Biogas appliances in Sub-Sahara Africa

Biogas appliances in Sub-Sahara Africa
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
Biogas appliances in Sub-Sahara Africa
Vianney Tumwesigea* David Fulfordb Grant .C. Davidsonc
a
Green Heat (U) Ltd, P.O. Box 10235, Kampala, Uganda and Center for Research in Energy and Energy
b
c
Conservation, Makerere University; [email protected] The James Hutton Institute,
Craigiebuckler, Aberdeen, AB15 8QH, Scotland, UK
*
Corresponding author: Vianney Tumwesige, Green Heat (U) Ltd, P.O. Box 10235, Kampala, Uganda.
Tel: (+256) 71 237 9889. Email: [email protected]
ABSTRACT
Biogas production technology has led to the growth of a number of biogas appliances for lighting,
cooking, heating, incubating and electricity generation. The most commonly used appliance for cooking
purposes in both households and institutions is the biogas stove. However, some households are using
biogas lamps for lighting their homes. The overall objective of this paper is to review biogas appliances
being used in the different National Biogas Support Programmes in Sub-Saharan Africa.
Several locally available biogas stoves were tested, but were found to have lower efficiencies than were
acceptable. The stoves were not made according to basic gas stove theory.
Key questions are: what biogas appliances are being used; what are the major areas where appliances
can be developed to improve their efficiencies; and what are the possible methods/mechanisms to do so?
1
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
1. Introduction
Energy is an essential ingredient for socio-economic development and economic growth. In developing
countries, around 2.6 billion people rely on traditional biomass such as fire wood, charcoal, animal dung
and agricultural residues, while 400 million use coal as their primary cooking fuel. Over 700 million people
without access to liquefied petroleum gas (LPG) and electricity for cooking live in the Least Developed
Countries and over 600 million in Sub-Saharan Africa (1).
Traditional biomass such as wood fuel, agricultural residue and animal waste accounts for over 80
percent of energy use in Sub-Saharan Africa (2). Over 90 per cent of the energy used in households is for
cooking and the rest of the energy is used for lighting (3). Ordinary kerosene lamps are the most common
type of fuel-based lighting in developing countries (4). The light output of kerosene lamps varies from 10
to 100 lumen, depending on the type of lamps and the wicks used. The recommended level of illumination
-2
required for reading is 100-200 lm m (5).
Global estimates of greenhouse gas emissions from fuel-based lighting places the value at 190 million
tonnes of carbon dioxide (CO2) per year (6). In Kenya, for example, 88 per cent of the population uses
kerosene as a lighting source (7). While kerosene is the dominant fuel in practice, diesel is used when it
is the cheaper alternative or where government programs limit the availability of kerosene (8). Despite
efforts at rural electrification, the number of people without access to grid electricity is growing in SubSaharan Africa (9).This is attributed to the high cost of supplying rural and peri-urban households,
population growth, weak implementation capacity, electricity generation shortage and lack of appropriate
incentives (10).
Lack of access to clean and efficient fuels in homes can impact health in many ways. The most important
direct health effects result from the air pollution caused by burning solid fuels, often indoors on open fires
and simple stoves (11,12). Over 1.6 billion people worldwide are exposed to particulate matter when
lighting and burning carbon based fuel such as wood, dung, candles, kerosene, to conduct business,
study, and perform household tasks after dark (6). The indoor use of inefficient stoves in households
releases large amounts of smoke from incomplete combustion of solid fuels. Breathing this smoke affects
the health of all members of the family, but especially that of women and their young children. While there
is very little literature on the impact of installing biogas digesters on household air quality there is
considerable evidence that homes burning traditional biomass fuels such as wood, charcoal, coal and
dried crop/animal residues have very high concentrations of fine particulate matter and carbon monoxide
(13). Evidence from studies homes in Nepal that burn charcoal and LPG suggests that fine particulate
concentrations in homes using LPG were about one-tenth of the concentration of those in homes burning
solid-fuels (14). It seems likely that similar order of magnitude reductions in indoor air pollutants could be
experienced in homes switching from traditional biomass fuel to biogas systems. Significant
improvements in respiratory and cardiovascular health of householders who experience such reductions
in indoor air pollution concentrations can be anticipated, given results obtained from stove-based
interventions in Guatemala (15,16).
The energy deficit in poor households results in practical constraints, such as inadequate lighting
(reliance on paraffin lamps, candles or wood fires), and inadequate cooking fuel and thus fewer hot,
cooked meals. The problems caused by energy poverty in turn have consequences in the standard of
living of the poor through more frequent illness (which impacts income), difficulty in doing schoolwork and
so on (17). Access to clean and convenient energy services that can meet the needs of both lighting and
cooking are therefore vital to the alleviation of poverty. Therefore, biogas could be an essential
component for socio-economic development.
Biogas technology is an integrated waste management system (18) that is a clean, renewable, naturally
produced and under-utilized source of energy. Biogas is produced in an air tight tank from a variety of
substrates, such as animal manure, food waste, energy crops and industrial wastes. Anaerobic digestion
is a multi-stage biological process, where the organic waste is mainly converted to a gaseous product
2
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
composed of 50-70% (by volume) methane, 25-40% (by volume) carbon dioxide and traces of hydrogen
sulphide, water vapour and ammonia (19–21).
134
135
136
137
and biogas contains 60% methane, while air contains 21% oxygen, the volume ratio for the stochiometric
mixture of air and biogas is 5.7:1 or a volume fraction of 17.5%. Biogas burns over a narrow range of
mixtures from approximately 9% to 17% of biogas in air (30). If the flame has too much fuel, then it will
burn incompletely, releasing carbon monoxide, which is poisonous, and soot particles. Therefore, the
Biogas energy has some advantages over other energy sources. Successful use of biogas technology
can result, not only in energy generation and bio-fertilizer production, but also in other social and
ecological benefits including improved sanitation from more efficient use of human and animal waste
products, and reduction of imported fuel oil (22). The technology has the potential to contribute to
mitigation of greenhouse gas emissions (23). Biogas systems lead to reduced enrichment of bodies of
fresh water by runoff of inorganic plant nutrients from applied fresh-waste, reduced air pollution and
improved utilization of crop nutrients (24). By eliminating the daily task of gathering firewood, biogas
technology could reduce the work-load of women (25). Further, biogas is produced mainly from raw
materials that are locally available and can be harnessed in controllable, containable and useable
quantities.
The most efficient way to use biogas is in a heat-power combination where 88% efficiency is achievable,
but this is only possible in larger installations and under the condition that the exhaust heat is used
profitably (26). The efficiency of using biogas at smaller scale is 55% in stoves, 24% in engines and 3% in
lamps
Biogas can lead to a reduction in greenhouse gas emissions by replacing fossil fuels; the carbon dioxide
emitted following burning of biogas includes only carbon accumulated during the life cycle of plants and
animals, and even without anaerobic digestion, this carbon would have been emitted over time by natural
decomposition processes. Biogas can lead to large CO2 emission reductions in the electricity sector, by
replacing fossil fuels and non-renewable fuels used in cooking or by substituting chemical fertilizer with
bio-slurry (27). If an engine is run on biogas instead of diesel, generation of 1 kW of electricity would
prevent 7,000 kg CO2 per year being added to the atmosphere (28). Biogas can be used for all
applications designed to run on natural gas. It can be used as a fuel in power generators, engines, boilers
and burners.
The main use of biogas technology in the world is for domestic use in rural areas, where animal dung
(mainly cattle or pig) is the main feed material. Small biogas plants are owned by individual farming
families, who own a few animals, and who use the gas for cooking and also lighting. There are about 40
million such plants in China, about 4 million in India, about 1/4 million in Nepal and another 1/4 million in
the rest of Asia (29). Both China and India had national biogas programmes, initiated by their
governments. The programmes in the rest of Asia were encouraged by the Netherlands Development
Organisation (SNV). In 1993, SNV were asked to take over a biogas programme that had originally been
set up in Nepal by Development and Consulting Services (DCS) of the United Mission to Nepal (UMN) in
1976 (30). DCS had developed their own design of biogas burner, which was cheaper than those
commercially available from India or China. As SNV extended their biogas programme to other countries
in Asia, this stove design has been copied and made by other manufacturers.
2. Biogas burners
2.1 Biogas burner theory
Biogas burns in oxygen to give carbon dioxide, water and energy content in methane is released.
Understanding the combustion process provides a basis of performance criteria and emission standards
used to regulate manufacturing and marketing of quality biogas stoves. Since the chemical reaction
biogas burning is:
CH 4  2O2  CO2  2H 2
3
138
139
140
141
142
143
144
145
146
147
designs of appliances should aim at maximizing the conversion of methane into carbon dioxide in order to
minimize the release of unburned methane and products of incomplete combustion. Stoves usually run
with a small excess of air to avoid the danger of the flame becoming rich. If too much air is supplied, the
flame cools off, thus prolonging the working time and increasing the gas demand (26).
148
149
150
151
152
153
154
155
156
157
158
159
Figure 1: Parts of a biogas burner
Biogas stoves and other equipment are made in two parts: the burner itself, which mixes gas and air and
feeds it to the flame ports, where it burns; and the frame within which it sits, which uses the flame to heat
cooking pots or to generate light or use the heat in some other way. The frame for a stove supports the
burner on legs and holds cooking pots the right distance away from the flame for effective heating.
The burner itself has several parts (30). The amount of gas that flows into the burner is controlled by the
jet, a hole which is carefully sized and defines the power output of the burner. Most burners are partially
aerated, which means that the gas is mixed with a proportion of primary air that is less than optimum for
combustion. The air and biogas are mixed and fed to a manifold which feeds the flame ports, where it
burns. Secondary air flows around the outside of the flame ports to complete the combustion process.
The burner ports are drilled into a shaped cap, which can be removed for cleaning, in case food gets
spilled into the burner ports.
The optimum amount of air to allow a fuel gas to burn is called the stochiometric mix and is 5.5:1 for
biogas. The flow of gas from the jet depends on the hole size and the gas pressure:
Q  3.16 Cd A0
160
161
162
163
164
165
166
167
168
173
174
175
176
177
178
179
180
181
182
183
(1)
2
where Cd is the coefficient of discharge of an orifice (jet), A0 is the area of the jet (m ), p is the gas
pressure (Pa), and s is the specific gravity of the gas. Typical values of Cd are between 0.7 and 0.9,
depending on how well it is made, s is 0.94 for biogas. An average value of the enthalpy of combustion (H
-3
- the heating value) of biogas is: 21.7 MJ m , so the power produced by a burner is simply Q × H or:
P  3.16 H Cd A0
p
1,000 in kW or P  3.16W Cd A0 p 1,000 ,
s
(2)
where W  H
s is called the Wobbe number of the gas (= 22.2 MJ m ).
As the gas emerges from the jet, it accelerates, which reduces the pressure according to Bernoulli’s
theorem:
-3
p  1  v 2  constant ,
2
169
170
171
172
p
1000 in dm3 s-1,
s
-3
(3)
-1
where p is the pressure (in Pa), ρ is the gas density (kg m ) and v is the gas velocity (in m s ). The
reduced pressure entrains (draws in) air, which mixes with the gas in the mixing tube. The entrainment
ratio r is given by Priggs formula:
 At

d

r  s 
 1 or r  s  t  1
 d0 
 A0

(4)
where At is the area and dt, the diameter, of the throat, the narrowest part of the mixing tube. Typical
values of r, which defines the primary aeration are between 50% and 75%. The entrainment ratio is
chosen to give an air flow to give a mix about twice that of stochiometric or theoretical air requirement.
The mixing tube can be made as a venturi, with a narrow throat with tapers leading in and out, or as a
straight tube. The length of a straight mixing tube should be at least 10 × dt, the diameter, while a venturi
tube can be 6 × dt. The total flame port area Ap should be between 1.5 and 2.2 × At for Priggs formula to
work.
Figure 2: Parts of a gas flame
4
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
The flame height at the flame ports are affected by the primary aeration. A low value of r means the gas is
seeking secondary air to burn, so the flames are long, “lazy” and do not burn properly. Poor combustion
generates carbon monoxide, which is poisonous, and carbon particles (which show as red flashes in the
flame). A high value of r means that the gas can burn with the primary air, so the flames are much
shorter. Full aeration is (r = 5.5) is inadvisable, as the flame can “flash back”, i.e. jump through the flame
ports, along the mixing tube and burn at the jet.
The flame ports need to be designed to allow easy access to secondary air flow. Various tricks are used
to stabilise a flame, such as ledges around the flame ports and using small secondary flames around the
main one. The potential heat contained in biogas can be released when sufficient quantity of air burns
with it. Insufficient air would lead to loss of potential heat by incomplete combustion while an excess may
give rise to an excessively large loss of potential heat. Biogas has a low laminar flame speed
-1
(vfl = 0.25 m s ) (30). The flow in a flame port is turbulent, so the actual flame speed is higher. The
velocity of the gas from the flame ports must be lower than the flame speed for the flame to be stable.
Good mixing increases the flame speed, so improves flame stability.
All gas burners follow the same principle; the force which drives the gas and air into the burner is the
pressure of gas in the pipeline (31). A biogas stove can have single or double burner with varying gas
3 -1
3 -1
consumption rates ranging from 220 dm h to 450 dm h at standard temperature and pressure (32,33).
This consumption rate results from the pressure provided by the biogas plant and the diameter of the inlet
pipe. The jet at the inlet of the burner increases the gas speed, so producing a draft that sucks primary air
into the pipe.
The stove must be designed to suit basic local requirements such as ease of cleaning, repair, good
burning properties, safe to use, versatility, attractive appearance (33). However, these requirements vary
from location to location and are linked to local dietary and hence cooking requirements. The gas demand
is higher in cultures with complicated cuisine and where whole grain maize or beans are part of the staple
diet.
The overall efficiency of using biogas is 45% in stoves (34,35), 24% in engines and 3% in lamps (26).The
efficiency of the given biogas stoves is not constant. It varies depending on the surrounding conditions;
wind, temperature, pressure, shape, specific heat capacity and weight of vessel, burner size of stove and
size of bottom face of cooking vessel, and the quality of the gas (36) (see Table 1).
Table 1 – Comparison of efficiency of different types of stoves
2.2 Tests on biogas burners
Eight biogas stoves were tested by the Centre for Research in Energy and Energy Conservation
(CREEC) at Makerere University in July 2012. CREEC was set up to test improved wood stoves using
equipment supplied by Aprovecho. The stoves were manufactured in Uganda and were supplied by
Heifer International-Uganda. Most of the stoves were made to a similar design, as shown in the
photograph in Figure 3. The stoves were supplied with gas from a fixed dome biogas plant and used to
boil a 5 litre pot of water (37).
Figure 3 – Photograph of the type of stove tested
The dimensions of the stoves were tested against the above theory. The measurements are given in
Table 2 and the results of the calculations in Table 3. The calculations were made assuming a gas
pressure of 137 Pa. The gas pressure from a fixed dome plant is variable, so the power output from the
burners is also variable. The flames are usually adjusted by a gas valve, which chokes the flow into the
burner and reduces the effective pressure at the burner jet.
Table 2 – Key dimensions of 8 stoves
Table 3 – Design checks on 8 stoves
5
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
The power output from these burners seems high. An electric kettle uses a 3 kW element, so a power of
over 5 kW seems excessive. This suggests that the jet sizes are too big. However, the calculations were
based on a value for the coefficient of discharge (Cd) of 0.8. If the jets are poorly made and have rough
edges, the value of Cd may be lower. The entrainment ratio seems reasonable for most of the stoves, as
the primary aeration is between 50% and 85%. The aeration for the Large Psem stove is much too high,
while that for the small Psem stove is too low. Aeration can be reduced by adjusting the size of the air
holes with a movable cover that partially blocks these holes.
None of these stoves meets the Priggs test, that the flame port area (Ap) is between 1.5 and 2.2 × the
throat area (At). This calls into question whether the primary aeration is working properly. Flame ports that
are too small give a high back pressure on the flow of the gas mixture through the burner. The flame port
areas should be much higher in all of the burners. Also the mixing tube length is too short, even for those
stoves that use a venturi shape; the mixing tubes should be at least 6 times the throat diameter,
preferably 10 times.
The stoves were used to heat 5 litres of water from ambient temperature to 96°C and the time it took
recorded. Taking ambient temperature as 27°C, this required 1.425 MJ of energy, ignoring the heat
capacity of the pan. The gas flow was measured and, assuming an average enthalpy of combustion for
-3
biogas of 21.7 MJ m , the actual power of each stove could be calculated, as shown in Table 4. Following
the Aprovecho protocol (designed for improved wood stoves) the tests were done when the stove was
cold and then again when the stove had been heated up by the first test.
Table 4 – Results of boiling tests on 8 stoves
The overall efficiency values seem low, as Chinese and India standards define 55% as the minimum
required (32). The variation between hot and cold results appears inconsistent, as the gas flow rate for
the hot tests are higher for some stoves and lower for others. However, the stove is turned off at the gas
tap between tests. It is very difficult to set the tap at exactly the same position between tests, so the
pressure at the jet may have been different for the cold and hot runs for some of the stoves. The two flow
rates were the same for the Bremmen stove, but the efficiency of the stove was lower for the hot test.
Stove efficiency has two components, the efficiency of combustion of the gas and the efficiency in which
the heat is transferred to the pot. Poor combustion is indicated by the formation of carbon monoxide,
which indicates the gas is poorly mixed with air and does not burn properly. Carbon monoxide and carbon
dioxide were measured in the tests and the results are shown in Table 5.
Table 5 – Results of combustion tests on 8 stoves
The results show very high carbon monoxide levels for most of the stoves. Indian and Chinese standards
require less than 0.05% of CO in the smoke (32), so all the stoves fail apart from the small Psem stove.
The small Psem stove has the lowest primary aeration. There is no correlation between the CO levels and
the overall stove efficiency. However, poor combustion is likely to be a strong contributor to the low
overall efficiency figures.
Poor heat transfer between the flames and the pot is a result of poor frame design. The frame supports
the burner and also holds pots at the right distance away from the flames. The optimum value for the
height of a pot above the flame ports is between 22 and 42 mm (38). The values for the stove on test all
fall within this range, apart from the Large Psem stove. However, the flame ports are on the side of the
manifold cap, so the distance between the flame ports and the pot is greater than the measured value
between the manifold cap and the pot.
The relative diameter of the circle through the flame ports and that of the pot is another factor. A large
source of heat under a small pot means that heat is wasted. A small heat source under a large pot allows
cold air from below to mix with the hot gases flowing up the side of the pot. The pot used in the tests was
270 mm in diameter, while the diameters of the circle on which the flame ports sit are much smaller in
most of the stoves. This suggests the second factor may explain some of the heat losses.
6
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
2.3 Safety of the stoves
Only one of the stoves had four legs, while the rest had three legs. The advantage of three legs is that the
stove remains stable, even if the surface on which it is placed is uneven. A four legged stove will tend to
wobble, unless it is on a flat surface. The three legged stoves also had a circular frame, but this meant
they could be tipped over more easily if they were knocked. Short legs meant the frame was more stable,
but at the cost of causing the surface on which the stove sits to heat up. The KEJ stove had the shortest
legs, but also had the smallest frame. A small frame means that it is close to the flame ports and the
edges can heat up and cause burns to people, especially children, who touch it.
2.4 Discussion and burner design modification
The results of these tests offer similar conclusions to the ones done by SNV on burners made by different
biogas projects, such as those in Nepal, Bangladesh, Lesotho and Rwanda (32). The quality of such
stoves does not meet the standards defined in China and India. However, the quality of the stoves tested
in Uganda seems to be worse than those tested by SNV.
In general, the stoves should be made larger, which would allow several problems to be overcome. A
larger diameter frame would make it more stable and allow the legs to be made longer. This would reduce
heat transfer to the frame and the surface on which the stove is placed. A larger frame would allow the
use of a longer mixing tube and a larger diameter for the burner manifold.
The gas flow through the jet is critical to the performance of a stove. If the flow is correct, the efficiency
can be as high as 60% (36). However, the pressure from a fixed dome plant is variable, so the flow needs
to be adjusted as the pressure changes. The tests done for SNV suggested that the jet sizes in those
stoves were too small (32). The jet diameters were between 2 and 3 mm.
The main parameters for designing a biogas stove are efficiency and safety. The main factors to be
considered in order to achieve high efficiency of the stove are composition and pressure of the gas,
velocity of the flame and pan to burner distance are important factors to be considered (30). However, the
stove should also meet the following criteria:






Gas inlet pipe should be smooth to minimize the resistance to flow of gas and air.
Spacing and size of air holes should match with the requirement of gas combustion.
Volume of burner manifold should be large enough to allow complete mixing of gas with air.
Size, shape and number of burner port holes should allow easy passage of the gas-air mixture,
formation of a stable flame and complete combustion of gas, without causing the flame to lift off
the burner port or back flash from the burner port to gas mixing tube and injector jet. The flame
should be self stabilizing, i.e. flameless zones must re-ignite automatically within 2 to 3 seconds.
Under ideal condition, the pot should be cupped by the outer cone of the flame without being
touched by the inner cone.
Size and shape of the burner should match the cooking vessels.
The tests on the Ugandan stoves suggest that the jet sizes (between 5 and 8 mm) are too big. An ideal
approach would be to use a needle valve that uses a fine tapered needle mounted on a threaded rod.
The rod can be turned by a knob to insert the needle into the jet to change its size. However, needle
valves are very difficult to make accurately.
Figure 4: Modified gas burner design
The main issue appears to be that the mixing tubes are too short and the flame port area is too small. The
mixing tube should be made longer, so that it extends well beyond the edge of the frame. The diameter of
the burner manifold needs to be larger, so that a greater number of flame ports can be drilled in the cap.
The burner manifold could be made in a donut shape, with a hole in the centre. Flame ports can be
7
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
placed on the inner surface of the ring as well as on the outside surface. Secondary air can then flow up
the centre of the donut ring as well as around the outside.
A modified burner design is shown in Figure 3. This uses a jet size of 4 mm, giving a nominal power
output of 3.3 kW at a gas pressure of 137 Pa. The throat size is 20 mm, which gives an entrainment ratio
of 3.88 and a primary aeration of 70%. The mixing tube is 200 mm in length (i.e. 10 times the throat
diameter). On the donut shaped manifold, there are 90 flame ports of 3.0 mm diameter, 60 on the outside
2
of the donut and 30 on the inside. This gives a total flame port area of 679 mm , which is 2.16 times the
throat area, which is within the Priggs range.
2.4.1 Modification of high pressure stoves
LPG stoves can be modified to fit the properties of biogas. However, the efficiency is not as good as with
a stove designed specifically for biogas. Compared to other gases, biogas needs less air for combustion.
Therefore, conventional gas appliances need larger gas jets when they are used for biogas combustion.
Overall efficiency of a stove depends on operating conditions, including temperature; pressure; wind
speed; specific heat capacity, bottom and overall shape, weight, and size of vessel; and amount of
specimen. Thus different tests for efficiency could yield different results for the same stove.
Biogas requires less air for complete combustion than LPG. This means that for the same quantity of air,
more biogas is required. To achieve this in a stove, the diameter of the jet nozzle should be increased
using a drill from 1.2 mm to 1.6 mm, so reducing the output speed of the gas. This will reduce the suction
2
3
of primary air, which will reduce the amount of air in the mixture. One dm of biogas requires 5.7 dm of
3
air for complete combustion, while butane and propane require 30.9 and 23.8 dm of air, respectively
(26).
Gas stoves can be manufactured by most blacksmiths or metal works. The gas burner is usually made of
high quality steel, cast iron or clay. The design of a pot-stand must be sufficiently strong to meet food
preparation methods of different communities, for example to allow stirring of thick foods such as millet
bread, rice, ugali, injera, matooke and stew.
3. Biogas lamps
In villages without electricity, lighting is not only a basic need, but also a status symbol. However, biogas
lamps currently provide little relief as they are not very energy-efficient and tend to get very hot, this
excess heat is a by-product.
Biogas lamps can be used to generate light by combustion of the gas (39). The gas lamp consists of gas
inlet hole, an air inlet hole, an air inlet adjustment valve, a mixing tube, a fire resistant clay head and
gauze mantle. The mantle holder consists of a gas nozzle for the flow of combustible gas and air holes for
proper mixing of gas and air. The burning gas heats a mantle until it glows brightly. Reflectors are fitted
on top of the lamp, heat and light produced at the mantle is reflected below and the flow of heat through
the lamp top is retarded.
The flame from the lamp has to be regulated in such a way that the hottest part of the flame matches the
form of the mantle. Proper air mixture and appropriate size of the mantle play the biggest roles. The
methane content of biogas sometimes changes. Therefore, brightness of the light will also change.
The performance of a biogas lamp is dependent on optimal tuning of the gas mantle and the shape of the
flame at the nozzle. The mantle should be surrounded by the hottest core of the flame at the minimum
gas consumption rate. If the mantle is too large, it will show dark spots; if the flame is too large, gas
consumption will be too high for the light-flux yield. The lampshade reflects the light downward, and the
glass prevents the loss of heat (40).
8
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
3
-1
Biogas lamps have a consumption rate of 120 - 150 dm h , with an average light output of 600 lumens
-1
and with an efficacy that varies from 0.48 - 0.94 lm watt (4,41) (see Table 6). A biogas lamp is only 3
percent efficient; most of the energy is lost in form of waste heat (42).
Table 6 – Ranges of luminous efficacy, flux and fuel use that can be expected from different flamebased and electric lamps
3.1. Modification of a biogas lamp
Biogas lamps are controlled by adjusting the supply of gas and primary air. The aim is to make the gas
mantle burn with uniform brightness and a steady, popping low sound. This can be checked by placing
the glass on the lamp and waiting 2-5 minutes, until the lamp has reached 1,000-1,500 C; the operating
temperature (43). Most lamps operate at a gas pressure of 0.49 - 1.47 kPa. If the pressure is any lower,
the mantle will not glow; if the pressure is too high (fixed-dome biogas plants) the mantle may tear.
Steps used to adjust a biogas lamp are as follows:
1. Pre-control of the supply of biogas and primary air without the mantle, to produce, at the outset,
an elongated flame with an extended inner core.
2. Fine tuning the flame with the incandescent body in place, to produce an intensely glowing
incandescent body, coupled with slight further fine-tuning of the air supply.
Kerosene pressure lamps can be modified to use biogas. The jet in the kerosene pressure lamps is
enlarged and a new mixing pipe is mounted. The gas is connected via the original pump opening. Instead
3 -1
3 -1
of a consumption rate of 0.09 dm h for kerosene, 186 dm h biogas is consumed (40).
3.2. Shortcomings of commercial-type biogas lamps
Commercially available gas lamps are not optimally designed for the specific conditions of biogas
combustion i.e. fluctuating pressure and variable gas composition. The most frequently observed
shortcomings are excessively large nozzle diameters and gas mantles, no possibility of changing the
injector, and poor means of combustion-air control. Such drawbacks result in unnecessarily high gas
consumption and poor lighting.
4. Biogas-fuelled engines
Biogas is an alternative fuel for internal combustion engines and gas turbines to generate electricity.
However, it has a low enthalpy of combustion (44). Electricity generation consists of burning the gas in an
engine; exhaust heat is generated and can be recovered for powering a refrigeration process (45).
Normally, a biogas driven engine requires a considerable amount of gas. Internal combustion engines are
used if the gas at the site is capable of producing 1–3 MW of electricity; otherwise a gas turbine is chosen
(46). As a rule of thumb, a biogas plant that will be used for fuelling engines should produce at least
3
10,000 dm of gas per day (33,42). Biogas will not auto-ignite; a pilot injection of liquid fuel is required to
start ignition (47).
Diesel engines draw in air and compress it at a ratio of about 17:1 to a pressure of approximately 3 Mpa
and a temperature of 700°C. The fuel charge is injected after the air is compressed and ignites itself at
this temperature. The power output is controlled by varying the amount of fuel that is injected, while the
air intake remains constant (42).
Traditional spark ignition engines draw in a mixture of fuel and the required amount of combustion air.
The charge is ignited by a spark plug at a lower compression ratio in the range of 8:1 to 12:1. Power is
controlled by varying the intake of air via the throttle, and the fuel injection into the air stream is controlled
by the carburettor.
4.1. Modification of engines
9
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
Biogas contains CO2, water vapour and hydrogen sulphide; the composition of biogas must be
considered while designing and or modifying spark-ignition engines. Appropriate engine types for
conversion to biogas fuels are four-stroke diesel engines or spark ignition engines, requiring little
modification compared to natural gas engines. These are available in standard versions with power
ratings from 1 kW to 100 kW. Less suitable for conversion to biogas fuel are two-stroke engines in which
the lubrication is achieved by adding oil to the liquid fuel (45).
Petrol engines can use biogas with relatively simple modifications, but the compression ratio of the
engine will be too low to produce optimum operation. Diesel engines can be converted to full biogas
operation by lowering the compression ratio and installation of the spark ignition system (42,45).
The pressure of the biogas used in an engine is important. A carburettor works by inducing a pressure
that is slightly lower than atmospheric to draw the gas into the engine. If the biogas pressure is too high
(more than 2-4 Pa), excess gas can be induced, making the engine inefficient. Gas pressure control
valves may need to be used, to keep the gas pressure in the right range.
Engines can be modified in the following ways:




Diesel engines in dual-fuel mode, for use with a mixture of diesel fuel and biogas
Diesel engines converted to a spark ignition engine, for use with pure biogas
Carburetted spark ignition engines converted for use with pure biogas
Fuel Injection gasoline engines converted for use with pure biogas
Small scale generator modifications can be very difficult to achieve. Large generators are not designed to
be portable or light weight, but personal sized units are. This can make modification very difficult, since
valves and other parts may be inaccessible, or may not have room available for modifications.
5. Refrigerators
An absorption refrigerator uses a heat source, for example biogas, solar or LPG to provide the energy
needed to propel the cooling system. The absorption refrigeration process uses three components; the
evaporator, the condenser, and the expansion valve. These components function in exactly the similar
manner as the vapour compression system. However, as an alternative of the mechanical compressor, it
exploits a thermal compressor.
5.1. Modification of commercial refrigerators
The burner in an absorption refrigerator must be modified to use biogas as the energy source. Whenever
a refrigerator is converted for operating with biogas, care must be taken to ensure that all safety features
function properly. Remote ignition via a piezoelectric element substantially increases the ease of
operation (48). A design of such a burner was successfully tested at Nepalgunj, Rupandehi district in
3
-1
Nepal (49). With a gas pressure of 8 cm water (785 Pa) and gas consumption of 100 dm hr , this burner
3
has worked to run a 340 dm refrigerator. Modifications must ensure that the combustion is safe and
controlled. Inadequate modifications may cause the performance of the equipment to deteriorate or may
even lead to total failure.
5.2. Biogas requirements at household level
3
3
For 100 dm refrigeration volume, about 2,000 dm of biogas is required per day, depending on outside
3
temperatures. A larger household refrigerator consumes about 3,000 dm per day (42).
6. Radiant heaters and incubators
Infrared heaters are used in agriculture for achieving the temperatures required for raising young stock,
such as piglets and chickens in a limited amount of space. The nursery temperature for piglets begins at
10
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
30-35°C for the first week and then gradually drops off to an ambient temperature of 18-23°C in the fourth
or fifth week. As a rule, temperature control consists of raising or lowering height of the heater. Good
ventilation is important in the stable or nursery in order to avoid excessive concentrations of carbon
monoxide or CO2. As a result, the animals must be kept under regular supervision, and the temperature
must be checked at regular intervals. Heaters for pig or chicken rearing usually require approximately
3 -1
200-300 dm h .
6.1. Thermal radiation of heaters
Radiant heaters develop their infrared thermal radiation through a ceramic body that is heated to 600800°C (red-hot) by the biogas flame. The heating capacity of the radiant heater is defined by multiplying
the gas flow by its net calorific value, since 95% of the energy content of biogas is converted to heat.
Small-heater outputs range from 1.5 to 10 kW thermal power.
6.2. Gas pressure
Commercial-type heaters are designed for operating on butane, propane and natural gas at a supply
pressure of between 3 and 8 MPa. Since the primary air supply is factory-set, converting a heater for
biogas use normally consists of replacing the injector. Experience shows that biogas heaters rarely work
satisfactorily because the biogas has a low net calorific value and the gas supply pressure is below
2 MPa. The ceramic panel, therefore, is not adequately heated, i.e. the flame does not reach the entire
surface, and the heater is very susceptible to draft.
6.3. Safety pilot and air filter
Biogas-fuelled radiant heaters should always be equipped with a safety pilot, which turns off the gas
supply if the temperatures falls i.e. the biogas is not burning. An air filter is required for sustained
operation in dusty barns.
6.4. Incubators
Incubators are used to imitate and maintain optimal hatching temperatures for eggs. They are used to
increase brooding efficiency. Indirectly warm-water-heated planar-type incubators, in which a burner
heats water in a heating element for circulation through the incubating chamber, are suitable for operating
on biogas. The temperature is controlled by ether-cell-regulated vents (39,50).
7. Discussion
The emphasis of this paper has been on the technical issues related to the use of biogas appliances in
SSA. Further work is required to consider other issues, which may also impede the use of biogas most
effectively in rural areas in SSA.
Issues that must be considered further to increase accessibility of biogas technology to the rural poor are:
a) Can the poor afford the initial investment and maintenance costs of biogas appliances?
b) Do the poor have access to finance/credit?
c) Is there commitment from national governments in support local manufacture of biogas
appliances?
d) Is there a way to ensure the manufacture biogas appliances is done to a good quality, so they
meet clearly defined design standards.
e) Is there potential for reducing cost of biogas appliances by working at a larger scale? What
potential is there for improving cost-effectiveness?
f) What is the efficiency of biogas appliances under operation in different locations of SSA?
g) What is the actual calorific value of biogas?
11
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
There is a need for further research into behavioural studies (choices and preferences), including
experimental economics, quantification issues (capturing various costs and benefits of components),
socio-economic design mechanisms, barriers to uptake, knowledge transfer (awareness, training, and
participation).
604
605
606
607
2.
Davidson O, Chenene M, Kituyi E, Nkomo J, Turner C, Sebitosi B. Sustainable Energy in SubSaharan Africa. Seychelles: International Council for Science: Regional Office for Africa; 2007 Mar.
Available
from:
http://www.icsu.org/africa/publications/ICSUROASciencePlanonSustainableEnergy.pdf.
608
609
3.
Mapako M, Mbewe A. Renewables and Energy for Rural Development in Sub-Saharan Africa.
Plymouth, UK: Zed Books; 2004. 395 p. Available from: http://zedbooks.co.uk/node/13217
610
611
612
4.
Mills E. The $230-billion Global Lighting Energy Bill. Nice, France: International Association for
Energy-Efficient Lighting and Lawrence Berkeley National Laboratory; 2002. Available from:
http://evanmills.lbl.gov/pubs/pdf/global_lighting_energy.pdf.
613
614
5.
Rajvanshi AK . R&D strategy for lighting and cooking energy for rural households. Curr Sci.
2003;85(4):437–43.
615
6.
Evans M. The specter of fuel-based lighting,. Sci J. 2005;308:1263–4.
616
617
7.
Kamfor Co Ltd. Study on Kenya’s Energy Demand, Supply and Policy Strategy for Households.
Nairobi, Kenya: Small-Scale Industries and Service Establishments, Ministry of Energy; 2002.
618
619
8.
Jones R, Du J, Gentry Z, Gur I, Evans M. Alternatives to fuel-based lighting in rural China.
Shanghai; 2005. Available from: http://light.lbl.gov/pubs/rl6_id058-final.pdf
8. Conclusions
Biogas has become an important alternative fuel because it is an integrated system with multiple benefits,
including diversification of cooking fuel supply, reduction of local pollutants, improved indoor air quality,
sanitation and crop yield improvement. The challenge does not lie in the development of the small-scale
biogas digesters; the processes of digestion are already well understood and different designs for lowcost digesters are operational. What is needed is the research to improve and document performances of
different biogas appliances in SSA.
Many of the locally available gas burners that were tested were shown to be of poor quality: they had low
efficiencies and were shown to be made to designs that did not follow gas burner theory adequately. The
range of potential applications for biogas, as a rural energy source, is high. If equipment can be made
locally that is of a higher quality and able to meet well defined design standards and also be affordable,
biogas technology can have a much greater impact on rural livelihoods in rural areas of SSA.
Acknowledgement
The authors are grateful to DFID for the financial support granted through The New and Emerging
Technologies Research Competition (NET-RC). We also want to thank numerous authors, staff at
CREEC and Uganda Domestic program who work tirelessly to provide the know-how, books, articles on
biogas technology whose works were made reference to.
REFERENCES
1.
Legros G, Havet I, Bruce N, Bonjour S. The Energy Access Situation in Developing Countries: A
Review Focusing on the Least Developed Countries and Sub-Saharan Africa. New York, USA:
UNDP/WHO;
2009
Nov
p.
130.
Available
from:
http://content.undp.org/go/cmsservice/download/asset/?asset_id=2205620
12
620
621
9.
IEA (International Energy Agency). World energy outlook. 2002. Available
http://www.worldenergyoutlook.org/media/weowebsite/2008-1994/weo2002_part1.pdf
from:
622
623
624
625
10.
World Bank Group Energy Sector strategy. Background paper addressing the electricity gap. 2010.
Available
from:
http://siteresources.worldbank.org/EXTESC/Resources/Addressing_the_Electricity_Access_Gap.pd
f
626
627
11.
WHO. Working together for health. 2006. Report No.: ISBN 92 4 156317 6,. Available from:
http://www.who.int/whr/2006/en/
628
629
12.
Bruce N, Perez-Padilla R, Albalak R. Indoor air pollution in developing countries: A Major
environmental and public health challenge. Bull World Health Organ. 2000;78(9):1078–92.
630
631
13.
Fullerton DG, Semple S, Kalambo F, Suseno A, Malamba R, Henderson GD. Biomass fuel use and
indoor air pollution in homes in Malawi. Occup Env Med. 2009;66:777–83.
632
633
14.
Kurmi OP, Semple S, Steiner M, George D, Henderson AJ. Particulate matter exposure during
domestic work in Nepal. Ann Occup Hyg. 2008;52(6):509–17.
634
635
636
15.
McCracken J, Smith K, Stone P, Díaz A, Arana B, Schwartz J. Intervention to lower household
wood smoke exposure in Guatemala reduces ST-segment depression on electrocardiograms. 2011.
Available from: http://ehs.sph.berkeley.edu/krsmith/publications/2011/2011_ST_EHP.pdf
637
638
639
16.
Northcross A, Mann J, Jenny A, Chowdhury Z, Canuz E, Smith K, et al. Wood Smoke Exposure and
Respiratory Health With and Without an Improved Chimney Stove in Rural Guatemala.
Epidemiology. 2011;22(1):S202–206.
640
641
17.
Austin G, Blignaut J. South African national rural domestic biogas feasibility assessment. Ministry of
Development Co-operation, the Netherlands; 2008.
642
643
644
18.
Verstraete W, Morgan-Sagastume F, Aiyuk S, Waweru M, Rabaey K, Lissens G. Anaerobic
digestion as a core technology in sustainable management of organic matter. Water Sci Technol.
2005;52(1-2):59–66.
645
646
19.
Singh KJ, Sooch S. Comparative study of economics of different models of family size biogas plants
for state of Punjab, India. Energy Convers Manag. 2004;45:1329–41.
647
648
20.
Igoni AH, Ayotamuno MJ, Eze CL, Ogaji SOT, Probert SD. Designs of Anaerobic Digesters for
Producing Biogas from Municipal Solid-Waste. Appl Energy. 2007;85:430–8.
649
650
21.
Angelidaki I, Ellegaard L, Angelidaki I, Ellegaard L. Codigestion of manure and organic wastes in
centralized biogas plants. Status and future trends. Appl Biochem Biotechnol. 2003;109:95–105.
651
652
22.
Ni J-Q, Nyns E-J. New concept for evaluation of biogas management in developing countries.
Energy Convers Manag. 1996;37:1525–34.
653
654
23.
Han JL, Mol APJ, Yonglong L, Zhang L. Small-scale fuel wood projects in rural China lessons to be
learnt. Energy Policy. 2008;36:2154–62.
655
656
24.
Lantz M, Svensson M, Bjornsson L, Bjornsson P. The prospects for an expansion of biogas systems
in Sweden incentives, barriers and potentials. Energy Policy. 2007;35(3):1830–43.
657
658
25.
Mwakaje AG. Dairy farming and biogas use in Rungwe district, south-west Tanzania: A study of
opportunities and constraints. Renew Sustain Energy Rev. 2008;12:2240–52.
13
659
660
661
26.
Sasse L, Kellner C, Kimaro A. Improved Biogas Unit for Developing Countries. GATE, GTZ
Germany;
1991.
Available
from:
http://www.gateinternational.org/documents/publications/webdocs/pdfs/g33ime.pdf
662
663
27.
UNEP (United Nations Environment Programe). PoA manual for mini biogas plants for households.
2009. Available from: cd4cdm.org/publications/poamanualbiogashouseholds.pdf
664
665
28.
Mateescu C, Barab G, Babutanu CA. Opportunities and Barriers for development of biogas
technologies in Romania. Environ Eng Manag J. 2008;7(5):603–7.
666
667
29.
Energy4All. Brief progress and planning report the Working Group on Domestic Biogas. 2012;
Available from: http://www.energyforall.info/domestic-biogas-working-group/
668
669
670
30.
Fulford DJ. Running a Biogas Programme: A handbook. London: Practical Action Publications
(Intermediate
Technology
Publications);
1988.
Available
from:
http://developmentbookshop.com/running-a-biogas-programme-pb
671
672
31.
Fulford DJ. Biogas stove design. University
http://www.kingdombio.com/BiogasBurner1.pdf
673
674
675
32.
Khandelwal KC, Gupta VK. Biogas stove and lamp test Report. SNV Publications, Netherlands;
2009. Available from: http://m.snvworld.org/en/publications/test-reports-on-biogas-stoves-andlamps-prepared-by-testing-institutes-in-china-india
676
677
678
33.
APCAEM. Recent developments in biogas technology for poverty reduction and sustainable
development. Beijing, China: ESCAP; 2007 p. 73. Available from: http://www.uncsam.org/publication/F-Biogas.PDF
679
680
681
682
34.
George R. Commercialization of technology for domestic cooking applications. New Delhi: TERI
(The
Energy
and
Resources
Institute);
1997.
p.
299–230.
Available
from:
http://www.worldcat.org/title/biomass-energy-systems-proceedings-of-the-international-conference26-27-february-1996-new-delhi/oclc/37443254
683
684
685
35.
Smith KR, Klialil MAK, Rasmussen RA, Apte M, Manegdeg F. Greenhouse gases from biomass and
fossil fuel stoves in developing countries: A Manila pilot study. Chemosphere. 1993;16(1-4):479–
505.
686
687
688
689
690
36.
Shrestha JN, Sing RB, Pradhan S, Paudyal BB. Final report on efficiency measurement of biogas
stoves. Center for Energy Studies Institute of Engineering, Tribhuvan University; 2004. Available
from:
http://www.snvworld.org/sites/www.snvworld.org/files/publications/efficiency_measurement_biogas_
stoves_nepal_2004.pdf
691
692
693
37.
Bailis R, Ogle D, MacCarty N, Still D. The Water Boiling Test Version 4.1.2 Cookstove Emissions
and Efficiency in a Controlled Laboratory Setting. Oregan: Aprovecho; 2009. Available from:
http://www.aprovecho.org/lab/index.php?option=com_rubberdoc&view=doc&id=231&format=raw
694
695
38.
Chandra A, Tiwari GN, Srivastava VK, Yadav YP, Khas H. Performance Evaluation of Biogas
Burners. Energy Convers Manag. 1991;32(4):353–8.
696
697
39.
Dishna S, Elmar D, George C B. Lighting technologies. 2005. Available from: http://www.ppre.unioldenburg.de/download/Downloads/Lighting_Technologies_1.pdf.
of
Reading,
UK;
1996.
Available
from:
14
698
699
700
40.
Kossmann W, Habermehl S, Hörz T, Krämer P, Klingler B, Kellner C, et al. Biogas Basics.
Germany:
ISAT
and
GTZ;
1999.
Available
from:
http://www.susana.org/langen/library?view=ccbktypeitem&type=2&id=526
701
702
41.
Rubab S, Kandpal TC. Financial evaluation of SPV lanterns for rural lighting in India. Sol Energy
Mater Sol Cells. 1996;44(3):261–70.
703
704
42.
Werner U, Stöhr U, Hees N. Biogas plants in animal husbandry. GTZ, GATE; 1989. Available from:
http://www.gate-international.org/documents/publications/webdocs/pdfs/g32bie.pdf
705
706
707
43.
Ter Heegde F. Appliances for domestic biogas plants. Oldenburg: SNV Netherlands Development
Organisation;
2011.
Available
from:
http://www.ppre.unioldenburg.de/download/Biogas/Biogas2011/presentations/04_20110427_Biogas_appliances.pptx
708
709
44.
Bari S. Effect of carbon dioxide on the performance of biogas/diesel dual-fuel engine. Renew
Energy. 1996 Sep;9(1-4):1007–10.
710
711
712
45.
Podorson D. Feasibility study on a biogas powered poly-generation system in rural and peri-urban
areas in Uganda. Master’s thesis: EGI-2011-024MSC EKV837. Stockholm, Sweden: Royal Institute
of Technology; 2010.
713
714
46.
Bade SSO, Guruprasath N. Landfill gas with hydrogen addition- a fuel for SI engines. Fuel.
2008;87:3616–26.
715
716
717
718
47.
Karim GA, Evans RL (Ed). The dual fuel engine. Automotive Engine alternatives. New York: Plenum
Press;
1987.
p.
83–104.
Available
from:
http://books.google.co.uk/books/about/Automotive_engine_alternatives.html?id=8KVTAAAAMAAJ&r
edir_esc=y
719
720
48.
Granryd E, Ekroth I, Lundqvist P, Melider A, Palm B, Rohlin P. Refrigeration Engineering.
Stockholm, Sweden: Department of Energy Technology, Royal Institute of Technology; 2005.
721
722
723
724
49.
Gurung B. Training report of slurry extension officers. BSP/N. SNV Nepal; 1996 p. 208. Available
from:
www.snvworld.org/files/publications/final_training_report_of_slurry_extension_officers_nepal_1996.
pdf
725
726
50.
Brew-Hammond A, Kemausuor F. Guidebook on modern bioenergy conversion technologies in
Africa. 2008. Available from: http://energycenter.knust.edu.gh/downloads/8/80old22.pdf.
727
728
15
729
730
731
732
733
734
735
736
737
738
Figures
Figure 1 – Parts of a biogas burner
Figure 2 – Parts of a gas flame
Figure 3 – Photograph of a typical stove
Figure 4 – Improved biogas burner
16
739
Jet
Throat
Air holes
Mixing tube
740
741
Burner
ports
Burner
manifold
Figure 1 – Parts of a biogas burner
17
Outer mantle
Combustion zone
Flame front
Secondary air
Inner cone
Burner port
Gas/air mixture
Mixing tube
Throat
Primary air
Air inlet ports
Gas
742
743
Jet
Figure 2: Parts of a gas flame
744
745
18
746
747
748
Figure 3: Photograph of the type of stove tested
19
More flame ports
dt
Longer mixing tube
Donut shaped manifold
749
750
751
Figure 4: Modified gas burner design
20
752
753
754
755
756
757
758
759
760
761
Tables
Table 1 – Comparison of efficiency of different types of stoves
Table 2 – Key dimensions of 8 stoves
Table 3 – Design checks on 8 stoves
Table 4 – Results of boiling tests on 8 stoves
Table 5 – Results of combustion tests on 8 stoves
Table 6 – Ranges of luminous efficacy, flux and fuel use that can be expected from different flame-based
and electric lamps
21
762
763
764
Table 1. – Comparison of efficiency of different types of stoves
Fuel/Stove
765
766
Combustion efficiency %
Overall efficiency %
Biogas
99.4
57.4
LPG
97.7
53.6
Kerosene
96.5
49.5
Wood
90.1
22.8
(Source [35])
22
767
768
Table 2 - Key dimensions of 8 stoves
Parameter
KEJS
Diameter jet d0 (mm)
Diameter throat dt (mm)
2
Area jet A0 (mm )
2
Diameter jet At (mm )
Diameter ports dp (mm)
Number ports N
2
Area ports Ap (mm )
Mix pipe length (mm)
769
770
771
Reo
Tusk
Bremen
Ideal
Psem
Double Psem L
5
6
6
5
6
8
6
5
24
27.5
28.2
28
26.8
26.5
24
38
19.6
28.3
28.3
19.6
28.3
50.3
28.3
19.6
452.4
594.0
624.6
615.8
564.1
551.5
452.4
1134.1
5
6
6
6
6
6
2
5
20
20
20
20
20
21
28
40
392.7
565.5
565.5
565.5
565.5
593.8
88.0
785.4
145
160
158
159
162
149
130
192
Note: Bremen is short for the Bremmen stove. Psem L is short for the Large version of the Psem stove.
23
772
773
Table 3 - Design checks on 8 stoves
Parameter
KEJS
-1
Gas Flow Q (litre min )
Reo
Tusk
Bremen
Ideal
Psem
Double Psem L
14.2
20.4
20.4
14.2
20.4
36.3
20.4
14.2
Power P (kW)
5.1
7.3
7.3
5.1
7.3
13.0
7.3
5.1
Entrainment r
3.68
3.47
3.59
4.46
3.36
2.24
2.91
6.40
Primary aeration %
67.0
63.2
65.2
81.1
61.1
40.8
52.9
116.3
0.868
0.952
0.905
0.918
1.002
1.077
0.194
0.693
6.042
5.818
5.603
5.679
6.045
5.623
5.417
5.053
2.82
2.69
2.76
2.28
2.63
3.30
15.13
2.23
Priggs test
Length/Diam mix tube
-1
Gas velocity (m s )
774
775
24
776
777
Table 4 - Results of boiling tests on 8 stoves
Parameter
KEJS
Reo
Power P (kW) theory
Tusk
Bremen
Ideal
Psem
Double
Psem L
5.1
7.3
7.3
5.1
7.3
13.0
7.3
5.1
Gas flow (litre/min) Cold
13.64
18.19
19.10
11.82
14.55
17.28
10.92
16.37
Gas flow (litre/min) Hot
15.46
13.64
20.01
11.82
12.73
10.92
10.01
14.55
Power P (kW) Cold data
4.89
6.52
6.84
4.24
5.21
6.19
3.91
5.87
Power P (kW) Hot data
5.54
4.89
7.17
4.24
4.56
3.91
3.59
5.21
Heat water (min) Cold
24.0
17.0
14.0
20.0
21.0
19.0
29.0
20.0
Heat water (min) Hot
21.0
19.0
15.0
24.0
29.0
27.0
30.0
22.0
Net Power (kW) Cold
0.99
1.40
1.70
1.19
1.13
1.25
0.82
1.19
Net Power (kW) Hot
1.13
1.25
1.58
0.99
0.82
0.88
0.79
1.08
Efficiency % Cold Data
20.2%
21.4%
24.8%
28.0% 21.7%
20.2%
20.9%
20.2%
Efficiency % Hot Data
20.4%
25.6%
22.1%
23.4% 18.0%
22.5%
22.1%
20.7%
778
779
25
780
781
Table 5 - Results of combustion tests on 8 stoves
Parameter
KEJS
Reo
Tusk
Efficiency % (average)
Bremen
Ideal
Psem
Double
Psem L
25.7% 19.8% 21.3%
21.5%
20.5%
20.3%
23.5%
23.4%
Carbon dioxide (g)
177
55
47
10
20
0
29
80
Carbon monoxide (g)
470
522
553
383
393
431
365
433
Ratio of CO/CO2 %
37.7
10.5
8.5
2.6
5.1
0.0
7.9
18.5
CO in smoke %
5.6
1.6
1.3
0.4
0.8
0.0
1.2
2.8
Pot above burner (mm)
35
30
30
25
35
30
26
10
Diameter of ports (mm)
65
67
70
72
70
70
130
160
Height of frame (mm)
94
120
120
120
121
118
137
160
Diameter frame (mm)
250
268
263
270
270
270
300
400
782
783
26
784
785
786
787
788
789
790
791
792
793
Table 6 Ranges of luminous efficacy, flux and fuel use that can be expected from different flame-based
and electric lamps
Luminous flux
Luminous efficacy
Light source
Fuel use
(lumen)
(lm/W)
Candle
1 to 16
0.02 - 0.22
5.5 - 7.2 x 10-3 kg h-1
Kerosene
Hurricane lamp
10 - 100
0.05 - 0.21
0.02 - 0.05 dm3 hr -1
Pressure lamp
220 - 1300
0.39 - 1.60
0.06 - 0.08 dm3 hr-1
Gas
LPG lamp
330 - 1000
0.94 - 2.35
2.8 - 3.4 x 10-3 kg hr-1
Biogas lamp
330 - 1300
0.48 - 0.94
100 - 200 dm3 hr-1
Electric
Incandescent (40W)
500
10 to 18
100 W
Halogen (12V/20W)
400
12 to 30
20 W
Fluorescent tube (13W)
600
35 to 77
13 W
(Source [53])
27
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement