Bio-gas Operated Domestic Cook-stove with PRB
5.2 Investigation of PRB Performance with Forced Air Supply
5.2.2 Result and discussion
According to the procedure discussed in section 5.2.1, PRB stable operating range was found in the ϕ range of 0.75-0.95. The measurements of burner surface temperature, efficiency, and pollutant emissions were done within this range of ϕ for biogas flow rates of 177, 353 and 530 l/h. Obtained results are presented below.
Fig. 5.9: Experimental setup used for thermal efficiency and emission measurements of Biogas cook-stove with PRB (Forced air supply).
The burner surface temperature distributions for different ϕ and biogas flow rates are shown in Fig. 5.10a-c. Biogas flow rate and ϕ increment show a similar impact on the trend of the maximum surface temperature of the PRB. Increment on both these parameters, ultimately increases the burner surface temperature. For all the cases, the maximum temperature was always occurred at the center of the burner and gradually decreases towards the periphery. The loss of heat by conduction and radiation from the burner casing to the environment are the main reason causing such heat distribution.
Due to the burner casing shape, in the central region, the air-fuel mixture encounters less flow resistance than the region closes to the periphery of the burner. The maximum temperature difference between center to the periphery was 83°C for ϕ of 0.95 and biogas flow rate of 530 l/h. Due to the increased amount of air, lower ϕ escalates proper mixing of fuel and air and resulting in lesser temperature difference between center and periphery. Higher biogas flow rate also displays a similar trend. Lower flow rate hinders the uniform distribution of heat causing a higher temperature difference between maximum and minimum values. Previous works by Devi et al. (2019) and Gao et al.
(2011, 2013) also showed the similar trends in surface temperature.
Fig. 5.10: Radial temperature distribution on top surface of the PRB (Forced air supply) with biogas flow rate of (a) 177 l/h, (b) 353 l/h and (c) 530 l/h.
The temperature variation of PRB in axial direction was measured for the combination of biogas flow rates of 177, 353 and 530 l/h and ϕ ratio of 0.75, 0.85 and 0.95. The behavior of temperature variation for all biogas flow rates shows a similar trend (Fig.
5.11). The temperature variation at all locations is always increasing with increase in biogas flow rate and ϕ. In PRB, continuous increase in temperature is seen up to position 2 and after that it gradually decreases. This region of maximum temperature (position 2 in Fig. 5.8) is the reaction zone where the combustion stabilizes. The
recorded maximum temperature for the biogas flow rate of 530 l/h and ϕ of 0.95 varied between 1001-1123°C, while the minimum temperature in the range of 63-73°C occurred in the position 0, which gave the thermal condition of the mixing chamber (Fig. 5.8).
Fig. 5.11: Axial temperature distribution of biogas operated PRB (Forced air supply) (a) ϕ =0.75, (b) ϕ =0.85, and (c) ϕ = 0.95.
At all situations, the temperature of the mixing chamber was far less than the ignition temperature of biogas (~650°C), which eliminated the possibility of occurrence of flame flashback. A considerable preheating of air-fuel mixture was observed because of solid to solid conduction and radiation. The temperature of the preheater was found in the range of 278-395°C. Because of enhanced heat transfer in PM, the temperature measured at downstream was lesser than the reaction zone, which also contributed in
reduced production of harmful NOx emission. Other combination of biogas flow rate and ϕ also shows a similar trend but temperature values was lower with decrease in biogas flow rate, which is obvious because of reduction in heat input. Thermal efficiency variation of PRB with fuel flow rate and ϕ is shown in Fig 5.12a. For a given flow rate of biogas, thermal efficiency shows a decreasing trend with an increase in ϕ.
Such behavior is because of leaner condition, which moves the flame front downstream due to higher air flow rates, resulting in maximum volumetric heat release. Within the biogas flow rate investigated, lowest flow rate yields the maximum efficiency. The maximum efficiency of 62% was obtained for the biogas flow rate and ϕ of 177 l/h and 0.75, respectively. Similarly, the minimum value (51%) was achieved for 530 l/h and 0.95, respectively. A comparison between the thermal efficiency of conventional cook- stove and PRB is shown in Fig. 5.12b. As the conventional cook-stove is designed for fuel rich condition, PRB operating at ϕ of 0.95 has been chosen for comparison. Within the biogas flow rate of 177-530 l/h, thermal efficiency for conventional cook-stove ranged between 52 to 43% (Section 5.1), whereas the same was 58-51% in case of PRB.
From efficiency data, it is clear that the percentage improvement in thermal efficiency of PRB is in the range of 11-16%.
Fig. 5.12a: Variation of thermal efficiency with respect to equivalence ratio in biogas operated PRB (Forced air supply).
Fig. 5.12b: Thermal efficiency of biogas operated PRB (forced air supply) and CB.
With an increase in biogas flow rate, conventional cook-stove efficiency decreases and decrement is higher than that of PRB. The reason behind such behavior is associated with the fact that, with an increase in flow rate, the height of the flame increases which results in more convective heat loss. Due to the combined effects of radiative and convective heat transfer of the highly emissive porous material, for all the cases, PRB shows higher thermal efficiency than conventional cook-stove. Since PRB has been explored for domestic cooking application, the measurement of CO and NOx emissions are very important due to direct contact of the burner flue gases with the user. In the present investigation, all the emission values are taken on dry-basis, with a correction to 3% oxygen level. The effects of ϕ and biogas flow rate on emissions of CO and NOx
are given in Fig. 5.13a. Similar to thermal efficiency, for comparison of emission values, ϕ of 0.95 in case of PRB was used. Comparison of CO and NOx values between conventional cook-stove and PRB is shown in Fig. 5.13b. In PRB, measured values of CO and NOx were found in the range of 29-80 ppm and 1-4 ppm, respectively, in the whole range of fuel flow rate. Whereas, in case CB same were 211-276 ppm and 9-15 ppm, respectively (Section 5.1). In PRB, increment in both ϕ and flow rate, increase CO and NOx emissions. In the case of higher ϕ, due to lower % of air, fuel mobility decreases, which in turn reduces the mixing rate of air and fuel and causes higher emissions. Another reason for high CO is associated with reduced residence time in the porous matrix, which leads to higher unconverted CO. In PRB, thermal NOx is predominant which is increased by an increase in temperature as the ϕ increases. Figure
5.13a, shows the impact of fuel flow rate on NOx emission, since NOx concentration reaches a threshold depending only on the ϕ. Similar, NOx emission pattern was also presented in previous work by Keramiotis (2013). From Fig. 5.13b, it is observed that with increase in biogas flow rate, both the CO and NOx emissions increase for both conventional cook-stove and PRB. Measured CO emissions of PRB are lower than that of conventional cook-stove, because of better combustion and more residence time.
Similarly, the NOx emission of PRB is also found much lower than that of the CB. In the PRB, lower global temperature (surface temperature of the burner) causes lower NOx emission. Whereas, higher NOx from CB is because of the fuel-rich combustion, which in turn results in high temperature in the reaction zone. From above investigation of biogas combustion in PRB, in the flow range of 177-530 l/h, the stable operation occurs within the equivalence ratio of 0.75 to 0.95. Within the operational range, leaner biogas combustion is possible. Compared to its conventional counterpart, a maximum of ~18% improvement in thermal efficiency and ~73% and ~80% reduction in CO and NOx, can be achieved in PRB. Overall performances of the PRB suggest that it can replace the conventional cook-stove provided if it is designed for self-aspirated operation. Therefore, in order to make the PRB work on self-aspiration mode, further modifications are required. Details of modifications and its impact on burner performance are discussed in the following section.
Fig. 5.13a: Variation of CO and NOx emissions with respect to equivalence ratio in biogas operated PRB (Forced air supply).
Fig. 5.13b: Comparison of CO and NOx emissions of biogas operated PRB (Forced air supply) and conventional cook-stove (CB).