Evaluation of fan-assisted rice husk fuelled gasifier cookstoves for application in sub-Sahara Africa

Renewable Energy 139 (2019) 924e935

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Evaluation of fan-assisted rice husk fuelled gasifier cookstoves for application in sub-Sahara Africa Sali Atanga Ndindeng*, Marco Wopereis 1, Sidi Sanyang, Koichi Futakuchi Africa Rice Center (AfricaRice), 01 BP 2031, Cotonou, Benin

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 November 2017 Received in revised form 3 February 2019 Accepted 25 February 2019 Available online 1 March 2019

To disseminate rice husk in rice-growing environments in Africa through the production of renewable thermal energy with less smoke for cooking, a water boiling test protocol was used to compare four fanassisted rice husk gasifier cookstoves [Rua (RRHS), Viet (VRHS), Paul Olivier 150 (PO150) and Paul Olivier 250 (PO250)] in comparison with a natural draft gasifier [(Mayon (MYN)]. Workplace emissions were compared for different cookstoves and burner types (pore and vent). Furthermore, the effect of mixing different proportions of rice husk and palm kernel shell on burning time and flame temperature was investigated. The feasibility of disseminating these cookstoves was evaluated by end-users using the controlled cooking test protocol. Fan-assisted cookstoves with vent-type burners were safer to use as they produced both lower concentrations of flue gases and particulate matter. The fan-assisted cookstoves also recorded better thermal indices. Rice husk mixed with palm kernel shell at varying proportions burned for a longer time than rice husk only, but this mixing had no effect on flame temperature. End-users preferred fan-assisted (RRHS, VRHS, PO150/PO250) gasifiers to the Mayon natural draft gasifier cookstove. Gasifier cookstove acceptability was positively correlated with five stove descriptors making them important in future fine-tuning and design. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Rick husk Stoves Descriptors Thermal indices Emissions End-user perception

1. Introduction Energy for household, commercial and industrial use is an important issue and has a major impact on human health and the landscape ecology of sub-Saharan Africa (SSA). Most rural households in developing countries still rely on traditional bio-fuels for cooking, including firewood, charcoal, and small amounts of crop residues. Firewood collection places a substantial timeelabor burden on families, particularly women, and local forest resources, particularly in places where wood is scarce [1]. The use of rice husk for household cooking and the local artisanal food

Abbreviations: AQG, Air quality guidelines; CCT, Controlled cooking test; CO, Carbon monoxide; IT, Interim target; MYN, Mayon rice husk gasifier stove; NO, Nitrogen oxide; NOx, Other Nitrogen oxide species; PM2, Particles with aerodynamic diameters of 2 mm; ppm, parts per million; PO150, Paul Olivier150 rice husk gasifier stove; RRHS, Rua rice husk gasifier stove; SSA, sub-Saharan Africa; SO2, Sulphur dioxide; THO, Thoracic particles; TLUD, Top-lit updraft; VRHS, Viet rice husk gasifier stove; WBT, Water Boiling Test. * Corresponding author. Africa Rice Center, M'be Research Station, 01 BP 2551, Bouake, Cote d’Ivoire. E-mail address: [email protected] (S.A. Ndindeng). 1 World vegetable Center, P. O. Box 42, Shanhua, Tainan 74199, Taiwan. https://doi.org/10.1016/j.renene.2019.02.132 0960-1481/© 2019 Elsevier Ltd. All rights reserved.

processing industry can be a suitable alternative to wood fuel and can be an important step towards utilizing the huge quantities of this milling byeproduct (rice husk) still considered a waste in SSA [2]. Some 5 million tons of rice husk are produced annually in SSA and mostly disposed by burning in open fields or abandoned behind rice milling facilities. With appropriate technologies and knowledge, it is possible to convert rice husk into high quality energy. In doing so, it offsets the use of other unsustainable energy sources, be it wood, charcoal or petroleum products. This can lead to financial, environmental and social benefit while also building greater resilience in the rice sector, thereby increasing food security and the incomes of rice value chain actors. Lim et al. [3] provides a detailed review on the potential and pre-treatments required for using rice husk and straw as renewable energy sources and these are already being exploited especially in developed countries. Several biomass gasification unit have been established in Europe [3] and Asia by different companies but very few have been piloted in Africa [4]. For African communities to start using agricultural waste especially rice husk as a renewable energy source, more studies, technology demonstrations, and awareness campaigns are needed in these communities to create demand for rice husk as fuel for domestic use and even the cottage industry in rural areas.

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Combustion and gasification remain the most important viable options for using rice husk as a source of energy for household cooking in SSA. For rice husk to be efficiently used in combustion stoves, the husk will need to be densified by briquetting or pelleting before burning in properly designed stoves [2]. Gasification, which is a thermoechemical conversion of biomass to produce combustible gas (syngas) can be used as an intermediate step in the production of energy for cooking processes [3,5] and for internal combustion engines [6]. Pure rice husk and rice husk pellets can be gasified to produce gases that can be burned to produce heat as they escape from the gasification bed or captured and used to power engines for electricity production. Pellet production from rice husk will require additional investments in equipment, labour and energy. However, the gasification of rice husk pellets in downdraft gasifiers have been shown to record higher gas heating value and efficiency than those from pure rice husk [7]. In addition, pure rice husk may have a shorter burning time compared to densified husk. Studies have been carried out to optimize rice husk pre-treatment for energy production [8], improve fuel properties especially using high gasification temperatures [9] and gasification unit design especially for household cooking in SSA [10]. Top-lit updraft (TLUD) gasification is one of the three types of fixed bed gasifiers with high thermal efficiency and is suited for materials with high moisture and ash content such as rice husk compared to down and crossflow gasification [5]. TLUD gasifiers appear promising because they are relatively cheap (less than $100) [11], simple to construct, operate, maintain and may be used for a variety of farm residues such as rice husk, palm kernel shell, groundnut shell, cashew nut shell, shear butter shell, wood chips, cow dung cake and coal. In localities where rice husk is the target residue, TLUD gasifiers will be more useful for users near rice milling facilities since the high volatility and low density of rice husk (96e160 kg/m3) can pose challenges in handling and transportation [10]. Several types of TLUD gasifiers have been developed and currently being introduced in Africa from Southeast (SE) Asia. Comparative studies on thermal performance and end-user perception of different types of cookstoves (including TLUD) are available but there is scanty literature on studies that compare both thermal performance and end-user perception amongst different types of TLUD gasifier cookstoves especially within the context of SSA. Comparative studies between different stoves operated on different fuels showed that force draft stoves tended to record higher concentrations of emissions compared with natural draft stoves [5]. In one of these studies, a similar observation was made for a TLUD gasifier stove running on wood pellets [11]. Basically, TLUD gasifiers cookstoves can either be fan-assisted or natural draft and the burner can either be vent-or pore-like. There is a need to investigate the performance characteristics of these different types of gasifier cookstoves as this will shed light on fuel use efficiency, health risks associated with the use of these unvented biomass stoves and potential greenhouse gases that may impact global warming. The use of traditional biomass systems has been linked to high fuelwood consumption [12] and household pollution [13] in developing countries. Jetter et al. [11] and MacCarty et al. [14] have used time to boil 5 L of water, burning rate, thermal efficiency, specific fuel consumption, firepower, turndown ratio, fuel and energy use efficiency of the water boiling test protocol [15] to compare the thermal performance of different stoves. Parmigiani et al. [10] reported a single phase (cold start) rather than the normal three (cold start, hot start and simmering) phases of the water boiling test for a batch fed TLUD gasifier cookstove where the fuel could burn to completion after ignition. This modification did not allow for the estimation of combustion efficiency and to properly simulate the cooking process. Gaseous compounds [(carbon monoxide (CO), nitrogen oxide (NO), nitrogen oxide species (NOx), and sulphur dioxide (SO2))] and particulate

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matter [(inhalable (INH), thoracic (THO) and respirable (RSP) fractions)] from stoves and other sources have also been used to evaluate workplace [16,17] and environmental [18,19] pollution. The inhalable fraction is the mass fraction of total airborne particles that can penetrate the nose and mouth, the thoracic fraction is the fraction of inhalable particles that can penetrate the bronchial region and the respirable fraction is the fraction of inhalable particles that reach the alveolar region of the lung [19]. Stoves with good thermal and emission performance need to be evaluated by potential users for suitability for different cooking processes. The controlled cooking test (CCT) [20] has been used to evaluate realworld performance, suitability of a design for local use [10] and acceptability [21,22] of the stove by local users. The objective of this study was thus to evaluate five different rice husk TLUD gasifier cookstoves using different thermal and emission performance metrices and end-user's perception and contribute towards the use of rice husk as a renewable energy source. The schematic block diagram below (Fig. 1) shows the multi-step experimental process involved in this study. 2. Materials and methods 2.1. Stoves Fig. 2, Table 1a and supplementary information 1 show respectively all the gasifier cookstoves and their characteristics used in this study. These stoves were: i. Rua rice husk stove (RRHS): This is a cylindrical fan-assisted gasifier cookstove that uses a 12-V BBQ-Dragon fire supercharger fan (BBQ Dragon, China) (Fig. 2a). The gasification bed and insulation chamber are made of stainless steel with the insulating material being fiber glass. The primary air supply chamber has a height of 9 cm and is separated from the gasification bed by a 6-mm diameter pore-size perforated plate. Secondary air supply comes in through a single row of pores at the uppermost part of the gasification chamber. The burner is a vent-like plate with a 12-cm orifice through which the flame passes and contacts the pot. This plate also has supports that hold the pot in place (Supplementary information 1a). ii. Viet rice husk stove (VRHS): This is a cylindrical fan-assisted gasifier cookstove that uses a 12-V BBQ-Dragon fire supercharger fan (BBQ Dragon, China) (Fig. 2b). The stove has an insulation chamber and a stand to improve stability. The gasification bed and insulation chamber are made of cast iron with the insulating material being air. The gasification bed has a height of 9 cm with a single 10-mm hole in the middle of the bottom plate of the cylindrical bed which serves for primary air supply. Secondary air supply comes in through a single row of 6 mm diameter pores which are located 10 cm from the bottom of the cylindrical gasification bed. The gasification bed is removable and sits inside an air chamber which is between the gasification bed and the insulation chamber. The burner is a vent-like with a 19-cm orifice through which the flame passes and contacts the pot. This plate also has supports that hold the pot in place (Supplementary information 1b). iii. Paul Oliver 150 rice husk stove (PO150): This is a cylindrical fan-assisted gasifier cookstove that uses a 12-V SanAce40 fan (Sanyo Denki, Philippines) (Fig. 2c). The stove has only a gasification bed made of stainless steel with no insulation chamber. Primary air supply chamber has a height of 9 cm and is separated from the gasification bed by 7 mm diameter pore-size perforated plate. The burner is a plate with two

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Fig. 1. Schematic block diagram showing the multi-step process involved in the experimental section.

rows of 6 mm pores at the outermost part of the plate that seals the top of the gasification chamber. This plate is held by a short cylinder that fits on the gasification chamber and contains spaces that allow the passage of secondary air to meet the gases escaping through the pores on the plate. This short cylinder also has supports that hold the pot in place (Supplementary information 1c). iv. Paul Olivier 250 rice husk stove (PO250): Is the bigger model of PO150 and is twice the size of PO150. v. The Mayon rice husk gasifier stove (MYN): This is a natural draft gasifier cookstove. The stove has a cylindrical gasification bed and an inverted cone-shaped fuel holding chamber both made of cast iron (Fig. 2e and supplementary information 1d). The burner is vent-like with stands that support the pot. The five (5) types of top-lit up draft gasifiers [Rua (RRHS), Viet (VRHS), Paul Olivier 150 (PO150), Paul Olivier 250 (PO250) and Mayon (MYN)] were acquired from SE Asia. RRHS, VRHS, PO150 and PO250 were gasifier cookstoves that had not been previously introduced in Africa while MYN gasifier cookstove had been introduced in West and East Africa. The maximum amount of fuel that could be fed into each gasifier and the fuel-refilling mode (batch or continuous) are indicated (Table 1a). For batch refilling, fuel was loaded into the gasifier, ignited and allowed to burn to completion. The biochar was then discarded, and new fuel was put in the gasifier to initiate another heat generation process. A batch is defined as the maximum amount of fuel each gasifier could hold. For continuous fuel refilling, fuel was added as was used by the gasifier.

shell and their calorific values are shown in Table 1b. 2.3. Optimization of fan dial position for each type of cookstove using flue gas and particulate matter analysis Flue gases and particulate matter were analysed for all the gasifier cookstoves using five different fan dial positions with each experiment replicated twice. The air velocity and flow rate ranges for the four fan-assisted cookstoves (RRHS, VRHS, PO150 and PO250) for each fan dial position was determined using an Air Velocity Meter (AirflowTM Instruments Model TA465, TSI, Inc, MN, USA) are shown in Table 1c. It is worth noting that for a given gasifier cookstove and fan dial position, the air velocity and flow rate was constant. Flue gas (CO, NO, NOx and SO2) concentration was measured every minute during the entire burning time of the gasifier (Table 1a) using a Testo350® Portable Emission Analyzer (Testo, Inc, NJ, USA) as per manufacturer's instruction and a dilution factor of 5. Flue gas was measured from a hood as described by Ref. [14] with a pitot tube factor of 1 with slight modification being the absence of a blower before the hood exhaust. Particulate matter (inhalable (INH), thoracic (THO), respirable (RSP) and PM2) concentrations were measured every two minutes during the entire burning time using a DustMate® (Turnkey Instruments Ltd., Cheshire, UK) according to manufacturer's instructions. INH, THO, RSP and PM2 particles were considered to have a diameter corresponding to 50% sampling efficiency (D50) of 100 mM, 10 mM, 4 mM and 2 mM respectively [17,18]. The mean concentrations of gaseous and particulate matter captured per minute during the stove burning period are reported. 2.4. Water boiling test

2.2. Fuels Rice husk (not densified) was used for all performance and enduser evaluations. Rice husk and rice husk mixed with palm nut shell at 25% and 50% weight basis was used to study their effects on flame temperature and burning rate for the different types of gasifier cookstoves. The compositions of rice husk and palm kernel

The WBT protocol version 4.2.2 [15] with slight modifications was used to comparatively evaluate rice husk gasifier cookstoves (RRHS, VRHS, PO150, PO250 and MYN). Briefly, the volume of water used was 5 L and the local boiling point was 99
C. The ambient temperature and relative humidity ranged respectively from 26.5 to 33.1
C and 80e87%, with light breeze. The time to start each stove

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Fig. 2. Different types of rice husk gasifier cookstoves evaluated for cooking applications in Africa. (a) Rua rice husk stove (RRHS) with vent type burner, (b) Viet rice husk stove (VRHS) with vent type burner, (c), Paul Olivier 150 rice husk stove (PO150) with pore type burner, (d) Paul Olivier 250 rice husk stove (PO250) with pore type burner, (e) Mayon rice husk stove (MYN) with vent type burner, (f) Vent type burner, (g) Pore type burner. MYN relies on natural draft while the others are fan-assisted.

Table 1a Description of rice husk gasifier stoves evaluated for application in Africa. Stove model

Bed volume Insulator (m3) thickness (cm)

Maximum quantity of rice husk per batch (kg)

Fuel refilling

Type of burner

Type of draft

Time to use fuel if it is rice Optimum fan* dial position husk (min) for gasifier

RRHS

0.0154

3

1.500

Batch

Vent

85

4

VRHS

0.0092

3

0.976

Batch

Vent

28

4

PO150

0.0065

3

0.776

Batch

Pores

30

5

PO250

0.0211

3

2.224

Batch

Pores

25

4

MYN

0.0085

absent

4.000

Continuous Vent

Fanassisted Fanassisted Fanassisted Fanassisted Natural

30

na

RRHS: Rua rice husk stove; VRHS: Viet rice husk stove; PO150: Paul Olivier 150 rice husk stove; PO250: Paul Olivier 250 rice husk stove; MYN: Mayon rice husk stove; na: not applicable, * Fan is a SanAce40 (Sanyo Denki, Philippines) or a BBQ-Dragon fire supercharger.

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Table 1b Ultimate analysis (% dry matter) and calorific value for rice husk [2,23] and palm kernel shell [26]. Parameter

Rice husk

Palm kernel shell

Moisture content (%) Volatile matter (%) Ash (%) Fixed carbon (%) Carbon (%) Oxygen (%) Hydrogen (%) Nitrogen (%) Sulphur (%) Chlorine (%) Calorific value (MJ/kg)

12.6 63.52 20.26 16.22 38.83 35.47 4.75 0.52 <0.2 <0.2 15.7e16.3

9.4 82.5 6.7 1.4 44.56 49.77 5.22 0.4 0.05 Na 15.6

Na: not available.

Table 1c Fan dial position, air velocity and flow rate ranges for the 12-V SanAce40 fan (Sanyo Denki, Philippines) and BBQ-Dragon fire supercharger fan (BBQ Dragon, China) used for studying the performance and end-user perception of five gasifier cookstoves. Fan dial position

Air velocity (m/s)

Flow rate (m3/s)

1 2 3 4 5

2.50e3.20 3.21e5.40 5.41e6.70 6.71e7.70 7.71e8.50

32e44 41e67 65e95 96e102 103e112

was 50 s and the starting material was 30 ml of kerosene. All three phases of the WBT [(High Power (Cold Start), High Power (Hot Start) and Lower Power (Simmer))] were done. Rice husk was collected from the rice processing unit at Africa Rice Center and stored in polyethylene bags to prevent moisture re-absorption. The calorific value of rice husk and rice husk char used for the calculations of thermal metrics were 16.1 and 9.6 MJ/kg, respectively. The moisture content of the husk at the time of the test was 12.6%. A standard 5 L flat-bottom aluminium cooking pot was used for the WBT. The pot was 0.5 mm thick, weighed 1 kg and could hold a maximum of 7.5 L of water. The stoves were randomized to receive three tests which were all performed by the same trained personnel. The optimum fan dial position previously determined was used for the cold and hot start while dial position 2 was used for the simmering phase for all fan-assisted cookstoves. At the end of each phase, the content of the stove was carefully removed, and the unburnt husk was separated from the husk biochar, weighed and used in the calculation of thermal indices. The data was entered in the water boiling test data calculation sheet version 4.2.2 [24]. The temperature corrected time to boil, burning rate, thermal efficiency, temperature corrected specific fuel consumption, temperature corrected specific energy consumption and firepower for each cookstove were computed using the data calculation sheet.

2.5. Determination of flame temperature and burning time for rice husk and rice husk-palm kernel mixtures The flame temperature and maximum burning time per batch were determined for RRHS, VRHS, PO150, PO250 and MYN rice husk gasifiers using three replicates. Rice husk and rice husk-palm kernel shell mixtures were the fuels used in this experiment. The flame temperature was recorded every 5 min until it was below 100
C using a Traceable® double thermometer (VWReUSA) equipped with thermal probes that could withstand temperatures of up to 1200
C. The time it took for the flame temperature to drop below 100
C was recorded as the maximum burning time.

2.6. Indoor emission analysis Room concentration of carbon monoxide (CO) and thoracic (THO) particles (PM10) were analysed for each gasifier cookstoves during the entire burning time using Testo350® Portable Emission Analyzer (Testo, Inc, NJ, USA) and DustMate® (Turnkey Instruments Ltd., Cheshire, UK) respectively. The room was 40 m3 with two windows of 1.82 m2 and a door of 1.58 m2 which were left open during the experiment. The rice gasifier cookstoves were placed at the center of the room, the gas captor was placed 90 cm directly above the stove while the particle captor was placed 50 cm above the burner and 50 cm horizontally from the stove (Supplementary information 2). CO and THO particle concentrations were measured every 30 s and 2 min respectively with two repetitions. The mean concentrations of CO and THO particles captured per minute during the stove burning period are reported. 2.7. End-user evaluation of fan-assisted rice husk gasifier cookstoves in comparison with widely disseminated Mayon cookstove The rice husk gasifier cookstoves; RRHS, VRHS, PO150, PO250, MYN were evaluated by potential end-users in Benin using the controlled cooking test (CCT) protocol [20] with slight modifications. Five (5) groups of women (assessors) with two women per group, were randomly assigned to cookstoves and each stove was used three times in different sessions for cooking rice and tomato sauce. The descriptors for stove evaluation and scoring of descriptors are shown in Table 2. These descriptors included portability, operation, rate of cooking, smoke emission, stove height, flame intensity, stove stability, stove structure, ash removal, safety, quality of stove material, start-up time, smoke odour, fuel refilling, fuel use, energy use, total time used to cook rice, and overall stove acceptability. After each session, each group was asked to complete a questionnaire (Supplementary information 3). This was done with the aid of trained interviewers and respondents placed a mark corresponding to their response on a fix scale. The value of their mark was then determined on a 5-point scale. Fuel and energy used were computed using the quantity of fuel used and the calorific value of rice husk. 2.8. Statistical analysis Multivariance analysis was used to study the flowing effects: 1) stove, fan dial position, type of burner, type of draft and the interaction between stove and fan dial position on flue gases and workplace particle concentrations. The difference between categories of the burners, draft and the interaction of stove and fan dial positions were compared using the Fisher least significant difference (LSD) multiple comparison test. This analysis led to the selection of the optimum fan dial position for each cookstove. The summary table of the least square means for the categories of burners, drafts and interaction between stove and fan dial positions is reported. 2) Stove and husk-palm kernel mixtures on flame temperature and burning time enabled the identification of huskpalm kernel mixtures that may be rewarding to users of these cookstoves. The mean flame temperature was plotted against burning time for each gasifier and for each fuel type tested. The mean and standard deviation of WBT results for each gasifier cookstove is reported. Charts of room concentrations of CO and THO particles for the different cookstoves captured per min against burning time are reported. The mean CO and THO particles captured per minute for each gasifier is also reported. Analysis of variance (ANOVA) was used to compare the score attributed to each cookstove by end-users. All means were further compared using

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Table 2 Gasifier cookstove variables used for end-user evaluation and descriptors for the different scores. Variables

Description of scores

Portability Operation Rate of cooking Smoke emission Stove height Flame intensity Stability Structure Ash removal Safety QSM Start-up time Smoke odour Refilling Fuel use (kg) Energy use (MJ) Total time use Stove acceptability

1

2

3

4

5

Very difficult Very difficult Very slow Way too much smoke Too high Poor Very unstable Poor Very difficult Very dangerous Very poor quality Very short Extremely bad Intermittent

Difficult Difficult Slow Too much smoke High Fair Unstable Fair Difficult Dangerous Poor quality Short Very bad e

ND/NE ND/NE Ok Some smoke Perfect height Ok Ok Ok ND/NE Ok Ok Ok Bad e

Easy Easy Fast Very little smoke Low Good Stable Good Easy Safe Good Long Fairly bad e

Very easy Very easy Very fast No smoke Too low Very good Very stable Very good Very easy Very safe Very good quality Very long No Continuous

Don't like at all

Don't like

NL/DL

Like

Like very much

QSM- Quality of material used to build stove; ND/NE-neither difficult nor easy; NL/ND-neither like nor dislike.

the Fisher multiple comparison tests (LSD). The statistical program used for the analysis was XLSTAT™ software for Windows® Version 18.6 (2017) (Addinsoft SARL, Paris, France). The multivariance analysis assumed that the variances of the error terms were independent, identically and normally distributed. XLSTAT allowed for correction of heteroscedasticity and autocorrelation to fulfil these assumptions. All analysis was done at 5% significance level.

Low fan dial positions tended to record higher CO emissions and particulate matter but lower NOx and NO. The rice husk gasifier cookstove optimization based on emissions, indicated that VRHS at fan dial position 4 was the best stove followed by RRHS at fan dial position 4, PO150 at fan dial position 5, PO250 at fan dial position 4 and the worst was MYN, a natural draft gasifier cookstove (Table 3d). The optimum fan dial position for each gasifier was used for subsequent experiments reported in section 2.2, 2.3, 2.4 and 2.5.

3. Results 3.2. Thermal performance metrics from WBT 3.1. Effect of type of gasifier cookstove, draft, burner and fan dial position on emissions Type of draft influenced the concentration of particulate matter (P < 0.05) but not that of flue gases (P > 0.05). Fan-assisted in comparison with natural rice husk gasifier cookstoves recorded lower concentrations of inhalable (INH) (427.75 against 3303.36 mg/ ml/min), thoracic (THO) (359.35 against 2966.03 mg/ml/min), respirable (RSP) (276.35 against 2376.66 mg/ml/min) and PM2 (40.50 against 380.98 mg/ml/min) emissions (Table 3a). Burner type on the contrary influenced flue gas (P < 0.05) but not particulate matter concentrations (P > 0.05). Vent burners in comparison with pores recorded lower concentrations of CO (110.03 against 1236.27 ppm/min), NOx (1.00 against 11.24 ppm/min), NO (0.95 against 10.61 ppm/min) and SO2 (0.86 against 3.88 ppm/min) (Table 3b). Type of rice husk gasifier stove (RRHS, VRHS, PO150, PO250 and MYN) and fan dial position both influenced flue gas and particulate matter concentrations recorded (P < 0.05) (Table 3c). Paul Olivier gasifier cookstoves tended to record higher gaseous emission while MYN recorded higher particulate matter emissions.

Thermal performance metrics during the entire WBT revealed that averagely, PO250 gasifiers took the shortest time to boil 5 L of water (6.15 ± 0.3 min), followed by PO150, RRHS and VRHS in that order while MYN took the longest time (29.15 ± 7.3 min) (Table 4). The above results suggest that PO250 and PO150 cook faster followed by RRHS, VRHS while MYN will take a longer time to cook. RRHS recorded the lowest burning rate (25.43 ± 2.1 g/min) followed by VRHS, PO150 and MYN while PO250 recorded the highest burning rate (72.23 ± 2.7 g/min). MYN stove had the highest specific fuel consumption (331.5 ± 48 g/L) while PO150 had the least (74.8 ± 8 g/L) although this value was similar to that recorded for PO250, RRHS and VRHS. The above results suggest that the fanassisted stoves performed better compared with MYN. PO250 recorded the highest firepower (17.76 ± 0.6 kW (KW)) followed by MYN (12.96 ± 1.6 KW) and least for VRHS and RRHS stoves (7.30 ± 1.3 and 9.35 ± 0.5 KW respectively). These results confirm that PO250 and MYN are burning fuel more quickly than the other gasifiers and in addition these cookstoves recorded higher outputs that reflected their size (bigger). PO150 recorded the lowest energy

Table 3a Least square means of workplace emissions captured per min from gasifier cookstove based on type of draft. Type of draft

CO(ppm)

NOx(ppm)

NO(ppm)

SO2(ppm)

INH(mg/m3)

THO(mg/m3)

RSP(mg/m3)

PM2(mg/m3)

Fan-assisted Natural LSD value Pr > F Significant

547.304 a 322.063 a 205 0.475 No

5.026 a 0.000 a 2.50 0.191 No

4.789 a 0.000 a 2.40 0.191 No

2.066 a 0.563 a 0.79 0.219 No

427.754 b 3303.363 a 210 0.000 Yes

359.357 b 2966.038 a 211 0.000 Yes

276.355 b 2376.663 a 205 0.000 Yes

40.507 b 380.987 a 22.3 0.000 Yes

Critical value for multiple comparison test ¼ 1.96; CO: Carbon monoxide; NOx: Other Nitrogen Oxide species NO: Nitrogen Oxide; SO: Sulphur Dioxide; INH: inhalable particles (D50 ¼ 100 mM); THO: Thoracic particles (D50 ¼ 10 mM); RSP: Respirable particles (D50 ¼ 4 mM); PM2: Particulate matter (D50 ¼ 2 mM). D50: Particle diameter corresponding to 50% sampling efficiency.

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Table 3b Least square means of workplace emissions captured per min from gasifier cookstove based on type of burner. Type of burner

CO(ppm)

NOx(ppm)

NO(ppm)

SO2(ppm)

INH(mg/m3)

THO(mg/m3)

RSP(mg/m3)

PM2(mg/m3)

Vent Pores LSD value Pr > F Significant

110.032 b 1236.279 a 185 0.000 Yes

1.003 b 11.144 a 2.40 0.000 Yes

0.957 b 10.619 a 2.26 0.000 Yes

0.867 b 3.888 a 0.76 0.000 Yes

534.030 a 470.229 a 225 0.588 No

470.581 a 373.833 a 223 0.409 No

376.130 a 271.625 a 213 0.349 No

56.688 a 39.729 a 24.2 0.181 No

Critical value for multiple comparison test ¼ 1.96; CO: Carbon monoxide; NOx: Other Nitrogen Oxide species; NO: Nitrogen Oxide; SO: Sulphur Dioxide; INH: inhalable particles (D50 ¼ 100 mM); THO: Thoracic particles (D50 ¼ 10 mM); RSP: Respirable particles (D50 ¼ 4 mM); PM2: Particulate matter (D50 ¼ 2 mM). D50: Particle diameter corresponding to 50% sampling efficiency.

Table 3c Effect of gasifier coskstove type and fan dial position on gaseous and particulate matter emissions captured per min. Source

CO

SECO

NOx

SENOx

NO

SENO

SO2

SESO2

Intercept MYN PO150 PO250 RRHS VRHS Fan dial-1 Fan dial-2 Fan dial-3 Fan dial-4 Fan dial-5

149.1 471.2 1214.4*** 1039.0*** 22.6 e 542.3** 366.8* 293.7* 2.5 e

132.5 305.5 141.3 137.7 122.8 e 149.6 142.4 148.1 153.8 e

5.7** 5.7 0.2 17.3*** 2.6 e 9.3*** 5.0** 0.5 1.8 e

1.6 3.6 1.7 1.6 1.4 e 1.8 1.7 1.7 1.8 e

5.4** 5.4 0.2 16.5*** 2.5 e 8.8*** 4.8** 0.5 1.7 e

1.5 3.4 1.6 1.5 1.4 e 1.7 1.6 1.7 1.7 e

0.3 0.3 2.1** 4.1*** 0.3 e 1.1 1.2* 0 0.2 e

0.5 1.3 0.6 0.6 0.5 e 0.6 0.6 0.6 0.6 e

Source

INH

SEINH THO

SETHO RSP

SERSP PM2

SEPM2

Intercept MYN PO150 PO250 RRHS VRHS Fan dial-1 Fan dial-2 Fan dial-3 Fan dial-4 Fan dial-5

227.4 3076.0*** 40.5 246.8 121.5 e 438.5* 142.6 69.4 84 e

151.9 350.1 161.9 157.8 140.7 e 171.5 163.2 169.8 176.3 e

152.7 351.9 162.8 158.6 141.4 e 172.4 164 170.6 177.2 e

147.8 340.7 157.6 153.5 136.9 e 166.9 158.8 165.2 171.5 e

16 36.8 17 16.6 14.8 e 18 17.2 17.9 18.6 e

147.3 2818.7*** 85.7 214.8 141.6 e 483.8** 156.2 93.7 63.7 e

53.4 2323.3*** 117.2 224.9 185.2 e 486.2** 121.6 120 47.3 e

14 367.0*** 8.2 27.4 27.6 e 56.0** 17.5 4.9 15.2 e

RRHS: Rua rice husk stove; VRHS: Viet rice husk stove; PO150: Paul Olivier 150 rice husk stove; PO250: Paul Olivier 250 rice husk stove; MYN: Mayon rice husk stove. CO: Carbon monoxide (ppm/min); NOx: Other Nitrogen Oxide species (ppm/min); NO: Nitrogen Oxide (ppm/min); SO: Sulphur Dioxide (ppm/min); INH: inhalable particles (mg/m3) (D50 ¼ 100 mM); THO: thoracic particles (mg/m3) (D50 ¼ 10 mM); RSP: Respirable particles (mg/m3) (D50 ¼ 4 mM); PM2: Particulate matter (mg/m3) (D50 ¼ 2 mM). D50: Particle diameter corresponding to 50% sampling efficiency. Type of draft and burner are included in the model as dummy variables. VRHS and Fan dial position 5 are used as references in the model. SE: Standard error derived from the model constants.

use benchmark value (15.6 ± 0.4 MJ) followed by VRHS, RRHS, PO250 in that order with MYN recording the highest (69.8 ± 2.1 MJ). Although MYN gasifier was burning fuel quickly, it had the lowest thermal efficiency (10 ± 0.9%) while PO150 had the highest (28 ± 2.2%). The other stoves had thermal efficiency values around 20%. The turndown ratios of all the gasifiers were low (1e1.3) and the lowest value was recorded for MYN. 3.3. Effect of rice husk gasifier cookstove and husk-palm kernel mixture proportion on flame temperature and burning time The Paul Olivier gasifier cookstoves (PO150 and PO250) recorded the highest flame temperatures reaching 750
C followed by VRHS (640
C), RRHS (580
C) and MYN (598
C) in that order (Fig. 3). In addition, no difference was observed in flame temperature between stoves even when different husk-palm kernel shell mixtures were used (Table 5). Except for MYN that could burn continuously (no restriction on burning time) due to its continuous

fuel recharging system (Fig. 3a), the other gasifiers had a restricted burning time due to batch filling. Amongst those gasifiers with batch filling, RRHS had the longest burning time (Fig. 3c) followed by VRHS (Fig. 3d) while Paul Olivier gasifiers (PO150 and PO250) (Fig. 3b) recorded the shortest burning time. Burning time was affected both by the gasifier cookstove used and husk-palm kernel shell mixture proportion (F ¼ 787; P < 0.0001). Rice husk only recorded the shortest burning time while husk mixed with palm kernel shell at 50% weight basis recorded the longest (Table 5). PO150 and PO250 produced a mixture of light yellow and bluish flame throughout the burning time with light soot on the pot. RRHS and VRHS produced yellow flame that caused some soot on the pot during the first 5 min of starting the fire after which the flame changes to blue during the rest of the burning time. MYN produced thick yellow flame with a lot of soot during the entire burning time. 3.4. Room concentration of carbon monoxide and inhalable particles CO concentrations fluctuated during operation for all gasifier cookstoves. PO250 recorded its peak CO concentration at the 18th min during burning (546 ppm), PO150 at the 39th min (386 ppm), MYN at the 23rd min (191 ppm), VRHS at the 13th min (102 ppm) and RRHS at the 63rd min (49 ppm) (Fig. 4a). THO particle concentration also fluctuated during operation for all gasifier cookstoves. MYN recorded its peak THO particle concentration at the 51st min during burning (6528 mg/m3), PO150 at the 45th min (509 mg/m3), RRHS at the 45th min (406 mg/m3), PO250 at the 6 min (222 mg/m3) and VRHS at the 42nd min (115 mg/m3) (Fig. 3b). PO150 and PO250 recorded the highest mean concentration of CO captured per minute during operation (96.6 ppm and 92.9 ppm) followed by MYN (68.7 ppm), VRHS (33.1 ppm) in that order while RRHS recorded the lowest (10.4 ppm) (Fig. 3c). In the same light, MYN recorded the highest mean concentration of THO particles captured per minute (1084 mg/m3) followed by PO150 (99.4 mg/m3), PO250 (96.5 mg/m3), RRHS (92.5 mg/m3) in that order while VRHS recorded the lowest (62.4 mg/m3). 3.5. End-user perception of individual gasifier cookstoves Stove operation, smoke emission, flame intensity, quality of material used to produce stove, fuel and energy use were important factors discriminating the rice husk gasifier cookstoves based on end-user evaluation (Table 6). End-users indicated that MYN was “difficult” (2.04) to operate (Tables 2 and 6) while the others were between “easy” and “very easy” (3.98e4.82) to operate. MYN also produced between “too much smoke” and “some smoke” (2.96), PO250 produced between “some smoke” and “very little smoke” (3.88), while PO150, RRHS and VRHS produced between “very little smoke” and “no smoke” (4.63e4.82). The Flame intensity of MYN was rated by end-users as “ok” (3.15) while that for the other stoves was rated as between “good” and “very good” (4.63e4.82). The

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Table 3d Interactive effect of fan dial position and rice husk gasifier cookstove on gaseous and particulate matter emissions captured per minute.

quality of material used to build MYN and PO250 was rated as between “poor” and “acceptable” (2.78 and 2.96 respectively), that for PO150 as “good” while that for RRHS and VRHS was rated as between “good” and “very good”. MYN and PO250 recorded the highest fuel and energy use followed by PO150 > VRHS > RRHS. End-users “did not like” MYN (2.22) but “liked” or “liked very much” (4.44e5.00) the other gasifier cookstoves.

4. Discussion In this study, the performance and end-user perception of different types of gasifier cook stoves were evaluated. Fan-assisted gasifier cookstoves (PO250, PO150, RRHS and VRHS) recorded higher flue gas emissions due to incomplete combustion of the fuel

and lower particulate matter (inhalable, thoracic, respirable and PM2) emissions compared with the natural draft stove (MYN). The fan-assisted stoves recorded higher thermal efficiency than the natural draft stove based on WBT (Table 4). These findings are consistent with observations by Ref. [16], who showed that increasing the overall stove efficiency tended to increase emission of products of incomplete combustion and by Ref. [5] who reported higher emissions (CO and PM2.5) for a force draft TLUD stove compared with a natural draft TLUD stove. Gasifier cookstoves with pore-type burners (PO150 and PO150) produced even higher amounts of these flue gases than the vent-type burner stoves. Ferek et al. [25] associated NOx and SO2 production to high burning temperatures during flaming combustion. In this study, PO250 and PO150 recorded the highest flame temperatures, NO, NOx and SO2

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Table 4 Thermal metrics of some rice husk gasifier cookstoves determined using the water boiling test. Rice husk gasifier cookstoves

Burning Thermal Temperature corrected Temperature corrected Temperature specific energy corrected time to rate (g/ efficiency specific fuel consumption (MJ/l) consumption (g/l) (%) min) boil (min)

High power test (Cold start) MYN 33.3 (5.0)a* PO150

11.0 (0.0)cd

PO250

7.3 (0.6)d

RRHS

18.3 (0.6)c

VRHS

20.0 (1.0)c

High power test (Hot start) MYN 25.0 (9.5)a PO150

7.3 (0.6)cd

PO250

5.0 (0.0)d

RRHS

11.7 (1.2)c

VRHS

13.3 (0.6)c

Low power (Simmer) MYN PO150 PO250 RRHS VRHS

Firepower Turn (KW) down ratio

Fuel use benchmark Energy use benchmark value to value to complete complete WBT (MJ) WBT (g)

49.7 (5.7)b 39.0 (3.6)c 76.3 (3.5)a 26.7 (0.6)d 28.7 (1.5)c

9.7 (0.6)d 358.0 (15)a

5.3 (0.2)a

12.3 (1.4)a

4727.0 (141)a

69.8 (2.1)a

28.7 (3.5)a 88.3 (8.4)b

1.3 (0.1)b

9.6 (0.9)c

1057.3 (26)e

15.6 (0.4)e

21.3 (1.2)c 118.0 (10)b

1.7 (0.1)b

18.8 (0.8)b

2911.7 (96)b

43.0 (1.4)b

26.3 99.0 (1.0)b (0.6)b 23.3 (0.6)c 119.7 (8.1)b

1.5 (0.0)b

6.5 (0.2)d

2070.3 (60)c

30.5 (0.9)c

1.8 (0.1)b

7.1 (0.3)d

1407.7 (35)d

20.8 (0.5)d

59.0 (11.8)b 41.0 (2.6)c 82.7 (1.5)a 29.3 (4.5)d 30.0 (1.0)c

10.3 305.0 (82)a (1.5)d 30.3 (1.5)a 61.3 (6.1)b

4.5 (1.2)a

14.5 (2.9)a

0.9 (0.0)b

10.2 (0.7)c

21.0 (1.0)c 83.7 (5.7)b

1.2 (0.0)b

20.4 (0.4)b

26.3 70.0 (5.3)b (2.1)b 24.0 (1.0)c 81.3 (4.0)b

1.0 (0.0)b

7.2 (1.1)d

1.2 (0.0)b

7.3 (0.2)d

49.3 (2.3)b 34.0 (13)c 57.7 (3.1)a 20.3 (1.2)d 31.3 (14)c

13.3 (0.6)d 27.7 (1.5)a

9.1 (0.6)a

21.3 (0.6)c

7.1 (0.1)b

26.7 (1.2)b 21.0 (1.0)c

4.9 (0.1)b

12.1 (0.5)a 1.0 (0.1)a 8.4 (3.1)c 1.2 (0.3)a 14.1 (0.7)b 1.3 (0.1)a 5.0 (0.2)d 1.3 (0.0)a 7.7 (3.5)d 1.0 (0.3)a

2.0 (0.0)b

2.7 (0.0)b

RRHS: Rua rice husk stove; VRHS: Viet rice husk stove; PO150: Paul Olivier 150 rice husk stove; PO250: Paul Olivier 250 rice husk stove; MYN: Mayon rice husk stove. * Values in the same column with different letters are different at the 5% level of significance. Numbers is the mean while that in bracket are standard deviation of the mean.

Fig. 3. Effect of rice husk and rice husk-palm nut shell mixture of flame temperature and burning time for (a) Mayon rice husk gasifier stove; (b) Paul Olivier 150 or 250 rice husk gasifier stove; (c) Rua rice husk gasifier stove and (d) Viet rice husk gasifier stove.

S.A. Ndindeng et al. / Renewable Energy 139 (2019) 924e935 Table 5 Effect of some rice husk gasifier cookstoves and husk-palm kernel mixtures on burning time and flame temperature. Source

Burning time (min) Value

SE

Value

SE

Intercept Gasifier cookstove MYN PO150/PO250 RRHS VRHS Husk-palm kernel mixtures Husk ¼ 50%: PKS ¼ 50% Husk ¼ 75%: PKS ¼ 25% Husk ¼ 100%: PKS ¼ 0% Model goodness of fit

43.11***

1.89

368.07***

37.70

NC 35.78*** 42.65*** e

NC 2.11 1.64 e

8.42 48.25 61.59 e

43.51 44.06 34.40 e

59.75*** 1.83 11.76*** 2.00 e e Adjusted R2 ¼ 0.95, F ¼ 547.4, P < 0.0001

Flame temperature (
C)

28.98 34.91 12.40 38.11 e e Adjusted R2 ¼ 0.02, F ¼ 1.79,P ¼ 0.117

PKS: Palm kernel shell; RRHS: Rua rice husk stove; VRHS: Viet rice husk stove; PO250: Paul Olivier 250 rice husk stove; MYN: Mayon rice husk stove, ***P < 0.0001; NC: Not considered in the model because the gasifier uses continuous fuel refilling and burning time was not restricted. VRHS and Husk ¼ 100% were used as references, SE: Standard error of the regression constants.

Fig. 4. Kitchen concentrations of (a) Carbon monoxide (CO) (b) Thoracic (THO) particles and (c) Mean CO and THO captured per minute during the entire burning time of different gasifier cookstoves.

concentrations compared with the other gasifier cookstoves (Fig. 2b & Table 3) thus confirming previous findings. Based on the above observations, it seems that fan-assisted gasifier cookstoves with vent-type burners were safer to use as they produced both

933

lower concentrations of flue gases and particulate matter (Table 3). PO250 took the shortest time (7.3 min) to boil 5 L of water while MYN took the longest (33.3 min). The time recorded with MYN is consistent with earlier finding by Ref. [11] using a similar gasifier stove. MYN recorded the highest specific fuel consumption followed by PO250 > RRHS > PO150 > VRHS. The specific fuel consumption is considered the preferred metric for comparing stoves as it considers both fuel use and amount of water left after the WBT [14]. All the gasifier cookstoves used more fuel to complete the WBT than the proposed benchmark, which is 15 MJ [14]. The four fan-assisted rice husk gasifier cookstoves also recorded higher thermal efficiencies than that of another rice husk stove tested in the region (18%) [10]. The flame temperatures and high output of PO250 provides an opportunity to exploit this stove and its variants for boiling processes where high flame temperature is required such as during rice parboiling [12]. All gasifier cookstoves also recorded lower turndown ratios compared with that recorded for an improved rocket brick stove (2.4) [12]. Low values of turndown ratio indicated a low range of power control in gasifier cookstoves and this may be inherent to the process of gasification where the gases produced needs to be burned to give heat energy as they leave the gasification bed. Although, gasifiers could be used for different cooking purposes, it was observed that all gasifiers needed some sought of fine-tuning to improve on overall efficiency. RRHS burned for 42.27 more minutes than VRHS while PO250 and PO150 burned respectively for 37.9 and 36.35 less minutes than VRHS (Table 5). A batch fuel refilling stoves with longer burning time appeared to be good although continuous fuel refilling was better since there was no restriction on burning time during cooking. Increasing the proportion of palm kernel shell in rice huskpalm kernel shell mixtures increased the burning time probably due to higher volatile matter, carbon, hydrogen content [26] and density of palm kernel shell compared with pure rice husk [27]. Therefore, users who may be concerned about the short burning time of pure rice husk operating on batch fuel refilling may overcome this huddle by mixing husk with palm kernel shell or similar agricultural waste. In addition, two stoves may be acquired and used in sequence so that when one is in use, the other is filled and ready to be ignited when the one in use quenches (stove switching). This can save cost associated with husk densification that is required to increase burning time of pure rice husk. Although stove switching was not experimented in this study, assessors indicated that this will be the best option they will adopt if they were cooking food with longer cooking time such as beans (1e3 h). Studies on TLUD stove switching has not been reported elsewhere and as such is worth investigating. PO150, PO250 and MYN produced higher concentrations of room CO compared with RRHS and VRHS. Since CO is a by-product of incomplete combustion of hydrocarbons [28], the high concentration of CO indicated that PO150, PO250 and MYN gasifier cookstoves suffered the most from incomplete combustion of hydrocarbons produced during the rice husk gasification and gas combustion process (Fig. 4c). The world health organization (WHO) air quality guideline (AQG) for CO during a 60-min period has been set at 30.56 ppm [29,30]. This AQG is based on 15 air exchanges per hour and a room size of 30 m3. The room that was used in this study is characteristic of the kitchens in SSA (a room with a door and windows with no modern air conditioning system). Except for RRHS that had CO levels below WHO benchmark (10.36 ppm) in a typical kitchen in SSA, the other gasifier cookstoves recorded values higher than the benchmark. WHO AQG for particulate matter during a 24-h period has been set at 50 mg/m3 for THO particles (PM10) [31]. WHO has also set three interim targets (IT) for THO particles, which are; IT-1 (150 mg/m3), IT-2 (100 mg/m3) and IT-1 (75 mg/m3). The average concentration of particles recorded per

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Table 6 End-user characterization of 5 rice husk gasifier cookstoves for cooking applications in Africa. Stove variables

Portability Operation Rate of cooking Smoke emission Stove height Flame intensity Stability Structure Ash removal Safety QSM Start-up time Smoke odour Refilling Fuel use (g) Energy use (MJ) Total time (min) Stove acceptability

Descriptive statistics

Stoves

Mean

SD

RRHS

PO150

VRHS

MYN

PO250

LSD value

Pr > F

4.19 3.94 4.76 4.22 2.63 4.43 4.69 4.30 3.52 4.07 3.85 1.35 4.50 2.52 1880.47 30275.62 49.00 4.15

1.03 1.14 0.38 0.89 1.06 0.75 0.46 0.68 1.12 1.37 1.04 1.13 0.76 1.76 723.45 11647.52 8.34 1.24

4.82a* 4.82a 4.82a 4.82a 3.52a 4.82a 4.91a 4.82a 3.43a 4.82a 4.44a 0.74a 4.82 ab 2.22 abc 967.3c 15568.9c 47.67a 5.00a

4.63a 4.44a 4.82a 4.63a 2.96a 4.82a 4.63a 4.44a 3.70a 4.82a 4.07 ab 1.3a 5.00a 1.3bc 1745.33b 28099.87b 51.33a 4.44a

3.89a 4.44a 5.00a 4.82a 2.78 ab 4.63a 4.82a 4.07a 2.59a 3.52a 5.00a 2.04a 5.00a 3.7 ab 1502.4bc 24188.64bc 48.00a 4.63a

3.15a 2.04b 4.44a 2.96b 2.59 ab 3.15b 4.63a 3.70a 3.70a 3.7a 2.78c 1.85a 3.8b 4.26a 2744.36a 44184.3a 57.67a 2.22b

4.44a 3.98a 4.72a 3.88 ab 1.30b 4.72a 4.44a 4.44a 4.17a 3.52a 2.96bc 0.83a 3.89b 1.11c 2442.63a 39326.4a 42.33a 4.44a

1.75 1.08 0.7 1.04 1.58 0.73 0.91 1.2 2.11 2.62 1.17 2.15 1.1 2.52 618.78 9962 15.3 1.52

0.28 0.001 0.54 0.011 0.095 0.002 0.813 0.355 0.578 0.628 0.007 0.589 0.072 0.065 0.001 0.001 0.422 0.016

LSD critical value is 2.22; QSM- Quality of material used to build gasifier cookstove; * For each gasifier cookstove variable, stoves with different letter indicate difference in the score attributed by the end-users. SD: Standard deviation of the mean; LSD: Least significant difference.

minute when MYN was in use was above WHO AQG and all interim targets (1032 mg/m3). Based on published risk coefficients from multi-center studies and meta-analyses, the use of this stove in kitchens in SSA may result in about 5% increase of short-term mortality over the AQG value [31]. RRHS, PO250 and PO150 recorded between 92 and 99 mg/m3 per minute which was within IT-2 (Fig. 4c). The use of this stove in kitchens in SSA may result in about 2.5% increase of short-term mortality over the AQG value [31]. VRHS recorded 62 mg/m3 which was within IT-1 and the use of this stove in kitchens in SSA may result in about 1.2% increase of short-term mortality over the AQG value [31]. Based on kitchen CO and THO particle concentration recorded for all gasifiers in use, it is recommended that these cookstoves be used only for out-door cooking and in areas with proper air circulation such as rice parboiling sites and open-air restaurants. In addition, program that intend to disseminate these rice husk gasifier cookstoves should not only consider stove acceptability and cost [11] as prescribed under certain scenarios [32] but also the level of pollutants since the stoves tested showed differences in pollutant levels that could have a short-term impact on health. MYN was rated as “difficult” to operate compared with the other gasifier cookstoves because it required a lot of attention during use otherwise the flame went off. The women indicated that they like stoves that allowed them to perform other task when in use. Endusers indicated that MYN produced “too much smoke” compared with PO250 (some smoke) and the other gasifier cookstoves (very little smoke). The smoke was perceived by observation, as pains in the eyes and blackening of the pot during cooking. Based on particulate matter analysis, MYN also produced high levels of particulate matter which could also explain the pains perceived in the eyes by end-users. Users considered the flame intensity of MYN to be weaker than that of the other gasifier cookstoves while its fuel consumption was comparable with that of PO250. These qualities of MYN made end-users not to like it (2.22) but to “liked” or “liked very much” (4.44e5.00) the other gasifier cookstoves (Table 6). The results of end-user evaluation are in conformity with stove performance metrics determined instrumentally and with the WBT, which consistently show MYN gasifier stove as an inferior gasifier cookstove although it had one positive attribute which was its capacity to burn continuously during use due to its continuous fuel refilling system. The following gasifier cookstove descriptors

positively correlated with stove acceptability: operation (R ¼ 0.68; P ¼ 0.005), smoke emission (R ¼ 0.58; P ¼ 0.024), flame intensity (R ¼ 0.82; P < 0.0001), stove structure (R ¼ 0.68; P ¼ 0.006), and fuel use (R ¼ 0.69; P ¼ 0.005). Other studies have indicated that women selected these descriptors as the basis of preference towards some improved cookstoves [21,22]. These descriptors should thus be given consideration during fine-tuning and future designs of gasifier cookstoves. The results of WBT, emissions and end-user evaluations obtained in this study are consistent across the different experimentations and with previous studies. Most of the variability observed was mainly due to differences in the cookstoves since the study was performed by trained technicians with two or three replications and random assignment of stoves for evaluation under similar ambient conditions (temperature, relative humidity and atmospheric pressure). The ambient temperature and relative humidity ranged respectively from 26.5 to 33.1
C and 80e87%, with light breeze. Although ambient conditions can influence test results, this was likely to have a similar effect on all the stoves. MYN tended to record higher standard errors more than the other stoves across the different experimentations probably because this stove was a natural draft and its thermal indices could be influenced by fluctuations in air circulation at the testing site.

5. Conclusions Fan-assisted gasifier cookstoves with vent-type burners were considered safer to use as they produced both lower concentrations of flue gases and particulate matter. In addition, fan-assisted gasifier cookstoves recorded better thermal indices compared with the natural draft cookstove. However, all gasifier cookstoves needed some fine-tuning of different aspects to improve the overall stove efficiency. Gasifier cookstoves with continuous fuel refilling were better because there was no restriction on burning time as was the case with batch filling systems. However, users who were concerned about the short burning time of rice husk operating on batch fuel refilling mode could overcome this huddle by mixing husk with palm kernel shell or similar agricultural waste. In addition, two stoves may be acquired and used in sequence so that when one is in use, the other is filled and ready to be ignited when the fuel in that in use is close to exhaustion (stove switching). These

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actions can save cost associated with husk densification that is required to increase burning time of pure rice husk. Based on kitchen CO and thoracic particle concentrations recorded for all gasifiers in use, it is recommended that these gasifier cookstoves be used only for out-door cooking and in areas with proper air circulation. Stove acceptability was positively correlated with ease of operation, smoke emission, flame intensity, stove structure and fuel use making these descriptors important in future fine-tuning and design of gasifier cookstoves. Since the level of pollutants released by the different rice husk stoves tested could have a short-term impact on health, stove acceptability, cost and pollutant levels should be considered as factors for their dissemination. Acknowledgements This work was supported by African Development Bank Support to Agricultural Research for Development of Strategic Crops in Africa (SARD-SC) (Grant Number 2100155022217) and by the CGIAR Research Program on Rice Agri-Food System (RICE-CRP [CRP 15]) from the CGIAR System Organization.

[13]

[14]

[15]

[16]

[17]

[18]

[19]

Appendix A. Supplementary data [20]

Supplementary data to this article can be found online at https://doi.org/10.1016/j.renene.2019.02.132.

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