DEVELOPMENT AND PERFORMANCE ANALYSIS OF POROUS RADIANT BURNERS FOR COOKING

INTRODUCTION: PROBLEM AND AIM

STATE OF THE ART

CONVENTIONAL DOMESTIC COOKING BURNER

The heated air moving away vertically draws the cooler secondary air in to the base of the flame. Combustion in the burner of a household gas stove takes place in a gaseous environment, and the flame stabilizes above the surface of the burner (Fig. 2.2).

POLLUTANT FORMATION

Carbon Monoxide
Nitrogen Oxides

At high air ratio numbers, the oxidation rate drops due to lower temperatures in the combustion zone. Any changes to the combustion process that reduce the peak temperatures in the flame can be used to reduce NOx emissions.

Table 2.1 Some important physical properties and effects of the primary pollutants Pollutant

BRIEF HISTORY OF POROUS MEDIUM COMBUSTION (PMC)

Early developments

The second was related to improving the combustion efficiency by a method where combustible mixture burned in the porous matrix. Your burner has an increase in pore size from the inlet to the outlet of the burner.

PRINCIPLE OF POROUS MEDIUM COMBUSTION

In matrix-stabilized combustion (Figure 2.6.a), the flame stabilizes near the inlet and combustion takes place entirely within the porous matrix. Unlike this, in surface-stabilized combustion (Figure 2.6.b), the flame stabilizes on the lower surface of the porous matrix and the volumetric heat release becomes the largest there.

Fig. 2.5 Heat transfer mechanism in a single layer PB

EXCESS AND SUPER EXCESS ENTHALPY COMBUSTION

The thermal efficiency and emission characteristics of the burner at different powers (Appendix-I) are also presented. Photographic view of the experimental setup (P1&P2 pressure gauges for LPG and air; R1 & R2 rotameters for LPG and air).

Fig. 2.9 Enthalpy comparison with and without heat recirculation

Properties of Porous Media

Thermal conductivity
Heat transfer coefficient
Radiative properties

COMBUSTION OF GASEOUS FUELS IN POROUS MEDIUM BURNERS

Experimental investigations
Numerical investigations

They found that the equivalence ratio determined the direction and speed of propagation of They found that flame stabilization depended on several parameters such as flow rate, heat transfer coefficient and thermal conductivity of the porous matrix. Sahraoui and Kaviany [1994] found that the flame speed was strongly influenced by the geometry of the porous medium.

With decrease in gas velocity, the time constant of the system was found to increase and it varied widely in the porous medium.

APPLICATIONS OF POROUS MEDIUM BURNER

Domestic Applications
Gas turbines and Boilers
Fuel cell and Hydrogen Production
Furnaces, process and IC engines
Combined Heat and Power Generation and Miscellaneous applications

2001] investigates the possible application of the PB in the combustor of the second stage of the chemical gas turbines developed by Arai et al. From their results, they concluded that using the porous medium in the chemical gas turbine would be a good choice. The experiments showed that the alumina beads had a long lifetime as foam and the conversion efficiency of the burner was also high.

It was found that the radiation efficiency of the burner was higher and the temperature inside the CZ was homogeneous.

Table 2.5 Light output and lighting efficiency of the light-PB [Qui and Hayden, 2006]

POROUS SURFACE COMBUSTION

1992] performed experimental investigations of surface combustion with methane and premixed air mixtures within and near a porous matrix. The highest rate of heat release was found at or above the surface of the porous matrix. Nakamura et al [1993] studied combustion with a premix of methane and air at the combustion surface and found that the maximum height at which the flame should be stabilized is 1 mm for a better balance between combustion speed and gas flow to prevent flashback.

The surface temperature was found to be proportional to the thickness of the porous matrix because the air-fuel mixture will have enough time to preheat to high temperatures.

CONCLUDING REMARKS

A good deal of work has been done to investigate the thermal performance of PBs made of different materials having different power outputs, different firing rates, equivalence ratios and optical properties. For a bilayer PB, it is found that the porosity of PZ should be lower than that of CZ. For an accurate analysis, these two aspects must be addressed in PMC radiation modeling.

Reduced temperature has been found to be one of the most effective means of controlling thermal NOx, and due to almost complete combustion, CO levels are significantly low.

OBJECTIVES OF THE PRESENT WORK

In the case of B7, the CO emissions reached below the lower limit of the household burner (shown in Figure 4.17). Temperatures were measured at the center of the burner at various heights using K-type thermocouples with a metal jacket. The thermal efficiency of the burner was evaluated by performing a water boiling test as described in Section 3.1.

The temperature profile measured on the burner surface shows a uniform combustion distribution.

FIRST PHASE DEVELOPMENTS

EXPERIMENTAL SET UP AND MATERIALS

With conventional burners, combustion takes place over the surface of the burner that is exactly above the head of the burner. The bottom and side of the mixing chamber were insulated with ceramic wool to minimize heat losses. The four combinations as shown in Table 3.1 were created based on the desired height and color of the flame.

Pebble and metal chip insulation with the reduced height between the burner surface and the bottom surface of the vessel.

Fig. 3.1 Conventional burners available in the market chosen for comparison (only burner heads are shown)

EXPERIMENTAL PROCEDURE

The diameter and height of the aluminum bucket used for the water boiling test were chosen according to the gas flow rate range used in the experiment. The thermal efficiency of the seven types of conventional burners selected from the Indian market was in the range of 60 - 65%. The detailed specifications of instrumentation/equipment used in the experiments are given in Annexure - IV.

Typical CO and NOx emission levels of a domestic cooking burner in the 0.9-1.0 equivalence ratio range are shown in the figure.

Fig. 3.3 Typical efficiency graph of the conventional domestic cooking burner

RESULTS AND DISCUSSIONS

Thermal efficiency
Exhaust gas analysis

This was mainly due to the lower air-fuel flow rates than the flame speed. The arrangement of the aluminum pan above the burner for burners B6 and B7 is shown in figure. CO emissions were still at the higher end of the lower limit of the conventional household burner.

The VMVs need to be further tested to understand the effect of burner surface temperature distribution on the thermal efficiency of the burner.

SECOND PHASE DEVELOPMENTS

EXPERIMENTAL PROCEDURE AND TEST SET-UP

Materials and specifications
Burner casing and mixing tube
Temperature measurement
Start up procedure

A schematic and a photo of the experimental setup used for testing the performance of PMBs are shown in Fig. The output of the thermocouples was obtained directly on the personal computer through a data acquisition unit (DAQ1), which converts the electrical output of the thermocouple and to the corresponding temperature. In the nomenclature, 'T' stands for temperature, the second letter stands for the position in the burner (C - combustion zone, I - interface of two zones, P - preheating zone and D - at the bottom of the wire mesh) and the numerical number stands for the serial numbers of the thermocouples.

The thermal efficiencies of LPG furnaces were determined according to Indian Standards (IS) 4246:2002.

Fig. 4.3 Photograph showing the basic materials used in the burnerSic foam

RESULTS AND DISCUSSIONS

Temperature Distribution
Emission characteristics
Thermal efficiency characteristics
Effect of air fuel distribution

The following paragraphs present the results of the effects of burner dimensions on the emissions and thermal efficiency of various PMBs at different powers and equivalence ratios. However, the CO emissions were slightly higher than the lower limit values of the CO emissions of the classic burner (90 mg/m3). For the B8 and B9, the arrangement of the aluminum pan above the burner has been changed as shown in fig.

The CO emissions were also found to decrease with the increase in the equivalence ratio, which is mainly due to the gap between the inside of the burner housing and the porous matrix.

Fig. 4.10 Temperature distribution of B6 burner showing the position of reaction zone at 1.11 kW and Φ = 0.54

THIRD PHASE DEVELOPMENTS

INTRODUCTION

There are only two main porous radiant burner technologies currently available viz; PMC and PSC. In this type, however, the flame stabilization is little difficult since the adiabatic burning velocity can increase by a factor of the order of 10 relative to the free burning velocity due to an increase in upstream energy transport [Bouma and De Goey, 1999]. The decreasing thermal efficiency due to the increasing radiation output (high radiation heat loss) is already explained in Section 4.2.3.

Therefore, an effort was made to develop a new burner with medium power output and low emissions for domestic cooking applications.

EXPERIMENTAL PROCEDURE AND SET-UP

Porous surface burner and Materials
Temperature and emissions measurement

The PM used above the surface burner was made of 10 ppi silicon carbide with a pore size of 5 mm. Temperature distributions inside the burner for the power range 2 - 3kW were presented at three different equivalence ratios and 0.71. Sampling was done at a height of 50 cm from the burner with a metal ring around the burner to prevent atmospheric air from entering.

The height of 50 cm was considered as the breathing height of a person from the household cooking burner in a crouching position [Kandpal et al., 1995].

Fig. 5.1 Schematic diagram and Photographic view of the CB experimental set up Combinational

RESULTS AND DISCUSSIONS

Emission characteristics
Temperature distribution
Thermal Efficiency characteristics

In this case, the CO emission levels just reached the lower limit of the conventional household burner. In the CB, the increased back radiation from the PM on the PSB increases the preheat temperature of the incoming air fuel mixture with the increase in watts which is different than in the PSB. Mital R, Gore JP and Viskanta R (1997), A study of the structure of submerged reaction in porous ceramic radiant burners, Combust.

Raviraj SD and Janrt LE (2006), Numerical and experimental study of the conversion of methane to hydrogen in a porous medium reactor, Combust.

Fig. 5.4 Emissions from the Combinational burner when the PM is at 4mm distance

CONCLUSIONS AND FUTURE WORK

CONCLUSIONS

During the first phase development, the modified conventional home cooking burner set a path for the application of the porous radiant burner for home cooking applications. NOX levels were drastically reduced to a maximum of 30 mg/3, which is well beyond the range of typical household combustion (mg/m3). The thermal efficiency of the combined burner reached a maximum value of about 50%, which is lower compared to the 65% of the common household burner.

It is suggested that the burner size should be reduced in the interests of end-user demand for low wattage down to 0.5 kW and also for the possible increase in thermal efficiency.

FUTURE WORK

Hoffman JG, Echigo R, Yoshida H and Tada S (1997), Experimental investigation of combustion in porous media with a reciprocating flow system, Combust. Sathe SB, Peck RE and Tong TW (1989a), A numerical analysis of combustion and heat transfer in porous jet burners, Heat Transfer Phenom. Sathe SB, Kulkarni MR, Peck RE and Tong TW (1989b), An experimental study of combustion and heat transfer in porous jet combustors, Meeting of Western Sates Section, The Combust.

Tong TW, Lin WQ and Peck RE (1987), Radiative heat transfer in porous media with spatially dependent heat generation, Int.