1. INTRODUCTION:

The biomass gasification system can be also connected to an engine with electrical generator set for small scale power applications in villages. The electricity produced from a biomass-based gasifier system can be used for lighting houses, powering irrigation pumps, and operating machines such as chillers. Biomass Gasification and Power Generation (BGPG) plants, with capacity ranging from 1000 to 5000 kW, is recommended for rice mills and timber mills in developing countries where large amounts of biomass wastes are available¹. Circulating Fluidized Bed Combustors (CFBC) offer fuel flexibility, broad turn down ratio, relatively fast response to load changes and lower SO₂ and NO_x emissions. Thus, several hundred CFB boilers are in use in process and utility industries worldwide². Fluidized bed gasification is considered to be the most advanced method for thermo chemical conversion of various biomass fuels to energy. CFBC boilers and gasifiers can utilise all grades of coal and lignite’s in an efficient and environmentally clean way³. In the present work experiments were carried out in a CFB biomass gasifier using wood residues as a biomass fuel. The effect of ER and reaction temperature on gas composition is investigated.

2. EXPERIMENTAL SET-UP:

The Schematic view of the developed CFB Biomass gasifier with it accessories was shown in Figure 1. The CFB Gasifier consists of a riser, smooth C-Shaped exit, cyclone separators, return leg, Fuel feeder and an electrical heater. Cyclone separators, standpipe and an L-Valve are used for recirculation of the solids back into the CFB riser. Additionally, the gasifier exit, cyclone separators, recycling line and L-valve are protected with outside insulation in order to minimise heat losses. The CFB gasifier is equipped with instruments like thermocouples, flow meters, pressure transducers with a data acquisition system.

Fig 1.Schematic diagram of CFB Biomass gasifier.

3. GAS-CLEANING AND COOLING SYSTEM:

The particulates in the producer gas like ash, char, tars and alkali metals are cleaned and removed by means of a gas cleaning system which contains (i) Cyclone separators (ii) Ceramic filter and (iii) water scrubber. The hot producer gas with particulates are passed through a first cyclone ,where the coarser particles in the producer gas are captured and its recycled back to the bottom of the riser through the L-Valve. The gas leaving the first cyclone is passed through a second cyclone where ash gets separated and gets collected in an ash drum. The hot producer gas leaving the second cyclone is again cleaned using a ceramic filter unit. For internal combustion engine applications the producer gas is cooled to ambient temperature in a wet scrubber.

4. GAS SAMPLING:

The gas sampling ports were located after the scrubber. Gas samples were collected using gas sampling bags for different operating conditions and the concentrations of producer gas were analyzed using a gas chromatograph.

5. EXPERIMENTAL PROCEDURE:

The Primary air required for gasification is supplied through a multiorifice type distributor plate at the bottom of the riser from an air blower. Wood residues are feed into the CFB gasifier using a screw type feeder. An electrical heater installed at the riser wall is utilized to preheat the gasifier to 400–550ºC to maintain a desired bed temperature level before biomass fuel is fed.

The atmospheric air supplied for gasification is measured and controlled by flowmeters and regulating valves. The operating temperature was maintained in the range 700–908ºC.The temperature at various points in the gasifier were measured for each experiment in a central control system. When all data collections were completed, the electric heater is turned off and other plant equipments were stopped. In order to cool the gasifier and to stop further reactions, air blower was operated continuously to supply air until the bed temperature drops to a desired level.

6. RESULTS AND DISCUSSION:

Tests were carried out in the CFB biomass gasifier with an operating temperature of 700–908ºC. Silica sand of mean diameter 250 μm with a particle density of 2400 kg/m³ is used as an inert bed material in the. The effect of ER and bed temperature on gas composition was investigated. Equivalence ratio (ER) and bed temperature is found to strongly influence the Gas Composition of the producer gas.

6.1. Effect of ER on bed temperature

Increasing the ER from 0.2 to 0.3 resulted in an increase in bed temperature from 600ºC to 810ºC as shown in Figure 2.This is due to the rate of exothermic reactions that occurs during increase in ER and fluidization velocity . Chuang-zhi⁴ reported an increase in gasifier bed temperature from 710ºC to 790ºC for an increase in ER from 0.18 to 0.24, this is due to the more input of air which elevates the bed temperature of gasifier and leads to complete combustion.

6.2. Effect of ER on gas composition

The main compositions of the producer gas are CO, CO₂, H₂ and N₂. ER is found to affect the gas composition of the producer gas. From Figure 3, it can be observed that an increase in ER decreased the Gas Heating Value and Gas Composition. Increase in ER increased the CO₂ concentration and decreases the concentration of CO, H₂ and CH₄. It was found that increase in ER from 0.26 to 0.3 decreases the concentration of CO, H₂ and CH₄ gradually. The effect of ER on gas composition is shown in Figure 3. At an ER of 0.3 the concentrations of CO decreased to 21.81%, which agrees with the experimental investigation by⁵ ,who reported that the concentration of CO get decreased from 25.71% at an ER of 0.22 to 21.46% at an ER of 0.34 using saw dust as a biomass fuel. The concentration of CH₄ was also decreased to 1.76% and H₂ to 7.8% at an ER of 0.3. At an operating temperature of 700 to 850ºC with an increase of ER from 0.22 to 0.30, the concentration of CO decreased from 25.8 vol.% to 21.81 vol.%, CH₄ decreased from 2.91 vol.% to 1.76 vol.%, and H₂ decreased from 14.61 vol.% to 10.23 vol.%. Whereas the concentration of CO₂ increased slowly from 7.14 vol. % at an ER of 0.2 to 19.1vol% at an ER of 0.30.

Fig 2. Effect of ER on gasifier temperature. Fig 3. Effect of ER on gas composition.

6.3. Effect of gasifier bed temperature on gas composition

The effect of CFB gasifier bed temperature on gas composition is shown in Table 1. Increase in reaction temperature favors the CO₂ formation. For an increase in gasifier temperature from 700–900ºC, the CO concentration gets decreased from 21 to 17 % and the concentration of CO₂ gets increased from 8% to 13% as shown in Figure 5 and Figure 6. Figure 4 shows the concentrations of H₂ at an operating temperature of 700–900ºC. For an increase in temperature from 800 – 900ºC, the H₂content is found to be improved from 10.6 to 14.4 %. This is due to the cracking of heavier hydrocarbons at higher temperatures above 800ºC as reported by⁶ and due to the improved tar cracking, reforming and water gas shift reactions that occurs above 750ºC as reported by⁷. This agrees with the investigations by other researchers⁸, ⁶ and ⁷. Yijun Zhao⁶ observed an increase in yield of H₂from 7.62% to 11.49% for a rise in temperature above 800º. H₂ yield of 7.5% was reported by⁷ at an operating temperature of 810 °C with ER of 0.31 using coir pitch. Gasification tests by^6,7,8 which also agrees with the present work.

Table 1: Influence of the temperature on the gas composition.

T_b (°C)

H₂(%)

CO₂(%)

CO (%)

CH₄(%)

N₂(%)

700

768

815

908

6.02

8.76

10.83

12.17

10.87

12.89

13.24

14.5

22.69

23.84

22.90

22.03

2.68

2.91

3.17

3.35

56.83

50.96

49.56

47.8

Fig 4. Yield of H₂ at different temperatures. Fig 5.Effect of T_b on CO₂ composition.

Fig.6. Effect of T_b on CO composition. Fig 7. Effect of T_b on CH₄ composition.

7. CONCLUSIONS:

Gasification tests were carried out in the CFB biomass gasifier using air as a fluidization agent and wood residues as a biomass fuel. ER is found to strongly influence the gas composition of the producer gas. The optimal reaction temperature is found to be 750 to 800°C, while the optimal ER is found to be 0.2-0.22. It is suggested that the fluidizing velocity in a CFB gasifier can be limited in the range of 0.2 to 0.5 m/s with an operating temperatures of 800ºC. Fluidized Bed gasification process offers considerable energy recovery from abundantly available biomass and industrial wastes. Thus biomass gasification system can be connected to an engine with electrical generator set for small scale power applications in villages.

8. NOMENCLATURE:

CO is the concentration of Carbon Monoxide in the fuel gas in (%).

CO₂ is the concentration of Carbon dioxide in the fuel gas in (%).

CH₄ is the concentration of Methane in the fuel gas in (%).

H₂ is the concentration of hydrogen in the fuel gas in (%).

T_bis the CFB gasifier bed temperature in (°C).

9. ACKNOWLEDGEMENT:

The authors acknowledge the guidance rendered by Professor Dr I Charles of Ballsbridge University, commonwealth of Dominica and Dr. R.Sundaresan, Principal, Kingston Engineering College, Katpadi, Vellore. The authors acknowledge the financial support provided by Department of Science and Technology (DST), Government of India, New Delhi and VIT University, Vellore. Also sincere thanks to the staff members and other supporting Staffs of CO2 Research and Green Technologies Centre of VIT University.

Authors parents Mr. A.S.G. Mohan, Mrs. Valarmathi Mohan, His wife Mrs. A. Abinaya B. Tech and his beloved brother Mr. M. Rajkamal B.E, were always supportive and encouraging in all his endeavors and he will never forget their sacrifice, moral and financial support through the period of his research studies for M.S (by Research) in VIT University and Doctor of Philosophy in Engineering and Management under the faculty of Ballsbridge University, Dominica. The author acknowledges his thanks to Dr. K. Siva Kumar HOD, Mechanical Engineering and Dr. A.M. Natarajan, Chief Executive of Bannari Amman Institute of Technology, Sathyamangalam for his motivation. Sincere thanks to Dr. D. Vasudevan, Dr. R. Kannan, Professors of Mechanical Engineering, and Mr. S. Karthik Asst. Professor and CAD CAM laboratory in charge of PSNACET for providing the necessary facilities in a timely manner. The author would like to thank Thiru. R.S.K. Sukumaran, Vice-Chairman Establishment, of PSNACET for giving the wonderful environment of academics par excellence in research.

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Received on 30.04.2015 Accepted on 25.05.2015

Int. J. Tech. 5(1): Jan.-June 2015; Page 69-73