State-of-the-Art
2.5 State of the art on plant oil cook-stove developments
Fig. 2.23: Schematic of the PRB used by Keramiotis et al. (2013, 2015).
favors the combustion. They also studied the variation in thermal efficiency with four different vessels viz., flat vessel and curved vessel with and without copper bottom, and found that flat vessel with copper bottom was resulted in maximum thermal efficiency.
Others have made attempts to use plant oils as cooking fuel by blending it with kerosene and changing the tank pressure. Maximum percentage of oil blends with kerosene was found to be 50-70% (Murthy et al., 2011; Pande et al., 2017) for cottonseed oil, 20-30%
for Jatropha (Singh, 2011; Kakati and Mahanta, 2017) and ~50% for waste cooking oil (Varun et al., 2018; Namoco Jr. et al. 2017). Similarly, different tank pressure such as 1.2 bar (Murthy et al., 2011), 1.4 bar (Singh, 2011), 1.5 bar (Jambhulkar et al., 2015) and 2 bar (Namoco Jr. et al. 2017) have been used for different oil/kerosene blends. By investigating the impact of tank pressure on thermal efficiency, Murthy et al. (2011) found optimum performance at 1.2 bar. The maximum operational blending of cottonseed oil and kerosene was found up to 40% in normal stoves, whereas, with modified stove due to improved preheating, blending moved to 70% (Fig. 2.26a). With cottonseed oil and kerosene blended fuel, the thermal efficiency reported by Murthy et al. (2011) was in a range of 45-47%, whereas the same reported by Pandey et al. (2017) was in the range of 13-15% (Fig. 2.26b). This difference was mainly due the method used for efficiency measurement and difference in tank pressure.
(a) (b)
Fig. 2.26: Modified horizontal pressurized kerosene stove used by (a) Murthy et al., 2011 and (b) Pande et al., 2017.
Using Jatropha oil and kerosene with different blend ratios viz., 10:90, 20:80, 30:70, 40:60, and 50:50, Singh (2011) found that the cooking pump stove yields a satisfactory performance only up to 30:70 blend. Kakati and Mahanta (2017) have also explored the use of Jatropha oil and reported similar observations. Jambhlkar et al. (2015) tested the performance of similar stove used by Murthy et al. (2011) but fueled with spent soya
bean cooking oil and kerosene blend. Experiments were performed with blends of various proportions viz., 25:75, 50:50, and 75:25. The maximum thermal efficiency of
~38.5% was found with 50:50 blend at 1.5 bar tank pressure. The thermal efficiency was found to increase up to 50:50 blend ratio, then started to decrease with increase in spent soya bean cooking oil. Also, at any given blending %, thermal efficiency gradually decreases with an increase in fuel consumption. Varun et al. (2018) have tested the performance of pressure stove fueled with waste cooking oil and kerosene blend and found that for waste cooking oil blending of 20-70%, thermal efficiency varied in the range of 4-20%.
Namoco Jr. et al. (2017) have also tested the performance of pressure stove (Fig. 2.27) fueled with waste cooking oil in terms of water boiling time and found that blending of 50% WCO results in a minimum boiling time of 6:74 min. Whereas, the same was 6:76 min, 7:50 min, and 8:19 min for 0 %, 20 % and 100% WCO blending with kerosene.
Fig. 2.27: The coiled copper tube with the flame holder used by Namoco Jr. et al.
(2017).
Fig. 2.28: Modified pressurized cooking stove, Suhartono et al.
(2017a, 2017b).
With Neem and Pongamia oil, Arvind and Bekal (2018) found that the decrement in calorific value and increment in viscosity (reduces fuel mobility and preheating) resulted in low thermal efficiency of the pressure stoves. Suhartono et al. (2017a, 2017b) modified pressurized cooking stove (Fig. 2.28) and experimented with vegetable cooking oils and found that higher calorific value and smaller droplet size increase the flame temperature. Efficiency measurements showed negative impact of flame temperature on thermal efficiency.
The undesirable oil physical properties viz., high auto-ignition temperature, density, flash point, and viscosity have been the main focus of the above research. All the above modified kerosene pressure stove developments have focused on increase in incoming oil temperature, thereby reducing viscosity and ignition time, which ultimately improves their combustion performance.
The second type of stove, i.e., wick stove, was also used by some researchers for demonstrating the applicability of plant oils as cooking fuel. In this type of stove, capillary action raises the fuel to the tip of the wick, which facilitates the combustion.
Experiments performed by Wagutu et al. (2010) with fatty acid methyl esters (FAMEs) were not very convincing and showed ~55% higher fuel requirement than with kerosene. Khan et al. (2010) found that in wick stove only 5% of Used Frying Oil (UFO) can blend with kerosene. Further, they extended their study for Karanja oil and found that maximum 10% Karanja oil could be blended with kerosene (Khan et al., 2011).
Such a low blending percentage could be due to the high density and gumming and deposit formation tendency of oils. With increase in percentage of oils in blend, they observed decrease in thermal efficiency. Nagaraju and Gopal (2013) reported that it was extremely difficult to ignite a commercial wick stove fueled with blends of kerosene and Pongamia oil. This is mainly due to the high distance of the burning tip from the oil reservoir and high viscosity of oil, which reduces percolation capacity of the oil. A comprehensive study on wick stove fueled with Pongamia oil was also carried out by Shetty et al. (2015). They tested blend of 10 to 90% Pongamia oil and kerosene and found a peak efficiency of only 5.6%. In view of the above, it is observed that high viscosity of plant oil possesses difficulty in proper combustion in these stoves.
However, Dinesha et al. (2019) observed stable flame for 50% blend of WCO and Sesame oil with wick stove (Fig. 2.29). With increase in percentage of oils, the transition of flame color from blue to yellow and red showed a negative impact of blending. Increase in blend percentage also reduces the flame intensity, which ultimately increases the time duration for water boiling, i.e., reduces thermal efficiency.
Such behavior could be associated with low capillary action due to the high viscosity and fire point of the oils.
Fig. 2.29: Schematic diagram of wick stove used in experimental work by Dinesha et al. (2019).
Very few works have been carried out using vegetable oils in PRBs. Lapirattanakun and Charoensuk (2017) used spherical ceramic ball inserts for waste cooking oil combustion (Fig. 2.30). Within the firing rate range of 325-548 kW/m2 with 0.16 kg/min flow rate of water, the maximum thermal and combustion efficiencies were approximately 28% and 99.5%. Similarly, CO and NOx concentrations were ~171 and
~40 ppm, respectively (at 6% O2).
Fig. 2.30: Waste vegetable oil burner with porous media, Lapirattanakun and Charoensuk (2017).
Mustafa et al. (2015, 2016) developed a PRB for kerosene-vegetable cooking oil (VCO) blend. They extensively studied the effects of equivalence ratio on the thermal performance and exhaust gas emissions of a PRB (Al2O3 porous medium) for thermoelectric power generation. Another carried out on a dual-layer microporous
burner reported that the burner performance was influenced by the size of vegetable oil droplets Janvekar et al. (2019).