7.1 Conclusions
This chapter presents the important conclusions obtained from the various stages PRB developments. Investigations have been done for different PRBs, which work on LPG, biogas and waste cooking oil (WCO).
While investigating the LPG based self-aspirated PRBs, developed by Mishra (2015) for domestic as well as medium-scale cooking application, the main objective was to demonstrate the PRB based cook-stove as a potential alternative to their conventional counterpart’s. The three main problems associated with conventional cook-stoves viz., high cooking energy requirement, the overall impact on environment and humans, and high cooking cost have been assessed by performing Control Cooking Test (CCT), Life Cycle Assessment (LCA) and Techno-economic Assessment (TEA), respectively. The cooking assessment (CCT) of self-aspirated PRB for domestic scale has clearly proved it to be a better choice than its conventional counterpart. It results in 30.23% saving of fuel and 29.98% saving in cooking time, on a daily basis. Life cycle economic assessment on domestic as well as medium scale cook-stove shows a yearly saving of
₹2204 (1-3 kW) for domestic and ₹73,219/- (10 kW) for commercial cooking applications, respectively. The calculated payback period over operating range are very less (maximum around 6 month) considering the life of the cook-stove (10 years). The PRB stove reduces the adverse impact on both human health and ecosystem significantly. This reduction is because of curtailment of both pollutant emission and fuel requirement.
In the next phase, focus has been paid on improving the existing design of self-aspirated PRB developed by Mishra (2017) for medium scale application. During testing, it was found that submerged mode of combustion was not very promising for cooking application as it was resulted in flashback due to overheating of burner casing.
Necessary design modifications have been made to work the PRB on partially submerged mode. With partially submerged mode of combustion, the operation was found stable and yield a maximum efficiency was found as 64%, against 55% reported
by Mishra’s (2017). The maximum temperature fluctuation between the center and periphery was found as 134°C in case of Mishra (2017), whereas it was only 74°C in the newly designed PRB. Compared to its conventional counterpart maximum of ~47%
thermal efficiency improvement was observed. CO and NOx reductions as compared to a CB were found to be ~90% for both. As the newly designed PRB works with unreduced pressure valve (applicable for Indian market), it can replace the existing stoves without any practical difficulties.
Third phase of work is devoted to the development for biogas cook-stove, and it has been carried out in two parts. In the first part, with external air supply, stability range of the burner was identified and then burner dimensions have been fixed for domestic power range application. With forced air supply, the range of biogas flame stability limit (equivalence ratio) in two-layer PRB was found as 0.75-0.95 for domestic power range. With selected parameters (diameter of burner, porous matrix porosity, thickness and burner casing dimensions), further experiments have been performed to convert it for naturally aspirated operation. Various combination of orifice diameters and port diameters have been tested. For stable combination, detailed performance investigations have been carried out and also compared with that of forced air supply to see its working equivalence ratio (ability of self-aspiration). Further, with self- aspirated mode, the newly designed PRB showed about 22% improvement in thermal efficiency compared to CB. CO emission is found as 36-48 ppm with self-aspirated PRB, whereas same is 235-276 ppm in case of CB. Similarly, NOx emissions are always found lower than 2 ppm in PRB. whereas the same is 15 ppm in case of CB.
In the last phase of work, investigations have been made to demonstrate WCO as an alternate option of cooking fuel. Comparative study of combustion characteristics of WCO in a pressurized kerosene stove with CB and PRB (with new vaporizer design) has been performed, with the following specific objectives. The first objective is to investigate the maximum % of the WCO, which can be blended with kerosene for domestic scale cooking application. The second objective is to measure the temperature distribution, thermal efficiency and emission with the obtained blending %. Third is to carry out CCT and TEA to get a proper understanding of cooking energy requirement and economic benefit of PRB. Maximum of 50% WCO and kerosene blend combustion can be sustained in both the test stoves. At condition of maximum blending (50%
WCO), maximum ~9% improvement of thermal efficiency has been found in case of PRB. The PRB reduces CO and NOx emission by 50-60% and 74-83%, respectively.
From CCT, it has been found that, on per day basis, PRB results in 49 minutes and
~59% saving in cooking time and fuel consumption, respectively, as compared to CB.
Techno-economic Assessment (TEA) indicates that, in a span of 10 years, PRB can offer a sum of ₹16,817 as net present worth of savings.
7.2 Scopes for future work
The following are the scope for the future research work in the development of porous burners.
Detailed life cycle analysis and kitchen performance test of newly developed PRB for medium scale cooking application.
Using the concept of partially stabilized combustion, the design of the domestic scale cook stove can be explored.
With improved LPG operated medium scale PRB for cooking stove
Detailed performance assessment for developed biogas PRB.
For further improvement in thermal efficiency of newly developed PRBs, optimization of operational and geometrical parameters (for self-aspiration) can be done with help of a numeral tool.