P55 to P70 per kg. Conventional cooking stoves, such as gas
and liquid burners, are convenient to use and to operate but
now prohibitive. Due to this, households particularly in the
villages adopt biomass and wood as fuel for their cooking
needs. This practice is affordable, but produces excessive
smoke and particulate emissions. It was reported by the
World Health Organization (2005) that indoor pollution
caused by too much smoke emission in the traditional
burning of wood and biomass stoves resulted in about 1.6
million deaths per year in developing countries due to
chronic respiratory diseases.
Rice husk, a by-product in rice milling is abundant. In the
past, it is disposed by burning along road sides and/or by
dumping on river banks. About 2 million metric tons of rice
husks is produced annually with enough potential energy as
fuel for domestic household (Belonio, 2005). A kilogram of
rice husk, contain about 3,000 kcal of heat (Kaupp, 1984).
Despite of the varied applications or usage of rice husks, the
abundance in supply of this waste material can still warrant
as an alternative source of energy for the rural people.
Gasifying rice husks is a good alternative to provide
households with low-cost but clean source of energy for
cooking (Anderson, et al, 2006 & Anderson, et al, 2008).
By limiting the amount of air used in burning rice husks,
combustible gas that is rich in carbon dioxide and hydrogen
are produced (Belonio, 2005). Several studies revealed that
a stove that operates on gasification has low particulates as
well as CO2
emission (Anderson, et al, 2006, Anderson, et
al, 2008 & Teenet, Undated). Among the various gasifier
stoves tested on spot during the US-ASEAN NewGeneration Stove Workshop at Asian Institute Technology
in Thailand, the rice husk gas stove obtained the lowest
black carbon emission of about 50ug/m
3
of gas (Hansen,
2009).
In 2005, a batch type, top-lit downdraft type rice husk
gasifer stove was developed at Central Philippine University
in Iloilo City (Belonio, 2005). The stove has caused the
widespread acceptance of the technology by the people not
only in the Philippines but also in other rice producing areas
in the world like Indonesia, Vietnam, India, and other
countries in Asia, Africa as well as Central and South
America (Belonio, 2009 & Minang, et al 2007). However,
because of the differences in cooking practices and needs by
households, a rice husk gas stove that operates in a
continuous mode was designed and developed.
This paper describes the design and performance of a
household-size continuous-flow rice husk gas stove aimed to
provide households a simple and clean burning stove for
cooking. The comparative operating cost analysis against
LPG and kerosene burners is also presented in this paper
2.1 Design Preparation
The design of the stove was based on the principle ofbottom-lit moving-bed down-draft type rice husk gasifier,
which was recently developed for industry application
(Belonio, et al, 2010). Instead of multiple locations
provided for fuel ignition, only one ignition point was
considered in the present design. The size of the reactor was
scaled down to nearly 1 kWt, just enough for a family with 3
to 4 members. The amount of air needed to gasify rice
husks was computed using an equivalence ratio of 0.3 to 0.4
with stoichiometric air for rice husk of 4.7 kg air per kg of
fuel as recommended by Dr. Albreacht Kaupp (1984).
After finalizing the conceptual design of the stove, a 3D
AutoCAD drawing was prepared to ensure consistency
throughout the different assemblies. A 2D drawing was also
prepared to serve as a guide in the fabrication of the
different parts of the stove.
2.2 Fabrication
Prior to fabrication, the design drawing was discussed with
the Fabricator to simplify the construction of the stove and
to make the unit durable and affordable.
The stove was fabricated at BMC in Pavia, Iloilo,
Philippines. Further revisions and improvement of the
design was done at BEST-Enterprise at the Science City of
Munoz, Nueva Ecija, Philippines. Regular shop visits were
made until construction of the stove was completed.
Performance Testing and Evaluation
The final proto-type of the stove was tested using waterboiling tests. Series of tests were conducted at BMC shop
as well as at CLSU-CRHET Rice Husk Project Office at the
College of Engineering, CLSU. During testing, fresh rice
husks were used as fuel for the stove. The time to ignite rice
husk fuel and the time to generate combustible gases were
also taken in each test. One liter and 2 liters of water were
boiled in the stove for more than an hour. In each test, the
time required to boil water was determined. During the test,
the temperature of water was measured at 2-minute interval
using a bimetallic thermometer. The gas temperatures as
well as the flame temperatures beneath the pot were
recorded every 10 minutes using a digital thermometer with
type K thermocouple wire sensor. The amount of water
remaining in the pot was also measured in each operation.
The following parameters were determined during the tests:
(1) Fuel consumption rate; (2) Specific gasification rate; (3)
Thermal efficiency; (4) Power output; and (5) Percentage
char produced.
Operating Cost Analysis
The cost of operating the stove was determined based on theinvestment cost, which is the actual selling price of the
stove. The investment cost and the costs incurred for the
rice husk fuel and electrical consumptions were computed
on a daily and on hourly bases. A comparative cost analysis
was done and the savings derived in using the rice husk gas
stove over conventional stoves was determined. The time
required to recover the investment for the stove was also
computed.
Design Description of the Stove
The stove, as shown in Figure 1 below, is a continuous-flowmoving-bed rice husk gasifier operating on a bottom-lit
down-draft mode. It consists of the following components,
namely: (1) Fuel Hopper; (2) Fan; (3) Fuel Reactor; (4) Gas
Duct; (5) Gas Burner; (6) Pot Support; (7) Support Legs; (8)
Char Pan; and (9) Push Rod. The fuel hopper holds the rice
husks in place before they are fed into the reactor. Rice
husks are gasified in the reactor by burning them with
limited amount of air supplied by a 12-Volt, 0.12-Amp
computer fan. The reactor is made of a 1.2 mm GI sheet
and has a diameter of 12 cm and a height of 30 cm. The gas
duct, where combustible gases are diverted into, is made of
10 cm diameter cylinder made slightly higher than the
reactor and the fuel hopper. On top of the gas duct is the
plate-type gas burner having 40 pieces 4 mm diameter holes.
The char pan and the push rod are used to remove char
during operation. The entire structure is supported by four
pieces of inclined legs.
Fabrication of Stove
The stove can be fabricated in a small shop using localmaterials and labor. Galvanized iron sheet or conduit and
bars are used as materials for the stove. Four units of the
stove can be produced from one standard size 1.2-m wide by
2.4-m long metal sheet. For these four units of stoves, two
pieces of round bars are needed for the legs and handle as
well as for the pot holder. One person can build one stove
in one day.
Test Performance
Results of the performance tests and evaluation revealed thatthe stove performs well as per design. As shown in Table 1
below, rice husk consumption of the stove is at a rate of 1.07
to 1.12 kg/hr, depending on the degree of char removal and
on the amount of air supplied. Ignition of rice husks is
achieved after a minute of dropping before combustible
gases are generated at the burner. It was found out during
the test, switching the fan to 12 volts will facilitate the
ignition of rice husks, shorten the generation time of
combustible gases, and minimizes smoke emission. It was
also observed that the use of dry and fresh rice husks
produces less smoke.
To boil a liter of water in the stove takes 5 to 7.6 minutes,
depending on the intensity of the flame. The higher the fan
voltage setting, the stronger the flame produced; hence,
shortening the boiling time of water. On the other hand,
boiling 2 liters of water requires between 10.4 to 15
minutes. Figure 4 shows the temperature profile of boiling
water in the stove. Gas temperature taken at the gas duct
using a thermocouple wire sensor, varies from 90 to 100°C;
whereas, the temperature measured beneath the pot varies
from 250 to 400°C. The specific gasification rate of the
stove varies from 90 to 102 kg/hr-m2. Furthermore, the
thermal efficiency of the stove varies from 18 to 25%. This
value is still acceptable since the gas burner operates
without a heat shield or a skirt in keeping the heat
concentrated at the bottom of the pot. The char production
rate or the amount of burned rice husk produced varies from
0.32 to 0.34%. In addition, the computed thermal output of
the stove is at the range of 0.69 to 1.01 kWt.
Operating Cost, Savings, and Payback Period
Table 2 shows the operating costs of using the RHG,kerosene and LPG stoves. The RHG stove, including the
12-volt fan and an AC-DC adoptor costs P2,000.00. A
typical household with 3 to 4 members will require an
average of one kilogram of rice husks per hour cooking.
The cost of electricity in operating the stove is very minimal
since the fan consumes only about 0.005 kw-hr. \
The computed fixed cost for the stove is P4.49 per day while
the variable costs, which are the costs incurred for the rice
husk fuel plus the small amount of electricity consumed in
running the fan, is P2.40 per hr. Considering a 3 -hour
operation per day, the computed operating cost per hour for
the stove is only P2.30. Comparing the rice husk gas stove
with conventional stoves, consumption of kerosene is
assumed at 0.25 liter per hour while for LPG is 0.15 kg per
hour. Investment cost for the kerosene stove is cheaper than
that of LPG stove, which is slightly higher than that of the
rice husk gas stove due to the cost of tank, hose, and
regulator. The computed fixed costs for kerosene and LPG
stoves are P0.79 and P5.69 per day, respectively. The cost
of fuel used per hour is quite expensive for the conventional
stoves giving P37.50 and P31.50 for kerosene and LPG fuel,
respectively. The cost to operate the kerosene stove per
hour is computed at P12.76 while P12.40 for LPG. The
households who will opt to use the rice husk gas stove can
have a daily savings of P10.46 over the use of kerosene
stove and P10.10 over the use of LPG stove. For the period
of one year, a total savings of P11,457.35 can be derived by
households over the use of kerosene stove and P11,059.50
over the use of LPG. The investment for the rice husk gas
stove can be recovered with 2 to 3 months.
CONCLUSIONS AND RECOMMENDATIONS
Based on the results of the study, the rice husk gas stoveperforms accordingly with the design. It can satisfactorily
provide combustible gases for continuous operation for
more than one hour of domestic cooking. It can be
energized either by direct connection into an AC-DC
calculator adoptor in areas where grid is available or by the
use of a 12-volt battery with 12- volt 5-watt solar panel in
off-grid situation. With proper operation, smoke is almost
completely eliminated and clean combustible gases for
cooking is achieved. The stove can be fabricated even in a
backyard shop using metal sheets and steel bars employing
the local people. The price of the stove is affordable to
many and households can generate substantial savings from
the use of biomass fuel over the use of conventional fuel.
Investment can be recovered within a short period of less
than a year.It is likewise recommended that further improvement on the
rice husk gas stove must be done to cater the specific needs
of households in terms of comfort and convenience in
cooking operation.
