Sustainable Cooking Based on a 3 kW Air-Forced Multifuel Gasiﬁcation Stove Using Alternative Fuels Obtained from Agricultural Wastes

: In this research work, a 3 kW stove based on biomass gasiﬁcation, together with a fuel obtained from agriculture wastes as an alternative to the commonly used charcoal, have been developed looking for sustainable cooking in poor communities. Alternative fuel (BSW) are briquettes obtained by carbonization and densiﬁcation of agricultural solid wastes. Two laboratory methods, water boil test (WBT) and controlled kitchen test (CCT) were used to analyze the performance of this approach by comparing the proposed improved stove (ICS-G) with the traditional one (TCS), when using both types of fuels: charcoal and BSW. Results indicate that consumption of charcoal decreases by 61% using the improved ICS-G stove instead of the traditional TCS. Similar fuel savings are obtained when using BSW fuels. BSW fuel allows for a carbon monoxide (CO) emission reduction of 41% and 67%, and ﬁne particles (PM) in a 84% and 93%, during the high and low power phases of the tests, respectively. Use of BSW fuel and ICS-G stove instead of the TCS stove with charcoal, provides a cooking time reduction of 18%, savings of $353.5 per year per family in the purchase of fuel, and an emission reduction of 3.2 t CO 2 / year.family.


Introduction
In recent decades, the demand for primary energy resources has considerably increased due to the high growths, both in world population and demand, while renewable resources, such as firewood, are often poorly managed in this scenario of increasing demand. Consequently, the proper use of these resources becomes mandatory for sustainable development [1]. Currently, in the world about three billion people depend on solid fuels, such as firewood and charcoal, for cooking food and heating, without having access to clean cooking methods [2][3][4][5]. Solid fuels are the main source of energy used for heating and cooking in many urban and suburban communities in sub-Saharan African countries, representing more than 80% of the primary energy supply of these areas [6][7][8]. The traditional cooking system most used in these communities is based on burning coal or firewood on stoves with the pots placed on top, resulting in a process of very low thermal efficiency and, therefore, in an excessive fuel consumption with the consequent environmental damage [9][10][11], including excessive CO 2 emissions, thus contributing to global warming and climate change [12]. Significant efforts are in progress to improve cooking stoves and limit the above-mentioned drawbacks. It has been found that fan-assisted cookstoves produce both lower concentrations of flue gases There are many different solid biomass residues from agricultural activities in the Democratic Republic of the Congo (DRC). In this work, we have selected as sources for fuel for cooking activities those with the better thermo-physical properties. The proposed fuel (BSW) for cooking are briquettes with a cylindrical shape of 2 cm in diameter and 3.5 cm in length (Figure 1), so they are well adjusted to the typical cooking stoves. The briquettes are made from solid agricultural wastes, such as peanut shells and rice husks. Manufacturing of briquettes entails the following steps: (1st) Carbonization: carried out in a traditional furnace composed of a cylindrical metal barrel 80 cm in diameter and 120 cm high. The metal barrel has about 30 vent holes, 3 cm diameter each, at its lower base. The removable upper base has a 10 cm diameter and 100 cm high chimney. Char waste is introduced from the top with a quantity of 20 kg of solid waste (rice husks or peanuts). The fire is lit from the top of this furnace. The carbonization system is endothermic in oxygen, evolving at temperatures between 250-500 • C for 2-3 h. After this, holes in the lower base are covered and the lid is closed until cooled, which can last 3-4 h. The carbonization yield varies between 18-20%. (2nd) Grinding: the char waste is placed in a mortar with an artisanal pestle to convert the charred waste into a fine powder with a grain size of 1 mm. (3rd) Binding: the resulting powder, combined with a binder biomass (paper pulp and cassava fibers), is mixed properly to have a good homogenization. (4th) Densification: this mixture is manually densified to form the briquettes. (5th) Drying: the briquettes are dried in the sun for three days before their use.   Figure 2a illustrates the traditional stove (TCS), currently used in DRC, taken as the reference for our study. It consists of a cylindrical combustion chamber, 100 mm deep and 280 mm in diameter, with holes, 10 to 12 mm in diameter, both in the base and the lateral side [31].

Stoves
Our improved gasification stove (ICS-G), shown in Figure 2b, generates combustion through two consecutive stages with a stoichiometric proportion of 6 kg of air per 1 kg of biomass to ensure Three types of briquettes were manufactured with the dosages detailed in Table 2. Their elemental composition and the different thermo-physical properties of these briquettes, respectively, are shown  Tables 3 and 4. These values were obtained following current regulations for this type of characterization [30]. From the cooking tests carried out summarized in Table 4, it results that BSW3 structure is the most adequate to be used, given its higher calorific value.   Figure 2a illustrates the traditional stove (TCS), currently used in DRC, taken as the reference for our study. It consists of a cylindrical combustion chamber, 100 mm deep and 280 mm in diameter, with holes, 10 to 12 mm in diameter, both in the base and the lateral side [31].   Figure 2a illustrates the traditional stove (TCS), currently used in DRC, taken as the reference for our study. It consists of a cylindrical combustion chamber, 100 mm deep and 280 mm in diameter, with holes, 10 to 12 mm in diameter, both in the base and the lateral side [31].

Stoves
Our improved gasification stove (ICS-G), shown in Figure 2b, generates combustion through two consecutive stages with a stoichiometric proportion of 6 kg of air per 1 kg of biomass to ensure total biomass combustion. The fraction of air that is introduced into the lower part of the reactor (ε), in respect to the total one used by the stove, is fixed to 0.3-0.4, with the purpose to gasify solid biomass into a gaseous element (syngas). The remaining quantity of air, known as secondary air, is introduced at the top of the reactor, and has the function to ensure a complete combustion of the biomass. The number of holes and, therefore, the primary and secondary air inlet sections, are such that they ensure these air proportions. The different components of this ICS-G are shown at the diagrams in the Figure 3. Calculation of the improved ICS-G stove dimensions takes into account different aspects [32][33][34][35][36]; in particular, the amount of energy needed to cook a meal for a six-person household was estimated to be around Q = 15.8 MJ [28,32,33]. Therefore, the minimum power requirement to cook food for a meal for a family of six persons with a burning time in the range 1.0-1.5 h [34] is about 3 kW. The rest of this stove design parameters are detailed in Table 5. Our improved gasification stove (ICS-G), shown in Figure 2b, generates combustion through two consecutive stages with a stoichiometric proportion of 6 kg of air per 1 kg of biomass to ensure total biomass combustion. The fraction of air that is introduced into the lower part of the reactor (ε), in respect to the total one used by the stove, is fixed to 0.3-0.4, with the purpose to gasify solid biomass into a gaseous element (syngas). The remaining quantity of air, known as secondary air, is introduced at the top of the reactor, and has the function to ensure a complete combustion of the biomass. The number of holes and, therefore, the primary and secondary air inlet sections, are such that allows secondary air to rise, sweeping through the reactor body. This allows preheating of the secondary air. c. Thermal insulation: a 4 cm layer of rock wool d. Fan: a small 3 W-12 V DC motor provides the primary and secondary air supply. e. Power supply: a small 5 W solar panel that charges a 9 Ah-12 V lithium battery. f.
Regulation: a potentiometric circuit allows varying the supply voltage of the small motor, to control the primary and secondary airflows. g. Outer shell: it is a 24 cm cube made of 1 mm thick sheet metal. The lower base is perforated to allow the motor to inject ambient air.

Instrumentation
The equipment used to characterize the proposed fuel and stove include:  Figure 4). This system consists of a bell (a1), inside which the stove to be tested (a2) is placed; an extractor (a3) absorbs all the polluting emissions and takes a sample of the emission gases to take them to the sensor box (a4). Finally, an interface with a data acquisition system allows for the data storage in a computer (a5). Calculation of the improved ICS-G stove dimensions takes into account different aspects [32][33][34][35][36]; in particular, the amount of energy needed to cook a meal for a six-person household was estimated to be around Q = 15.8 MJ [28,32,33]. Therefore, the minimum power requirement to cook food for a meal for a family of six persons with a burning time in the range 1.0-1.5 h [34] is about 3 kW. The rest of this stove design parameters are detailed in Table 5. The following ICS-G characteristics are deduced: (a) Fuel Consumption Rate (FCR): amount of biomass fuel to be used by the stove to provide the required energy, it is deduced by using the relationship (1).
where LCV represents the fuel low specific calorific power, η th accounts for the gasifier thermal efficiency and P is the power reactor. For this gasifier, the thermal efficiency was initially assumed as 60-70% [28,33,35].
The reactor diameter is a function of the fuel consumption rate and the specific gasification rate (SGR), this one defined as the amount of fuel used per unit of time and per unit of area in the reactor. (110-210 kg·m −2 ·h −1 ) The diameter can be determined using expression (2).
(c) Reactor Height: the height of the reactor determines the operation time of the combustion chamber once the fuel is loaded. It is deduced by using Equation (3) [32,[34][35][36].
where, SGR is the specific gasification rate, ∆t is the estimated reactor operation time and ρ f is the fuel density.
(d) Amount of air needed for gasification (Q PA ): this magnitude refers to the air flow rate needed to gasify the fuel and it is given by Equation (4).
where, Q PA is the airflow rate, ε is the gasification equivalence ratio (0.3 to 0.4), FCR is the fuel consumption rate, SA is the stoichiometric amount of air required by unit of biomass (6 kg air per kg biomass) [33] and ρ a is the air density. The total amount of air needed for total combustion in the stove is deduced from the above-mentioned Q PA , by dividing it by the equivalence ratio ε.
The parameters of a 3 kW stove are deduced using the abovementioned equations and are detailed in Table 6. Air is introduced in the combustion chamber (Figure 3a) through the inputs A1 (primary air for gasification) and A2 (secondary air for total combustion). The total air flux A ST is guaranteed by a 3 W fan at 12 V DC. A small speed controller allows for the regulation of the airflow in the reactor. A lithium-ion battery (12 V; 9 Ah) and a solar panel (5 W) provide the necessary power for the system. The primary air enters through 10 small holes of 2 mm in diameter located 10 mm from the bottom of the reactor. The secondary air enters the reactor through 20 small holes 2 mm in diameter at the top of the stove (Figure 3b).
ICS-G stove includes the following components ( Figure 3c): Thermal insulation: a 4 cm layer of rock wool d.
Fan: a small 3 W-12 V DC motor provides the primary and secondary air supply. e.
Power supply: a small 5 W solar panel that charges a 9 Ah-12 V lithium battery. f.
Regulation: a potentiometric circuit allows varying the supply voltage of the small motor, to control the primary and secondary airflows. g.
Outer shell: it is a 24 cm cube made of 1 mm thick sheet metal. The lower base is perforated to allow the motor to inject ambient air.

Instrumentation
The equipment used to characterize the proposed fuel and stove include:  Figure 4). This system consists of a bell (a1), inside which the stove to be tested (a2) is placed; an extractor (a3) absorbs all the polluting emissions and takes a sample of the emission gases to take them to the sensor box (a4). Finally, an interface with a data acquisition system allows for the data storage in a computer (a5).

Instrumentation
The equipment used to characterize the proposed fuel and stove include:  (Figure 4). This system consists of a bell (a1), inside which the stove to be tested (a2) is placed; an extractor (a3) absorbs all the polluting emissions and takes a sample of the emission gases to take them to the sensor box (a4). Finally, an interface with a data acquisition system allows for the data storage in a computer (a5).

(a) Laboratory tests
Performance of the ICS-G and the TCS stoves were evaluated by using the WBT 4.2.3 [37] and CCT v.2 [38] laboratory methods. WBT 4.2.3 protocol is a laboratory simulation of the energy efficiency of the cooking process using water in three sequential phases, as detailed in Figure 5. The first phase, High Power Cold Start (HPCS), begins by heating the stove, filled with water from room temperature, until the water reaches boiling point. In the second phase, High Power High Start (HPHS), with the stove already hot from the previous phase, a new refill with fresh water is made and heating starts to reach again the water boiling temperature. In the third phase, Low Power (LP), the water is maintained for 45 min at a temperature close to the boiling point. In the three phases, the amount of fuel used for each process is carefully measured. Performance analyses were performed in order to compare the traditional stove TCS with the improved ICS-G stove using the new BSW3 fuel. The performance indicators used to compare the stoves are those officially recognized by the International Workshop Agreement (IWA), in order to ensure consistency of the selection with the ISO/IWA11:2012 guidelines [39].

(a) Laboratory tests
Performance of the ICS-G and the TCS stoves were evaluated by using the WBT 4.2.3 [37] and CCT v.2 [38] laboratory methods. WBT 4.2.3 protocol is a laboratory simulation of the energy efficiency of the cooking process using water in three sequential phases, as detailed in Figure 5. The first phase, High Power Cold Start (HPCS), begins by heating the stove, filled with water from room temperature, until the water reaches boiling point. In the second phase, High Power High Start (HPHS), with the stove already hot from the previous phase, a new refill with fresh water is made and heating starts to reach again the water boiling temperature. In the third phase, Low Power (LP), the water is maintained for 45 min at a temperature close to the boiling point. In the three phases, the amount of fuel used for each process is carefully measured. Performance analyses were performed in order to compare the traditional stove TCS with the improved ICS-G stove using the new BSW3 fuel. The performance indicators used to compare the stoves are those officially recognized by the International Workshop Agreement (IWA), in order to ensure consistency of the selection with the ISO/IWA11:2012 guidelines [39]. Many studies and researchers suggest that WBT laboratory tests do not necessarily predict the performance of stoves in real domestic kitchens [40][41][42][43][44]. For these reasons, in this investigation the WBT tests were complemented by the CCT tests, in which real meals were prepared. In these CCT tests the cooking of a real meal commonly consumed by the population of the area under study is carried out under strict controlled conditions. During the CCT process, three cooks prepared the same meal under identical conditions and with the same amounts of ingredients and water. Total needed time and the amount of fuel used for the food cooking were measured. To allow for the reproducibility of the results and to minimize the margin of error, all the pots used for these tests have the same characteristics and dimensions [45][46][47], and the cooks prepared six times the same amount of food (n = 6). The ingredients used in the tests are shown in Table 7. Each test is based on a total of 12.45 kg of raw material, including cooking water. Final weight after cooking should be 6.58 kg (Standard deviation, SD = 0.10). Many studies and researchers suggest that WBT laboratory tests do not necessarily predict the performance of stoves in real domestic kitchens [40][41][42][43][44]. For these reasons, in this investigation the WBT tests were complemented by the CCT tests, in which real meals were prepared. In these CCT tests the cooking of a real meal commonly consumed by the population of the area under study is carried out under strict controlled conditions. During the CCT process, three cooks prepared the same meal under identical conditions and with the same amounts of ingredients and water. Total needed time and the amount of fuel used for the food cooking were measured. To allow for the reproducibility of the results and to minimize the margin of error, all the pots used for these tests have the same characteristics and dimensions [45][46][47], and the cooks prepared six times the same amount of food (n = 6). The ingredients used in the tests are shown in Table 7. Each test is based on a total of 12.45 kg of raw material, including cooking water. Final weight after cooking should be 6.58 kg (Standard deviation, SD = 0.10). This test provides reliable performance indicators of the behavior of the cooking stove when used on the field. These performance indicators are the specific fuel consumption (SFC) and the total cooking time. SFC represents the amount of fuel needed to cook the quantity of food needed for usual meal and it is calculated by: where, W f uel is the mass of fuel used for cooking the meal and W f is the cooked meal mass.
(b) Estimates of CO 2 Emission Reduction (ER-CO 2 ) for the ICS-G stove.
The calculations of the ER-CO 2 resulting from the use of non-renewable wood in kitchens are carried out using the AMS-II methodology [48] according to the United Nations Convention on Climate Change (UNFCC). These emissions savings are given by: where, B Savings is the amount of woody biomass, in tons, used by the ICS-G during the year; f NBR is the fraction of non-renewable biomass (it can be obtained from some study results or government data, default value for DRC is 90%); NCV biomass is the low specific heat value of the non-renewable woody biomass that has been replaced (in the case of wood, this value is 0.015 TJ/t, using the gross weight of the air dried wood); and EF is the fossil fuel emission factor that is expected to be used for the replacement of non-renewable woody biomass with other commonly available fossil fuels, its value is 63.7 t CO 2 /TJ. When charcoal is used as fuel by the reference TCS stove or by the new ICS-G stove, the amount of woody biomass is determined using a conversion factor of 5 kg of wood (wet) per 1 kg of charcoal (dry base). All these values are obtained from [48]. B Savings can be determined from the results of the CCT tests by using the following relationship: where, SFC olds and SFC new are the specific fuel consumption for the TCS and the ICS-G stoves, respectively. Table 8 shows the results from the WBT tests for both types of stoves when using charcoal as a fuel. Table 9 details the results when the used fuel is BSW3. All these values have been obtained directly from the PEMS system. In both cases, a significant improvement in energy efficiency of 134% and 153%, respectively, is obtained by using the ICS-G stove, together with a very significant reduction of CO and PM emissions. Comparing the TCS stove using charcoal with the new ICS-G stove using BSW3 briquettes, (Table 10), there was a 150% increase in energy efficiency, savings in fuel of about 67% and CO emission reductions of 41% and 67% during the high and low power test phases, respectively, while PM particle emission reduction reached 84% and 93%, respectively. Therefore, a significant decrease of pollutants and an increase in performance due mainly to the new design of the stove and the new fuel was observed. In a recent study done in a Kenyan village on the impact of a gasifier on improving energy efficiency and reducing polluting emissions, Gitau, J.K. et al. [15] underlines a reduction in CO and PM emissions of 57% and 79%, respectively, when compared with the traditional model. Our improved performance of the ICS-G is mainly due to the improved combustion quality due to the adjustment of the stoichiometric air quantity, which leads to an almost complete combustion of the solid biomass. Forced ventilation (ICS-G) always results in better combustion than natural ventilation (TCS), all other things being equal. In addition, the ICS-G combustion chamber is thermally insulated; this prevents heat loss on the sides of the ICS-G stove and therefore concentrates all the heat produced and directs it towards the pot with the movement of forced air. Natural ventilation does not ensure perfect combustion because of its random nature and high dependence on the external atmospheric conditions, as the combustion chamber is not closed, and heat losses are uncontrolled and widespread.   Obtained improvement in emissions are in agreement with the results published in [16], where at lower air supply rates, low emissions of both PM and CO are achieved. Tables 11 and 12 show the results of the CCT tests carried out in the preparation of the typical meal consumed in the city of Bandundu. Table 11 summarizes the comparison between the ICS-G and TCS stoves using charcoal as fuel. A fuel saving of 61% is observed as well as a 20% decrease in the time used for cooking when the improved ICS-G stove is used. This is an improvement on the 40% fuel economy reported in [15] for a natural air gasifier. Table 12 shows the test results using BSW3 as fuel. In this case, ICS-G has very similar fuel savings in relation to the TCS independent of the type of fuel, charcoal or BSW3, than in the previous case, 61%. Similarly, cooking time saving is almost the same for the two kinds of fuel: 18% compared to the traditional system. However, BSW3 main advantage comes from the fact that this fuel is obtained from agricultural residues, so no cutting down of trees, as in the use of charcoal, is needed. Besides this, there is a saving in fuel consumption mainly due to the fact that in an ICS-G, the firepower can be fully controlled; i.e., during the simmering phase of the food, the power is reduced with the corresponding fuel saving. For a TCS, it is impossible to vary the fire power during the different phases of the cooking process, given that it is based on natural ventilation. Besides this, the ICS-G includes a greater thermal insulation, especially in the lateral surface.

Environmental Analysis
By using data from the results in Tables 11 and 12, we can deduce that the fuel savings by the use of the ICS-G stove instead of the TCS is 1.21 kg when using charcoal, and 2.06 kg when using BSW3 briquettes. CO 2 emission reductions have been calculated according to the AMS-II methodology. Table 13 indicates the annual reduction in wood consumption and CO 2 emissions for a household and for the entire city of Bandundu, where around 90% depend on biomass for cooking food. We are considering that 1 kg of charcoal is equivalent to 5 kg of firewood.

Socioeconomic Analysis
The use of ICS-G with BSW3 fuel will provide significant economic benefits to the households in developing countries. The price of one kilogram of charcoal is estimated at 0.6 $/kg in Bandundu and the price of BSW3 could be around 0.2 US$/kg. In accordance with the fuel consumptions deduced in the CCT tests, the daily fuel purchases under current conditions (TCS stove using charcoal) reaches 1.23 US$/family/day. This would be reduced to 0.48 US$/family/day when using the ICS-G stove with charcoal and up to 0.25 US$/family/day if the fuel for this stove would be BSW3. Therefore, monthly savings of US$ 22.6 will be obtained by the introduction of ICS-G stoves using charcoal and US$ 29.4 when the fuel used is BSW3. Taking into account that the purchase price of this ICS-G stove is in the order of US$ 50, the return periods are 2.2 and 1.7 months, respectively. Therefore, the savings for the first year are US$ 222 and US$ 303.5 for the both cases of ICS-G under consideration.

Conclusions
The causes of deforestation and greenhouse gas emissions and pollutants in developing countries, such as those in sub-Saharan Africa, are diverse, but they include in a high percentage from the use in cooking activities of traditional fuels with low energy efficiency stoves. A possible solution to reduce deforestation and the rate of polluting and greenhouse gases emissions would require the improvement of the stoves and the fuels used for those cooking activities. In this work, an improved stove based on gasification and a new fuel obtained from agricultural wastes have been designed and built to address these goals. Results using standard protocols, such as BWT and CCT, indicates fuel savings up to 61% and cooking time reduction of 18% by the introduction of these improvements in stove and fuel. Environmental impact remediation is obtained by wood savings of 2.05 Mt/year, from the substitution of this wood by agricultural wastes, and 1.9 Mt CO 2 /year emissions in the case of the Bandundu City in the DRC. Economic improvement can also be obtained with these new elements, reaching, for a standard family with six members, annual savings up to US$ 303 by the introduction of ICS-G stoves with BSW3 fuel, and a return period for the investment in the new stove of less than 2 months.

Conflicts of Interest:
The authors declare no conflict of interest.