of naturally
by the lower combustion temperatures of biomass-air sys-
tem as thermal NO
x
is the predominant source. Limiting
the velocities in the fuel residing zone will help limit the
particulate carry over.
The stove designs that have (i) free convection-based
air ingestion and (ii) biomass loaded as and when it is
thought fit into the hot combustion zone, will function
with air-to-fuel ratios that can be very rich or very lean.
The former condition leads to sooting and the latter to
smoking. Also, parts of the fuel over the stove may still
be cold leading to air flow out of the stove body without
participating in the combustion while other areas are
deprived of the needed air for combustion. Coupled to all
these, the biomass may either be dry or very wet. The
latter condition will lead to poor combustion with heavy
smoking. Thus, improvement of biomass-based stove
combustion involves biomass quality and combustion
aerodynamics, a view that has not been adequately
reflected either in the stove literature or in the develop-
ment of improved cook stove initiatives in the past.
Typical specifications of a domestic stove can be deri-
ved from comparable stoves based on kerosene or LPG
that have an input power level of about 2.0 kWth
(~3 g/min of LPG). Accounting for efficiency differ-
ences, the biomass stove input power level can be set at
about 3 kWth (12 g/min of biomass). The stove must
allow continuous operation and if not, must last the
complete cooking duration that has been estimated as
about one hour. One would expect to have clean combus-
tion with minimal emissions. Minimizing emissions of
CO and other unburnt products of combustion calls for
maximizing the combustion efficiency, and minimizing
CO
2
emissions implies maximizing the utilization effi-
ciency (or water boiling efficiency on vessels of practical
relevance). One would desire a smooth start-up and some
control on the power level. The life of the stove must be
large, at least two to three years. In doing all these, the
cost of the stove must be affordable.
The work described here is arranged as follows: (i)
principles of gasifier stove, (ii) free convection versus
forced convection operation, (iii) gasifier stove versus
in-situ combustion stove, (iv) fuel for the stove, (v) the
stove, efficiency and emissions, (vi) power pack related
aspects, (vii) stove performance evaluation by others, and
(viii) comparison with other developments.
The broad implications of these developments to the
country and other countries that have similar require-
ments or challenges are also brought out.
Principles of gasifier stove
Gasifiers are essentially devices that enable converting
solid fuel to gaseous fuel by a thermochemical conver-
sion process. This process involves substoichiometric
high temperature oxidation and reduction reactions
between the solid fuel and an oxidant – air in the present
case. This is arranged such that air and the gas passes
through a fixed packed bed.
The gasification knowhow has a long history; it be-
came particularly important in the European scene during
the World War II when fossil fuel availability was scarce.
Research and development into gasification process came
to a low after fossil fuel availability became normal and
their prices low. The area has become active in the last 30
years particularly in oil-importing countries like India.
Research relevant to the objectives of this article can be
traced to rice hull gasification systems that used a vertical
cylinder filled with rice hull and air drawn through the
packed bed from the top. While these were first deve-
loped in China as early as in 1967 (see Bhattacharya
5
),
the scientific study was conducted by Kaupp
6
. In this
reactor, shown on the left side of Figure 1, one can use
biomass like wood chips, and pellets apart from ricehulls
for which it was first conceived. If now the reactor is op-
erated such that air flows from the bottom and the fuel
surface at the top is lit, one would get combustible gases
that will burn above the top surface with ambient air or
with additional air supplied towards the top. Such a con-
figuration, shown on the right side of Figure 1 termed
reverse downdraft gasifier, constitutes the essence of a
gasifier stove. An important consequence of this mode of
operation is that the gas exiting from the top of the
packed bed bears a fixed ratio to the amount of air intro-
duced for gasification (primary air owing from the bot-
tom). The reduction reactions following the oxidation
limit the amount of fuel to be consumed due to the endo-
thermic nature of these reactions. The interesting feature
is that the relative amounts of fuel consumed and air
introduced remain the same and increased amount of
Figure 1. a, Schematic view of a rice hull gasifier. Note that the air
flows from the open top towards the bottom. b, Schematic representa-
tion of a reverse downdraft gasifier stove where the gasification air
flows from bottom to top.
GENERAL ARTICLES
CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010
629
solid is consumed when primary air flow rate increases.
Thus, the power of the stove is proportional to the pri-
mary air flow rate. The gases coming out of the bed will
be at a temperature of 800–1100 K and will be composed
of CO, H
2
, CH
4
, H
2
O (as gas), some higher hydrocarbons
and N
2
. These gases are burnt to CO
2
and H
2
O with a
second stream of air which is introduced in the top region
for this purpose.
Based an early work of La-Fontaine and Reed
7
, Reed
and co-workers developed a free convection-based gasi-
fier stove
8
and have subsequently discussed the develop-
ment of forced convection-based gasifier stove
9,10
. The
biomass is largely wood chips and the forced convection
depended on a fan; it formed the basis of camp stove and
this is indeed how it got marketed. While the broad prin-
ciples were clear at this time, the aspects of process opti-
mization and development of components needed to be
addressed to deal with the affordability issue in emerging
economies like India with fuels depending on agro-
residues rather than wood chips. One of the key drivers
for the development was the evolution of low cost storage
battery based fan; the idea of building a domestic stove
became meaningful when the fans for supplying air were
being built in numbers for computer applications and at
this stage, low power fans could be obtained or built at
less than 5 USD per system including the electrics. Work
at the IISc laboratory on gasification systems had begun
in 1983 (ref. 11) and reached reasonable maturity by the
end of nineties, and the work on the development of
stoves of high efficiency began in 1999 with a study to
determine the dependence of various parameters on water
boiling efficiency and emissions with several agro-
residues.
Free versus forced convection stoves
Nearly all the domestic cook stoves over the world are
free convection driven. In several of these designs
the free convective driving potential is so small that the
smallest of ambient disturbances can affect the air flow
through the stove. Even in better of these designs with a
column of hot zone (due to the combustion chamber) that
enables stabilize the flow through the stove, flow distur-
bances caused by varying heat release across the section
of the combustion chamber due to randomly located and
moved-around pieces of ‘firewood’ leads to widely vary-
ing air-to-fuel ratios locally. This kind of a variation re-
duces the peak flame temperature most of the time and
leads to emissions of CO, unburnt hydrocarbons (UHC)
and particulates significantly. Even when better efficien-
cies are obtained, the stove operation becomes rich and
significant sooting will result. This is particularly
because faster cooking is understood to be obtained at a
larger fuel burn rate and this is achieved by introducing
more fuel into the combustion space. Getting good emis-
sion performance from such stoves is usually a difficult
task. Thus laboratory test results and operational data
from realistic environment can be widely different. Much
of the current debate with regard to the standards and pro-
tocols for testing stoves arising from the fact that the
laboratory tests are different from real experiments is due
to this fundamental aspect of the free convection stove;
by design, the efficiency as well as emissions are depend-
ent on the user and the ways the stove-fuel combination is
used. Free convective stoves were and even now, are con-
sidered inevitable because of the (i) perception that such
stoves provide the ‘choice’ and flexibility and allow the
users to continue to use whatever non-processed raw
biomass and/or wood they were using, (ii) use of any
forced draft that requires electricity is seen as difficult
and/or unavailable. The point (i) needs to be challenged
given the variable and distinctly poorer performance on
efficiencies and emissions that have been observed and
recorded (to be brought out subsequently). As far as point
(ii) is concerned, improvement in electricity, infrastruc-
ture and technology of smaller fans and power sources
have given the real option to provide forced convection in
the stove designs to improve performance and reduce
emissions. All these do not imply that the rural environ-
ment enjoys the availability of electricity all the time;
perhaps, they have it over few hours a day. Yet the fact
that electricity is available over some period can be made
use of for the use of electricity-enabled stove designs.
Also, it is possible that nothing better than free convec-
tion stove can be contemplated in regions deprived of
electricity totally. This should not mean that other regions
that have electricity support even over the part of day
should be deprived of modern technology interventions.
In-situ combustion stove versus gasifier stove
Another stove design that has been revealed in recent
times that has had a limited dissemination is an in-situ
combustion stove. Such a stove is not different from a
furnace in principle. One supplies enough air for combus-
tion directly around the fuel zone so that some parts of
the fuel will undergo volatilization and the volatiles will
burn with the air around, other parts that have become
char will also simultaneously be oxidized. In some stoves
(like Priyagni in India) the air flow occurs through the
grate and over it just from around a chamber (of short
height) to complete the combustion process. While direct
combustion systems are common at large power levels
where fuel feeding can be automatic and combustion
management is relatively easy, at small throughputs like
in a domestic stove, fuel loading has to done at frequent
intervals (typically every few minutes apart) manually to
ensure complete combustion with smaller packets of fuel
and this may be considered a burden on the user in com-
parison to dealing with conventional stoves. The power
variation with time is dependent on this attention. If per-
chance, the amount loaded at one time is more than what
GENERAL ARTICLES
CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010
630
the system can carry, significant emissions will result.
Here again it may turn out that a trained operator in a
laboratory can get good emission performance, but users
in the field may end up with poor emission performance.
In comparison to the above mentioned designs, gasifier
stoves with loaded fuel provide an alternative. Gasifier
stove that has loaded fuel pellets/pieces can be conceived
from the end user as a starting point. Starting with the
amount of cooking energy needed to cook one meal, the
design of the stove has been arrived at in combination
with amount of fuel to be loaded in such a way that the
operation is relatively foolproof. The stove operation is
dependent on thermo-chemistry which modulates the gas
production process. At any fixed power level, the
gas composition and temperature remain steady and with
appropriate secondary air flow, the flame will get main-
tained at its peak temperature in a near-steady mode.
However, when power variation is demanded, it takes
a transition time (of about a few minutes) much unlike a
gas stove since the thermal inertia in the hot fuel bed has
to be overcome in the process of reaching a new steady
state. This design while providing flame control to the
users, also makes it less user-dependent for achieving
desired efficiency and emissions, and it may be expected
that field results would be close to those obtained at the
laboratory.
Fuel for the stoves
Historically, biomass stoves imply ‘firewood’ stoves. It is
generally thought that firewood of any size, shape should
be acceptable; moisture in the firewood is known to be
undesirable, but there is no rigour in ensuring dry bio-
mass use in stoves. The national statistics on fuel use in
cook stoves in rural and urban environments is presented
in Table 1. This table is composed of the data provided
by various workers
12–15
. The differences between various
studies and the data of Table 1 can be expected to have
inaccuracies up to 15%.
Table 1. National fuel usage in rural and urban households and
energy efficiency
Rural Urban Fuel used Unit fuel Unit energy
Fuel type (mhh) (mhh) (mmt/yr) (t/yr/hh) (GJ/yr/hh)
Firewood 87 15 250 2.5 40.0
Agro-residue 20 2 120 5.5 77.0
Cowdung cake 20 2 95 4.3 55.0
Coal, coke 2 2 6 1.5 27.0
Kerosene 2 8 5 0.5 21.0
LPG 9 25 8 0.24 5.7
Others 1 2 – –
Total 141 66 465
a
3.2
a,b
47
a,b
a
Solid bio-fuels only;
b
National averages; hh, household; mhh, million
household; mmt, million metric tonnes; yr, year; t, tonnes; GJ, Gega
Joules.
The data in the table is revealing. While wood and
agro-residues are both biomass, the amount of agro-
residues used on a per household basis is nearly twice
that of wood. While it is generally understood that wood
use itself is inefficient, the degree of wastefulness of
agro-residues is enormous, a fact about which there is
little appreciation all-round. If developing improved cook
stoves on firewood is considered important, it is far more
important to develop stoves to burn agro-residues that are
light and odd shaped to obtain high efficiency and reduce
the emissions. The magnitude of the use of cowdung cake
as a source for fuel is non-insignificant, but its use is
about as energy-inefficient as agro-residues. However,
the emissions from its use are significant and any
improvement in the use of cowdung cake should address
this aspect as well. Coal is used in a wasteful way largely
because of ignition problem. Many of the stoves are lit in
the open for the volatiles to escape (about 30% in
comparison to biomass with 70% volatiles) until coal
becomes virtually coke and its combustion becomes
vigorous. China that has encouraged a large production of
coal-powder based beehive briquettes has serious indoor
air pollution problems related to this fact
16
.
LPG and kerosene are more sought-after fuels and are
in the upper region of the energy ladder. Hence, they turn
out to be important as reference for performance com-
parison. Kerosene is a fuel used to a larger extent by the
urban poor with relatively small kitchens and cannot
afford the costs of LPG for cooking. Both these have
higher water boiling efficiency. Laboratory experiments
have shown water boiling efficiencies of 70–75% for
LPG stoves and 60–65% for kerosene stoves. Kerosene
use as fuel on a per-household basis appears large
whereas the usage of LPG seems not unreasonable (one
14 kg cylinder for three weeks for a family of five).
The calorific values of biomass, kerosene and LPG are
16, 42 and 45 MJ/kg and on this basis, one would have
expected kerosene usage to be about 30% higher than
LPG allowing for differences in the utilization efficiency.
Perhaps, the magnitude reported on kerosene use for
cooking may be inaccurate as it is generally known that
significant amount of kerosene bought under public
distribution scheme at subsidized prices is sold away as
either cooking fuel or fuel for adulteration with gasoline
at higher prices.
The calorific value ratio coupled with efficiency differ-
ences allows speculation on how much of solid biomass
is needed for domestic cooking on a national scale. The
equivalent of 0.24 tonnes/yr/hh of LPG translates to bio-
mass of 1.20 t/yr/hh [0.24 × (45/16) × (70/40)]. Achiev-
ing this implies that one would aim at a total solid biofuel
use for cooking of 253 mmt/yr as against the current
estimate of 465 mmt/yr. The magnitude of the task can be
understood if we note the current efficiencies of firewood
stoves as 20%, of agro-residue based stoves (or their use
in the same firewood stoves) at 10% and cowdung cakes
GENERAL ARTICLES
CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010
631
Table 2. Average food consumption, energy intake, fuel required for cooking and cooking energy
cpm cpd Fuel/food Fuel/person/day Food cals Calories
Food item (kg/p/m) (g/p/d) (g/g) (g/p/d) (cals/g) (cals/d)
Rice 7.2 240.0 0.23 55.2 1.5 360
Wheat 4.4 146.7 0.23 33.7 3.0 440
Other cereals 1.3 43.3 0.23 10.0 1.0 43
Pulses 0.9 30.0 0.15 4.5 0.7 1
Dairy 4.2 140.0 0.04 5.6 1.0 140
Edible oil 0.5 16.7 0.06 1.0 9.0 150
Meat/fish/egg 1.6 53.3 0.30 16.0 4.0 213
Veg/fruit 10.2 340.0 0.03 10.2 0.4 136
Sugar/spices 1.5 50.0 3.5 175
Processed food 1.6 53.3 3.0 160
Beverages 2.3 76.7 3.0 230
Total 136.2 2048
cpm, Consumption per month; cpd, consumption per day; m, month; p, person; d, day.
as 12%. Enhancing the efficiency of the use of agro-
residue based fuels and cowdung cakes must occupy the
highest attention, next to which is firewood. Cowdung
can perhaps be integrated into the strategy for better fuel
making without any special stove design for cowdung
cakes; fuels based on agro-residues and cowdung should
be dealt with as a separate development task.
Lastly, it is noted from Table 1 that the average energy
per year per person is estimated at 5–10 GJ (ref. 12). The
present estimate of 8.9 GJ/person/year (the average
household in rural India is 5.3 as per the recent census
documents) falls within the range obtained earlier.
One can get an assessment of the cooking energy needs
by determining the amount of food cooked per day,
assessing how much of solid biofuel is needed to achieve
this. Table 2 gives data of the average monthly consump-
tion of food items in India drawn from a recent study
17
.
The amount of fuel required for cooking these items
(excepting for meat/fish/egg item) as determined from
actual cooking experiments
4
in a biomass stove with 40%
water boiling efficiency are also shown in Table 2. Using
these data, the amount of fuel required to cook food for
one person for a day is obtained (food/p/d). The last two
columns indicate the calories in each of the food items
and the calorie intake in the food per day. As can be seen
from the table, the total fuel per person per day is 136.2 g
and for a household this translates to 722 g of biomass
fuel/day which implies 0.26 t/yr/hh. This will perhaps
be an absolute lower bound of the possibility. The differ-
ence between this value and the one deduced earlier
–1.2 t/yr/hh is not small and may be related to the differ-
ence in food intake between LPG users and others and
needs reconciliation. At this point, it is appropriate to
conclude that a more realistic target would be 1.2 t/yr/hh
of biomass.
Issues with agro-residues and cowdung
Most agro-residues are characterized by seasonal avail-
ability and low intrinsic as well as bulk density. Typically,
intrinsic density varies from 50 to 200 kg/m
3
whereas
bulk density would be 50–70% of this value. There is
also substantial moisture (up to 50%) at the time the
material is harvested. The fact that densities are low per-
mits loss of moisture even in open air storage, the extent
of loss depending on the ambient conditions. Transporta-
tion and combustion are affected by these aspects. Draw-
ing from the practice of firewood transportation, it is
understood to be economical to transport about 10 t of
firewood over hundreds of kilometers, shorter the dis-
tance the better. A truck carrying 10 t of firewood whose
typical density is upwards of 500 kg/m
3
can carry less
than a few tonnes of agro-residues economically. The
region to be covered to maintain a continuous supply of a
variety of these residues becomes large, many times
larger than economics can support. Hence, densification
is an important element in the process. A question then
arises – is it not good enough to densify the material to as
much as firewood (implying about 500 kg/m
3
)? The
answer to this question comes from the stove that uses
this material. Approaching high efficiencies of a LPG
stove demands that the combustion volume be brought
down to as low a value as possible. This can be obtained
by increasing the density of the fuel. Also in a stove with
a fixed storage volume for the fuel like the one consi-
dered here, increasing the density helps in reducing the
overall chamber volume that would have benefits of
lower inert material content and associated heat loss. A
question that next arises is the level to which the material
must be densified. Here, practice in industries suggests
that binderless briquetting achieves a density of 1000–
1100 kg/m
3
. This would form an upper limit to which the
material must be densified. Since the requirement in
stoves will turn out to be small-sized pieces, one needs to
produce pellets, typically of 10–12 mm diameter and up to
40 mm long to obtain good packing density. Producing
pellets of this size at large throughputs in an economical
way has many challenges in the process as well plant
throughput sizing both of which have not been addressed
adequately in this country, yet. Pellet making and bri-
GENERAL ARTICLES
CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010
632
quetting have different process fundamentals. Briquetting
process uses very high pressures, of the order of
1200 atms to generate heat due to friction between the
material and the die wall to raise the temperature up to
350°C to enable lignin to be released. This lignin acts as
a powerful binder. Pelleting process uses moisture or
steam in extruding the material through a small die; the
temperatures achieved are not high, typically, around
100°C to 120°C. Under these conditions, the crude pro-
tein present in the biomass is softened and helps in the
binding process. The densities achieved are usually lower
than in binderless briquetting and are typically about
600–800 kg/m
3
. The presence of crude protein is impor-
tant and hence any kind of ingredients having some
amount would aid in the process and enhancing the
throughput of the system.
Cowdung is a major cooking fuel in north India, perhaps
in conjunction with agro-residues or firewood. Huge
mounds of dry cowdung are set around houses to enable
peal-off and use in the stoves. Cowdung itself has com-
ponents that could help pelleting in both the intrinsic feed
material (to the bovines) and due to what happens in the
rumen of the bovines. This is an important task for the
future.
Typical agro-fuels used currently are bagasse and
groundnut husk as primary fuels along with tamarind
husk, deoiled ricebran, sawdust and other seasonally
available wastes as secondary ingredients. The team at
FEPL has had wide experience in using fodder pellet
making machines. The fuel pellet throughput comes down
to 50–60% of the fodder pellet throughput with increased
maintenance. After much consideration, two pellet
machines were obtained from Europe and they have been
set up, one of 20 t/d Dharwad (Karnataka) and another of
50 t/d at Islampur (Maharashtra). These installations are
producing pellets and with other procurements a total of
1600 t/month is being supplied into the market.
The stove, efficiency and emissions
The development was carried out over a period with
different fuels, different sized combustion chambers and
designs of air supply to establish the performance as a
function of various parameters on efficiency. Actual
emission measurements were undertaken in the last stage
as it was clear that obtaining the highest combustion effi-
ciency (and accompanying utilization efficiency) with
relatively short visible flame heights was needed to be
achieved first; it was inferred that this would also imply
reducing the emissions. The amount of secondary air was
varied to obtain a short visible flame. This meant that the
ambient air demanded for combustion was minimal. The
relevant dimensions of the stoves built for this purpose
and their performance are shown in Table 3. The
experiments were made with the stove and vessel with
water kept on a balance. The mass of the system was
continuously measured along with the temperature of the
water after it was vigorously stirred once in a while. Fig-
ure 2 shows the plot of the mass of the fuel burnt with
time. As can be noticed, there are two distinct phases of
heat release. The first part is due to flaming and the
second part due to char combustion. The power output
during the second phase is about a fourth of the power in
the first phase. The slope of the mass versus time plot
gives the mass loss rate that is the same as burn rate; this
quantity multiplied by the calorific value will give the
power of stove.
Three cooking vessels were used for determination of
the thermal utilization efficiency. The consideration
behind this choice is that small families may use smaller
vessels and larger families, larger vessels. It would be
valuable to determine the efficiency with vessel size. It
can be expected that larger diameter vessels extract more
heat compared to smaller vessels and hence designs that
allow greater heat extraction from the same stove would
Table 3. Initial experiments – stove geometry, fuels and performance
ρ
bulk
Moisture Ash Biomass Burn time
+
Power*
η
wb
Biomass (kg/m
3
) (%) (%) loaded (g) (min) (kWth) (%)
Stove diameter = 100 mm, chamber volume = 0.6 liter, vessel = 10 liter
WC 220 10 0.8 130 14 + 5 2.3 49.3
RHB + WC 500 7 18.3 250 + 50 27 + 10 1.9 49.3
CS + WC 430 9 0.6 230 + 30 30 + 10 2.2 53.3
MP + WC 366 12 11.3 225 + 30 18 + 7 2.5 49.1
Stove diameter = 125 mm, chamber volume = 0.9 liter, vessel = 10 liter
WC 200 10 0.8 170 30 + 8 5.0 48.0
MP + WC 366 14 10.0 325 + 25 40 + 11 3.5 51.5
+
The split-up is between flaming and char combustion times; *Power over the flaming time.
WC, Wood chips; RHB, rice husk briquette pieces; CS, coconut shell pieces; MP, Marigold waste pellets.
GENERAL ARTICLES
CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010
633
be the appropriate choice. The cooking vessels were alu-
minium vessels of 10 liter volume (diameter of 320 mm,
height of 160 mm and 0.96 kg weight), 6 liter volume
(diameter of 260 mm, height of 130 mm and 0.61 kg
weight) and 2.5 liter volume (diameter of 205 mm, height
of 105 mm and 0.34 kg weight).
The lighting up process used two approaches. First one
used about 15 ml of kerosene on top region of the fuel
bed and the second one about 25 g of fine wood chips
over which about 10 ml of kerosene was doused. The
kerosene soaked fuel was lit with a match stick and the
primary air was turned on. After a 2 min of vigorous
combustion, the secondary air was turned on. The vessel
containing water was kept just about a minute after the
lighting. Procedure used in the tests was based on
Indian standard specifications except that the tests were
done with vessels larger than indicated in the specifica-
tions; the vessel diameters and volumes of water to be
taken for the tests at various power levels were set in the
1991 specifications of the Bureau of Indian Standards
keeping in mind lower efficiencies that were expected in
the better of the stoves at that time. The currently
achieved efficiencies are nearly double of those in earlier
times; this is the reason for the choice of larger vessel
sizes in the current tests.
Figure 2. Mass of fuel with time during the stove efficiency tests.
The slope of the curve gives the mass loss rate.
Figure 3. Effect of vessel diameter on efficiency, the lower plot is for
the stove named Swosthee
4
.
Table 3 shows the results of efficiency measurements.
The efficiency values are close to 50% in most cases. The
results on vessel diameter effect with the 100 mm dia
stove are also shown in Figure 3. It is clear that there is
significant enhancement in the efficiency with vessel
diameter. This implies that given an option of a cooking
vessel with a desired volume, it would be better to select
a vessel with as low a height-to-diameter value as is prac-
tical. This result was also found in the earlier study
4
,
albiet with lower efficiencies with a stove named ‘Swo-
sthee’ (for Single pan WOod SToves of High Efficiency)
that is similar in configuration and performance to the
currently prevalent rocket stove.
Considerations for the practical stove
The geometric parameters of the stove relate to the dia-
meter and height of the combustion chamber. These were
decided based on the following considerations.
The power level of the stove was set at 3 kWth
amounting to a nominal pellet fuel consumption rate of
12 g/min. For a one hour burn time with a 40 min normal
burn, some duration of a 9 g/min burn and remaining of
char burn, it was inferred that 600 g of fuel would be ade-
quate for a cooking cycle. At a bulk density of 400, 450
and 500 kg/m
3
, 600 g of fuel translates to a fuel loading
volume of 1.5, 1.33 and 1.2 liters. Based on a nominal
pellet bulk density of 450 kg/m
3
, a fuel loading volume of
1.3 liters was chosen. This implies that if fuel of a higher
density was loaded, one gets larger burn time. On the
other hand, if wood chips of a bulk density of 200 kg/m
3
were loaded (260 g), the burn time of the stove would be
25 min. Thus, while the stove would operate with many
fuels in the form of small pieces, the functional role of
cooking would be met with fuels with bulk density more
than 450 kg/m
3
.
To make a choice of the combustion chamber diameter,
two consistent viewpoints were synthesized. The first one
is that the primary airflow had to be about 1.5 times the
fuel consumption rate for the gasification process. Hence
the air flow rate would be 18 g/min. The other parameter
of importance is the superficial velocity through the com-
bustion chamber. If this was large (say, of the order of or
more than 0.1 m/s) then particulate carry-over would be
large and this would directly affect the particulate emis-
sions from the stove. If the velocity was too low (say, less
than 0.04 m/s), the combustion process in the char mode
could either be extinguished or worse, so weak that CO
emissions would be large, perhaps beyond acceptable
limits. The choice of 0.05 m/s for superficial velocity was
made after some trial studies. This would correspond to
95 mm diameter. Thus, the choice of combustion cham-
ber diameter between 95 and 105 mm would meet the
objectives of power, burn time, and minimum emission of
particulate matter.
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CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010
634
A second view arises from comparison to a typical gas
stove burner; this has an outer dimension of about
100 mm. Since these stoves have been used very effi-
ciently widely, it is prudent to make this choice. Hence,
the inner diameter was chosen as 100 mm.
Scaling the design of this stove for other power levels
is straightforward. We first recognize that at the design
superficial velocity of 0.05 m/s, the power level (P) of
the stove is 3 kWth for a inner diameter of 100 mm. The
power level scales as the area of the combustion chamber
(A
c
). Thus P = 3(A
c
/78)
2
kWth. One can use a circular or
square combustion chamber.
If the burn time has to be increased at a fixed power
level, the combustion chamber depth is to be increased
linearly.
The air for combustion of the gases is provided above
the top of the bed. The amount of air (secondary air) that
has to be provided here is the difference between the
stoichiometric combustion air for the biomass and air
supplied for gasification. For biomass, the stoichiometric
combustion air depends on CHNO analysis of the fuel.
For the range of fuels considered here, the stoichiometric
air-to-fuel ratio is about 6.0 (the fuel is allowed 10% ash
with corresponding reduction in the air-to-fuel ratio). For
the current design, for 12 g/min of burn rate, the primary
air is 18 g/min and the secondary air will therefore be
54 g/min. This is supplied through a large number of
holes of small diameter. In the current design, 18 holes of
6.5 mm dia (an area of 597 mm
2
) are provided. This leads
to an inward air velocity of 1.8 m/s through the holes.
The criticality of this air flow is more towards determin-
ing the emission rather than efficiency. For, if this air
flow is inadequate, some part of the air from the ambient
atmosphere is drawn in for combustion; but the oxidation
of carbon monoxide in the product stream is affected, for
it is very slow to combust. Hence, the provision of a
slightly larger secondary air flow will not affect the per-
Figure 4. Variation of the volumetric ratio of CO
:
CO
2
during a stove
operation. The operation till 30 min is in flaming mode; char mode op-
eration begins after a short transition.
formance. Also, the fuel pellets made from a wide variety
of agro-residues cannot be expected to have the same
CHNO composition. As such, some variations in the
stoichiometric air requirement can be expected; thus,
if by design a slight excess air is introduced, it would
of carbon monoxide in the product stream is affected, for
it is very slow to combust. Hence, the provision of a
slightly larger secondary air flow will not affect the per-
formance. Also, the fuel pellets made from a wide variety
of agro-residues cannot be expected to have the same
CHNO composition. As such, some variations in the
stoichiometric air requirement can be expected; thus,
if by design a slight excess air is introduced, it would
account for these variations in limiting the CO emissions.
Before release of the stove for developments involving
engineering and production, emission measurements
carried out at fuel consumption rates of 12 and 9 g/min
showed that the CO emissions were 1 and 1.3 g/MJ
whereas particulate emissions were 10 and 6 mg/MJ for
the two power levels. Figure 4 shows the variation of the
volumetric ratio of CO
:
CO
2
with time of operation. The
first phase – flaming mode shows a low CO/CO
2
(less
than 0.01). The transition to char mode of operation
increases the emission of CO significantly. The fact that
char combustion in stoves has significant CO emissions is
well known in stove literature (see for recent data, Smith
et al.
18
). There are also test-to-test variations in the CO
emissions. These are largely because of the differences in
packing of pellets in the bed. The overall CO
:
CO
2
was
found as 0.01 (volumetric) even though char burn alone
creates far more CO. These meet the requirements of the
Indian standards and hence are considered acceptable.
The power pack
The first choice was to integrate the power pack based on
commercially available elements. The first version has
6 V, 4 amp-hour lead-acid battery that has a life of at
least two years (about 2000 cooking cycles). A single full
charge permits 10 cooking cycles and would need about
4 h of charging before fresh use. To eliminate the use of
lead-acid battery and also reduce the demand of fan
power, after much in-house research and development at
FEPL, a new version that allowed local manufacture of
the fan and the power pack was developed. This is based
on a 1.2 V Ni-Mh battery with 1.2 amp-hours that has
also lasted more than two years. A single full charge can
last about 5–6 cooking cycles and the charging time is an
hour and a half. The fan itself was simply rebuilt with
special bearings to the same specifications. Now the team
is evolving Ni-Mh battery pack with feedback of con-
sumers to take the same battery at 2.3 amp-hours (from
1.2 amp-hours). The battery charger is supported by
circuitry that: (i) boosts the voltage, (ii) starts warning
the users with a beep when the voltage drops to 0.7 V
GENERAL ARTICLES
CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010
635
giving consumers ample time to finish the cooking and
charging it; the user will never have an issue of the
battery draining and/or fan stopping since they will be
alerted well before.
Stove in the market
Figure 5 shows the photograph of the stove in two orien-
tations. The inner wall is made of ceramic composition
that removes the limitations of the material limited life
issues in the combustion chamber. The bottom grate is
made of cast iron that ensures long life. The primary air
comes through the grate and the secondary air issues out
of holes seen at the top. The design ensures that the outer
wall temperature does not exceed 60°C complying with
Indian standard requirement even though there is a warn-
ing on the outside for the user not to touch the body. The
combustion process inside the stove is seen in Figure 6.
The cup-like flames are those formed around air jets issu-
ing from the wall. Except for the initial lighting process
during which flames are yet to acquire the character of
the combustion of a gasified fuel, all subsequent flame
behaviour is similar to what is seen in Figure 6.
These stoves were first built with metal version. The
metal was enamelled to extend the life. Extensive com-
Figure 5. The Oorja stove. Notice the power pack towards the left
bottom section of the left plate. The ceramic combustion chamber with
grate at the bottom and secondary air holes towards the top.
Figure 6. Combustion process in the Oorja stove.
bustion tests in the factory showed that the life of this
version was about 12 months. This was considered inade-
quate. The next version included an inner cast iron
sheathing. This version gained wide acceptance and is
expected to have a life of more than two years. This was
also considered inadequate primarily because it was
concluded that the life could be affected by rough use and
this might leave the life question unresolved. After much
research and development by FEPL in coordination with
private ceramic industries, ceramic combustion chamber
was developed (the stove using the ceramic combustion
chamber is termed Oorja-plus). This has provided a quan-
tum jump in the quality of the product. Over the last two
years, at this time of writing, about 420,000 stoves
(including the advanced metal and ceramic versions) have
been sold in the states of Maharashtra, Karnataka and
Tamil Nadu. Based on the feedback from users, another
version of the stove with a square ceramic combustion
chamber has been developed including an ash removal
tray (termed Oorja-super). This stove has still to see
large-scale commercialization. The Oorja-plus stove has
been subject to laboratory testing nationally and interna-
tionally.
While the stove configuration at the time of technology
transfer to BP (India) had specified the secondary air hole
area of 550–600 mm
2
, there were no measurements on the
influence of a choice of this area on efficiency and emis-
sions even though the qualitative influences could be
deduced. The results of the study on the variations in the
area of secondary air introduction are summarized in
Table 4 (more detailed results can be seen at http://cgpl.
iisc.ernet.in
). The ash content of the fuel is on the outer
boundary (it is expected to be less than 10%). The power
level of the stove is 2.7–3 kWth over the flaming duration
shown in the third column of Table 4. The water boiling
efficiency is independent of the secondary area in this
range. The amount of CO
2
/unit fuel and NO in mg/MJ
basis are roughly constant and SO
2
seems insignificant as
expected for biomass but CO decreases with increase in
the secondary hole area significantly. This is vividly clear
in Figure 7.
Efficiency and emissions
Two studies on the stove performed by outside groups on
this stove are Bryden and Taylor
19
as well as Datta
20
.
They have used the procedure developed by the Univer-
sity of Berkeley discussed in detail by Smith et al.
18
.
Table 5 shows the results. They both report similar effi-
ciencies – 64% for nominal power and 65% for low
power. These appeared large to the present authors and
when the details of the measurements were examined,
one quantity that seemed inappropriate was the heat of
combustion; Bryden and Taylor
19
used a low value for the
lower heating value resulting in unjustified enhancement
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CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010
636
Table 4. Emission and efficiency performance with secondary air entry area variation, fuel used = 650 g
A
s
Ash t
b
WBE CO CO
2
NO SO
2
CO NO SO
2
(mm
2
) (%) (min) (%) (g) (g) (g) (g) (g/MJ) (mg/MJ) (g/MJ)
320 11.5 40 52.2 59.1 1039 0.027 1.18 6.5 2.9 0.13
320 11.0 47 53.2 62.3 1008 0.037 2.13 6.8 4.1 0.23
320 10.5 44 53.6 954 0.030 1.10 5.8 3.2 0.12
382 12.1 40 52.0 32.5 1029 0.035 0.53 3.6 3.9 0.06
510 12.0 38 51.7 9.6 879 0.037 – 1.1 4.1 –
510 11.3 41 52.6 10.6 815 0.037 – 1.2 4.0 –
A
s
, secondary hole area, t
b
, Power during flaming time; WBE, Water boiling efficiency with 10 liter vessel, emis-
sions in g and g/MJ of fuel energy.
Table 5. Comparison of the results on efficiency and emissions on
Oorja
Item Present BT08 KD08
Power, kWth (n) 2.7 2.3 nr
WBE (%) 51 64 64
Fuel to boil (g/liter) 36* 45
+
nr
Low power (kWth) 2.0 1.7 nr
Low power WBE (%) – 65 65
CO, n (g/MJ) 1 1.2 nr
PM
10
nominal (mg/MJ) 8 1.8 nr
PM
2.5
nominal – 1.2 nr
*10 liter vessel;
+
5 liter vessel.
WBE, Water boiling efficiency; BT08, Bryden and Taylor, 2008;
KD08, Karabi Datta, 2008; n, nominal; nr, not reported.
Table 6. Efficiency and emissions of various classes of stoves
WBE CO PM
Nature of stoves (%) (g/MJ) (mg/MJ)
Free-convective based designs 15–35 1.5–15 30–1000
mud, ceramic, metal
Fan-based stoves 35–45 0.8–1.2 2–20
Optimized gasifier fan stove 40–50 0.8–1.0 2–9
Figure 7. CO, NO and SO
2
emission indices with secondary air hole
area.
of the efficiency. Fortunately, Bryden and Taylor report
the amount of fuel required to heat 1 liter of water. This
value (45 g/liter) seems to match with the results from
this laboratory if we take note of the different vessel sizes
and the influence of vessel size on the efficiency. The
results of emissions reported by Datta
20
seem to relate to
indoor air pollution rather than from the stove directly.
Emissions of CO and particulate matter (at 10 and 2.5 μm
diameter) seem to match roughly.
Comparison with other studies: Most early studies are
on free convection-based stoves made of metal, mud and
ceramics with single, two and three pots. Smith et al.
18
have conducted study of the emissions of a variety of
stoves in India. There are a number of studies by Kirk
Smith and colleagues on the greenhouse gas emissions
from domestic stoves in several countries (see the publi-
cation list of Kirk Smith). Bhattacharya et al.
2,3
have pre-
sented the results of similar stoves from south east Asia
and India. Still
21
has compiled the measurements of effi-
ciency of and emissions from about 20 stoves, only six of
which are relevant here (several stoves are with chim-
ney). These stoves contain the data of fan-based stoves as
well. A class of stoves termed TLUD (top lit up-draft)
around the development of Reed and co-workers
8–10
has
been popularized by Anderson
22
. Recently, a combustion
stove with small continuous feed and supply of air from a
fan has been developed and is in marketing trials by Phil-
lips. The results are given in Table 6 and in Figure 8 with
data from the above sources.
The wide range of efficiencies and emissions in free
convective-based designs is not unexpected since there is
no possibility of controlling the emissions due to free-
convective mode of operation. The key problem of free-
convection based stove is that while a certain arrange-
ment of fuel sticks on the grate and tending will provide
reasonably good efficiency and low emissions, it is never
clear what tending will provide good results. At least
sooting can be observed and controlled. However, gaseous
emissions cannot be observed and hence no observable
physical control strategy can be devised. A well con-
trolled laboratory test may provide good performance and
a whole range of field test data may indicate bad-to-
average results. Rigorous protocols for testing are not of
any great use since they will not represent an average
user. What is amply clear from the plot is that fan-based
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CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010
637
Figure 8. CO emissions (g/MJ) versus water boiling efficiency from
many studies.
stoves that promise near stoichiometric operating condi-
tions for combustion perform in a far superior way both
with regard to efficiency and emissions. A further opti-
mization brought about in the Oorja design relates to the
choice of a high-density fuel in the form of pellets. This
feature is emphasized by characterizing the Oorja as a
fuel optimized forced convection (FOFC) stove. The
choice of high density for the fuel pellets helps reduce the
volume of combustion chamber, provides guidance as to
the amount that would normally be required for cooking
by needing to fill a fixed amount and reduces opportuni-
ties to obtain an inferior performance by not having to
demand periodic loading or tending. Piece-by-piece load-
ing is resorted only to extend the cooking by another
10–15 min when required rather than a basic need to do it
at undesirably short periods as in Phillips stove operation.
Attaining high combustion efficiency appears to be a
pre-requisite for a ‘new generation’ stove to not only
meet the requirements of cooking, but also meet the obli-
gations of low greenhouse gas emissions, a fact clearly
brought out by Kirk Smith in most of his writings on
indoor air pollution (see for instance, Smith
23
). Keeping
away from fan-based designs by invoking the lack of
electricity is continuously getting weakened with larger
emphasis on rural electrification; availability of electri-
city even for a small period during the day or night is
adequate to charge the batteries used for cooking.
Concluding remarks
The development a new cooking solution that is based on
‘gasifier’ stove that uses an engineered ‘solid fuel’ based
on agricultural residues in the form of high density pellets
is reported here. Several aspects of the stove design and
the fuel are brought out. The high utilization efficiency
and low emissions are a consequence of the generation of
near-constant throughput of gaseous fuel due to gasifica-
tion and a correct air-to-fuel ratio used for combustion of
the gases. The role of secondary air in strongly control-
ling the CO emissions is emphasized. An important
aspect brought out is that while free convection-based
stoves may be appropriate in totally unelectrified areas,
forced convection stoves may be the only way to the
future in improving the quality of the environment around
the stove, apart from higher efficiency; the minimum
demand on ‘tending’ is met with by the current design.
There appears to be a rising demand for this class of
stoves in the ‘rural’ user groups that do not lack the fuel
as well as the ‘urban’ users who have difficulty in sourc-
ing any kind of fuel in an affordable manner. This
demand is traced to a clear feeling of the user in owning
something that operates in a manner close to what LPG
stove does. The journey from concept and laboratory
studies to production and commercial outreach to more
than a thousand users a day hoping to move to a million
users soon has been possible due primarily to many prob-
lem solving sessions over the last several years.
A view that emerges from the present effort is this: the
crucial aspect of producing adequate amounts of dense
pellet fuel and making them available in an affordable
manner forms the primary limiting feature in resolving
the cooking fuel problem of our country (as well as other
similar countries). In other words, any progress in main-
streaming solid biofuels will alleviate the cooking pro-
blem of a large segment of the Indian population. The
role of the current efforts in making positive contribu-
tions to the climate change problem and related debate
has been outside the scope of this study.
1. Prasad, K. K., Sangen, E and Visser, P., Wood burning cook
stoves. Adv. Heat Transfer, 1985, 17, 159–317.
2. Bhattacharya, S. C., Albina, D. O. and Khaing, A. M., Effects of
selected parameters on performance and emission of biomass-fired
cook-stoves. Biomass Bioenergy, 2002, 23, 387–395.
3. Bhattacharya, S. C., Albina, D. O. and Salam, A. P., Emission
factors of wood and charcoal fired stoves. Biomass Bioenergy,
2002, 23, 453–469.
4. Mukunda, H. S., Shrinivasa, U. and Dasappa, S., Portable single-
pan wood stoves for high efficiency. Sadhana, 1988, 13, 237–270.
5. Bhattacharya, S. C., Biomass gasification in Asia. In Handbook
Biomass Gasification (ed. Knoef), Biotechnology Group, BV,
2005, chap. 7, pp. 162–176.
6. Kaupp, A., Gasification of Rice Hulls. GTZ, Eschborn, Germany,
1984; also see Kaupp, A., The gasification characteristics of rice
hulls for the generation of electricity and shaft power on a small
(5–30 hp) scale, 1st International Producer Gas Conference, Co-
lombo, Sri Lanka, 1982.
7. La Fontaine, H. and Reed, T. B., In An Inverted Downdraft Wood-
Gas Stove and Charcoal Producer, in Energy from Biomass and
Wastes XV (ed. Klass, D.), Washington DC, 1993.
8. Reed, T. B. and Larson, R., A Wood-Gas Stove for Developing
Countries, in Developments in Thermochemical Biomass Conver-
sion (ed. Bridgwater, A. V.), Blackie Academic Press, 1996.
9. Reed, T. B. and Walt, R., The ‘Turbo’ Wood-Gas Stove in Bio-
mass. In Proceedings of the 4th Conference of the Americas in
GENERAL ARTICLES
CURRENT SCIENCE, VOL. 98, NO. 5, 10 MARCH 2010
638
Oakland, Ca (eds Overend, R. P. and Chornet, E.), Pergamon
Press, Oxford, 1999.
10. Reed, T. B., Anselmo, E. and Kircher, K., Testing and modeling
the wood-gas turbostove. In Progress in Thermochemical Biomass
Conversion Conference, Tyrol, Austria, 17–22 September 2000.
11. Shrinivasa, U. and Mukunda, H. S., Wood gas generators for small
power requirements. Curr. Sci., 1983, 52, 1094–1098.
12. Ravindranath, N. H. and Hall, D. O., Biomass Energy and Envi-
ronment – A Developing Country Perspective from India, Oxford
University Press, Oxford, 1995.
13. Shukla, P. R., Biomass Energy in India, Conference Proceedings –
Biomass Energy: Key Issues and Priority Needs, International En-
ergy Agency, Paris, France, 1997.
14. Kishore, V. V. N., Preety, M. B. and Gupta, P., Biomass energy
technologies for rural infrastructure and village power – oppor-
tunities and challenges in the context of global climate change
concerns. Energy Policy, 2004, 32, 801–810.
15. Anon., TERI energy data directory and yearbook (TEDDY),
2004/05.
16. Sinton, J. E. and Smith, K. R., Indoor air pollution database for
China, WHO/ENG/95.8, UNEP, Geneva, 1995; also see FAO
document RWEDP Report 32, GCP/RAS/154/NET, Proceedings
of the regional expert consultation, 1997.
17. Chatterjee, S., Rae, A. and Ray, R., Food consumption and calorie
intake in contemporary India. Department of Applied and Interna-
tional Economics, Discussion Paper No. 07.05, Massey Univer-
sity, 2007; http://www.fao.org/docrep/006/Y5022E/y5022e04.htm
and http://www.weightlossforall.com/food-calories-list.htm
18. Smith, K. et al., Greenhouse gases from small-scale combustion
devices in developing countries, Phase IIa, Household stoves in
India, USEPA, EPA-600-00-052, 2000.
19. Bryden, M. and Taylor, P., Emission test results for BP-Oorja
stove, April 2008.
20. Datta, K., A comparison of stove emission levels in rural house-
holds using various models of improved stoves for at least a year
and the constraints influencing the utilization of improved stove in
a rural community in Maharashtra state, Better air quality 2008,
Air quality and climate change: scaling up win-win solutions for
Asia, Bangkok, Thailand, November, 2008.
21. Still, D., MacCarty, N., Ogle, D., Bond, T. and Bryden, M., Com-
parison of stoves, Aprevecho Research Center, Shell Foundation,
Partnership for clean indoor air, 2006.
22. Anderson, P. S., Reed, T. B. and Wever, P. W., Micro-
gasification: what it is and why it works. Boiling Point, 2007, 53,
35–37.
23. Smith, K. R., Health, energy and greenhouse gas impact of bio-
mass combustion in household stoves. Energy Sustain. Develop.,
1994, 1, 23–29.
ACKNOWLEDGEMENTS. We thank Ms Manikankana Sharma who
performed most of the early experiments and Mr Srinath who per-
formed all the tests on many variants of Oorja stove, both on efficiency
and emissions.
Received 10 December 2009; accepted 27 January 2010
