Performance Evaluation and Simulation of Pressurized Gasification

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ISSN : 2319 – 3182, Volume-1, Issue-2, 2012

26

Performance Evaluation and Simulation of Pressurized Gasification

C. Mohanraj & J. Kesavan

Dept. of Aeronautical Engineering, Periyar Maniammai University, Thanjavur, S.India E-mail : [email protected], [email protected]

Abstract - The emergence of biomass based energy warrants the evaluation of syn-gas from biomass gasification as a fuel for power systems. The earlier investigations reveal that the operating parameters strongly affect the syn gas quality. The gasifier performance was investigated with different operating pressure. The downdraft gasifier has tested with silver oak woodchips of size approximately 12mm×12mm×12mm. The total feed of 8-8.5kg of wood was fed into the system and an airflow rate of 130 lpm supplied by compressor and the gasifier was tested different pressure conditions. The main variables namely oxidation zone temperature, combustible gas contents (H₂, CO & CH₄), calorific value, gas production rate and conversion efficiency was studied. The percentage of total combustible gas is varied between 30.60% - 35.97% and the average composition is N₂ = 44.29% – 54.78%, CH₄ = 0.62% – 1.51%, H2 = 15.7% – 25.48%, CO = 7.96% – 11.4%, CO2 = 11.37% – 19.70%. The calorific value of syn gas was found to vary between 3.860 MJ/m3 – 4.374.94 MJ/m3. The conversion efficiency varied between 86.8% - 73.7%.Computational fluid dynamics (CFD) method was used to predict the performance of the down draft biomass gasifier. For simulation purpose the combustion zone of the gasifier was separately modeled and analyzed.

Keywords - Pressurized gasification; Conversion efficiency, Calorific value; CFD Simulation

I. INTRODUCTION

Most gasifier development work has been carried out with common fuels such as coal and wood. It has been recognized that bio-fuel properties such as surface area, size, and shape, moisture content, volatile matter and carbon content influence gasification. Theoretically, almost all biomass with moisture contents from 5 to 50% can be gasified. Realistically however, gasification becomes difficult when moisture contents of bio-fuels are high [1]. The moisture content of bio-fuels is important for syn-gas quality. High moisture content reduces the thermal efficiency, therefore heat is used to drive off the water from biofuels, and consequently this energy is not available for the reduction reactions and for converting the thermal energy into chemical bond

energy in the syn-gas. Thus, high- moisture content bio- fuels result in low heating values in syn-gas. In downdraft gasifiers, high moisture content gives rise to lower temperatures in the reactor, which leads to insufficient tar conversion, and thus further affects syn- gas quality. Operational problems in gasifiers occur if the fuel moisture content is too low or too high[1].

It is generally known that gasification produces three main products: syn-gas, char or ash, and oils or tars. The proportions of these products depend on gasifier type, process parameters and chemical composition of biomass [2] and the composition of these three major reaction products vary under different operational conditions. Many investigations have indicated that operating parameters of gasifiers strongly affect the quality of syn gas. Typically, an increase in temperature leads to higher syn-gas production and lower ash fraction. Increasing the residence time of the volatile phase results in increasing syn gas yield, but after a certain point this increase is reduced. It was also found that syn-gas yield is strongly influenced by the temperature in the gasifier, the mechanical or chemical pre-treatment of the bio-fuel and the geometrical configuration of the gasifier [3]. The formation of tars during gasification has been found to decrease with an increasing air ratio (the ratio of actual air supply to stoichiometric air requirement for complete combustion). Most important parameters affecting the pyrolysis process are temperature, residence time, the method of heating bio-fuels and air supply [1].An increase of gasification temperature by operating pressure leads to higher syn gas production and lower ash fraction. So our reactor design was focused for higher operating pressure to increase the gasification temperature and gas production rate.

II. EXPERIMENTAL SETUP

It consists of an air compressor to supply the quantity of air. The air flow rate was measured using an orifice meter at the entrance to the gasifier. The primary

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27 air enters to the gasifier through the nozzles around the periphery of the cylindrical vessel and the secondary air from the top of the feed stock. Type K thermocouples were used to measure the temperature distribution inside the reactor.

Fig. 1 : Schematic diagram of the biomass gasifier experimental setup Two thermocouples were used to measure the exit temperature of the producer gas after the reactor and the scrubber. Knowing the air flow rate and the time taken for the experiment, the amount of air used in each experiment was calculated. The amount of producer gas obtained is evaluated by using an orifice meter. Keeping the air flow rate constant for the particular pressure, experiments were conducted by maintaining the pressure. Experiments were conducted at atmospheric, 0.5, 1, 2 and 2.5 bar pressures.

A. Reactor Preparation

Fig. 2 : Reactor

Internal diameter 196mm and thickness 4.2mm with flanges on both sides of each 10mm thickness. A stainless steel throat of height 300mm and of equal

thickness is provided below the vessel. Both the vessels are lined with a refractory material of 5 mm thickness.

Fire clay is used as refractory material which is resistant to high temperature and it has a fusion point higher than 1600°C.Thus the heat loss from the furnace was prevented in order to create a high internal temperature and to reduce the tar formation. It has an ash pit at the bottom of the throat and a wood feeding section at the top. The reactor is a throated down draft gasifier and the air is admitted through circumferential nozzles (primary air).Major part of the air supplied for sustaining the combustion was made through the primary air nozzles provided around the circumference of the gasifier.

The primary air is controlled using needle valves.

Through the top part of a reactor, feedstock is fed.

Reactor is made of mild steel and joined to the cylindrical part with flange joint. The cylindrical part is made of stainless steel (ss304). The various zones such as drying, pyrolysis and oxidation are identified by knowing the temperature history inside the reactor. The inner surface is lined with refractory material. Primary air is fed into the oxidation zone at four different points around the circumference.

B. Experimental Procedure

The batch experiments were carried out as described. The reactor was filled with 8 kg of wood pieces of size approximately 12x12x12 mm and density 589.33 kg/ m³ through the top and sealed. The fuel bed was torched, at the pre-set port. The temperature is recorded at different locations at an interval of 60 s during the experiment using data acquisition system.

Gas samples are collected at regular intervals, when steady state is reached approximately in the combustion zone. The gaseous products were analysed using AUTO-CHRO WIN gas chromatograph. Knowing the air flow rate and the time taken for experiment the amount of air used in each experiment was calculated.

The amount of producer gas obtained is evaluated by using an orifice meter. The experiments were repeated for different operating pressure.

III. SIMULATION

This study of gasification/thermal flow interactions and investigate the effects of these different input parameters on the performance of down draft biomass gasifiers by modelling the gasification process and employing the Computational Fluid Dynamics (CFD) technology would contribute to the industry resolving concerns and improve gasifier efficiency and reliability.

Investigation of down draft biomass gasifier has been carried out with help of ANSYS FLUENT, a CFD tool to simulate the effect of different operating pressure

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28 on the temperature distribution. The combustion chamber was separately modeled and simulated.

Through temperature distribution of the reactor, only the combustion zone was modeled in GAMBIT 2.4 software package. The simulation was done through the concepts of Species transport phenomena, eddy dissipation and turbulent model as k-epsilon.

Fig.3 : Two dimensional simulation model of combustion zone

IV. RESULTS AND DISCUSSION A. Experimental Results

The objective of the study is to model the Biomass gasification process which is considered to be a very complicated process. There are many parameters that affect the efficiency of producer gas production in Biomass gasifiers, such as deferent operating pressure, fuel type, moisture content in the fuel and equivalence ratio. This study of gasification and thermal flow interactions investigate the effects of different operating pressures as input parameters on the performance of high pressure down draft biomass gasifiers by experimentation and Computational Fluid Dynamic simulation. The following are the different parameters on which the study is focused: 1. Proximate and ultimate analysis of feed stock, 2.Gas composition, 3.

Temperature of each zone, 4. Calorific value of the gas produced, and 5. Conversion efficiency.

1) PROXIMATE AND ULTIMATE ANALYSIS OF FEEDSTOCK

The ultimate and proximate analysis of the silver oak wood chips (12×12×12mm) used for the experiments were carried out and the results are shown.

TABLE I : PROXIMATE ANALYSIS OF WOOD CHIPS

Moisture Ash Volatile Matter

Fixed carbon

7.15% 2.04% 82.56% 8.25%

TABLE II : ULTIMATE ANALYSIS OF WOOD CHIPS

C H₂ O₂ N₂ S

Gross Calorific

value 42.55 % 4.22% 43.65% 0.33% 0.06% 16052 KJ /kg

2) GAS COMPOSITION

The gas composition is an important indicator of gasifier performance. The composition of the fuel gas was determined using AUTO-CHRO WIN gas chromatography. After the gasification process was established, the gas samples were collected every 15 minutes. During the sampling period the gas was burnt without the help of pilot burner. Minimum four samples were collected in each run for the analysis.

Fig. 4 : Overlapping chromatography for

Fig.5 : Overlapping chromatography for O2, N2, CO, CH4, CO2

Thermal conductivity detector and carrier gases N₂ and H₂were used to detect the volumetric compositions of gases like O₂, H₂, N₂, CO, CH₄ in a molecular sieve

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29 column and CO₂ was determined using chromosorb102 column. The effects of experimental conditions on the gas composition were evaluated for different modes and are as given in the tables. Gas analysis results are presented to show the development of the gas composition in the gasifier starting from cold as shown in Table III.

TABLE III. GAS COMPOSITION FOR DIFFERENT OPERATING PRESSURES

Pressure (bar)

Gas composition (%)

H2 N2 O2 CH4 CO CO2

ATM 18.95 49.043 1.101 1.449 10.304 18.812 0.5 22.725 52.41 1.145 0.925 10.465 12.087 1 23.572 52.02 1.003 1.035 10.147 12.20 2 23.74 51.352 1.056 1.140 10.787 12.063 2.5 23.82 50.56 1.015 1.165 10.99 12.325 The producer gas contains both combustible gases and non-combustible gases. The combustible gases contain gases such as H₂, CO, CH₄ and traces of C₂H₂ and C₂H₆, while the non-combustible gases include N₂ and CO₂.The composition of combustible gases increases with increase in operating pressure.

3) TEMPERATURE DISTRIBUTION

As a result of the testing at various pressure conditions the maximum temperature obtained were between 998-860 K. The high temperature at the drying zone helps in driving out the moisture present in the wood chips.

TABLE IV : TEMPERATURE DISTRIBUTION ALONG THE HEIGHT OF THE REACTOR Locatio

n of the thermo couple in m

Tempera ture in K at ATM

Temper ature

in K at 0.5

bar

Temper ature

in K at 1 bar

Tempera ture in K at 2 bar

Tempera ture in K at 2.5

bar

0.05 756 789 822 855 910

0.15 780 820 860 890 934

0.25 827 865 920 950 998

0.35 852 889 955 992 1025

0.45 865 920 987 1118 1042

0.65 650 780 765 852 822

0.8 442 488 462 515 525

0.95 351 367 381 408 421

1 315 318 328 344 352

In the reduction zone, owing to endothermic reactions, the temperature has been dropped down. The degree of temperature drop depends upon the extent of the reactions. The extent of the reaction depends upon the reactivity of the char and the thermal history. For

higher char reactivity, the reduction zone temperature drops faster and the reaction completes rapidly. The increased char reactivity is essential along the reduction zone. Higher the temperature inside the reactor helps in better tar cracking. Since it is not possible to reduce or remove tar effectively from the gas once it comes out of the reactor, the only way to achieve it is to maintain high temperature inside the reactor.

Fig. 6 : Temperature distribution plot along the height of the reactor

4) CALORIFIC VALUE OF PRODUCER GAS The calorific valve of the fuel gas is calculated from the concentration of the combustible components.

Calorific value in kJ/kg for average composition of samples is calculated from following equation [4].

Calorific value = ρ× × 0.001 × M^-1 ρ – Density of the producer gas, in kg/m³

i - molar fraction of each combustible component in the producer gas

- Standard heat of combustion, in kJ/mol M - molecular weight in, kg/kmol.

Fig. 7 : Variation of calorific value with pressure

300 400 500 600 700 800 900 1000 1100 1200

0 0.2 0.4 0.6 0.8 1 1.2

Temperature in K

Length of thermocouple in m ATM

0.5 bar 1 bar 2 bar 2.5 bar

3800 3900 4000 4100 4200 4300 4400

atmospheric 0.5 1 2 2.5

calorific value in KJ/m3

pressure in bar

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30 Compared to atmospheric pressure the calorific value has improved at higher pressures. The increase in calorific value with increase in reactor pressure may be due to increase in temperature in the combustion and reduction zone which results in better reaction.

5) CONVERSION EFFICIENCY

Fig. 8 : Variation of conversion efficiency with pressure Conversion efficiency, ηc = (m_gas x HHV of fuel gas) / (m_woodx HHV of wood)

It gives the information about how much energy is extracted from the wood. In our case, the conversion efficiency is decreased with increase in pressure. This is mainly due to decrease in the residence time of gas B. Simulation Results

1) TEMPERATURE DISTRIBUTION

The temperature profile is plotted for length along the combustion chamber vertically which is taken in x axis and the static temperature in y axis. This shows the static temperature in the combustion chamber from 0.25 meters to 0.55 meters length of the gasifier.

Fig. 9 : Temperature plot with different pressure conditions

The filled temperature contour exactly predicts the temperature distribution inside the combustion chamber.

The contour shows the combustion temperature in the combustion chamber around 808 K at ATM conditions.

There is decrease in the temperature below and above the combustion chamber. There is a very high temperature near the primary air supply in the combustion chamber.

The predicted values almost match very close with the experimental values. The thermal vibration inside the combustion chamber is clearly seen in figure 10.

Fig. 10 : Temperature contours with different pressure conditions As said earlier, the temperature near the primary is high because there is rich availability of oxygen content and mainly due to this wood volatiles readily burn and produces efficient temperatures at this phase. The maximum temperature reached is 946 K at 2 bar pressure condition. There is vertical probe in the gasifier wherein ten thermocouples are attached which is kept inside the combustion chamber to measure the experimental temperature values. At the same height in the combustion chamber, the simulated temperature

65 70 75 80 85 90

1 2 3 4 5

Conversion efficiency in %

Pressure in bar

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31 values are taken and compared with the experimental values.

2) VALIDATING EXPERIMENTAL VALUES WITH SIMULATED VALUES

The predicted values show a close match with the experimental values. The simulation shows that the physical combustion in the combustion chamber is actually the same as the validated one. This also helps us to validate the experimental values. Some of the results obtained from this simulation are not accurate. This may be due to several parameters such as the model’s parameters and discretization that play an important role in the result accuracy.

Fig. 11 : Validated chart for 2.5 bar pressure condition

V. CONCLUSION

The high pressure gasifier system was operated at atmospheric, 0.5, 1, 2 and 2.5 bar pressure conditions.

The effect of operating pressures on the gasification process were evaluated by assessing the main variables namely, gas composition, calorific value of the fuel gas, quantity of gas produced, product gas energy and conversion efficiency . Gas samples were taken for every pressure at equal interval of time after the gasification reached steady state. From the analysis of the gas samples using gas chromatograph, it was observed that the percentage of the combustible contents namely CO, CH₄, H₂improved during the operation at 0.5, 1, 2 and 2.5 bar respectively as compared to atmospheric pressure. Also the calorific value of the producer gas improved from 3.86MJ/m³ at atmospheric pressure to 4.37MJ/m³ at 2.5 bar respectively. The gas production rate is increased from 9.4 m³/hr to 13.2 m³/hr at pressurized conditions respectively. The conversion efficiency is in range of 73.7% to 86.8% with the maximum at atmospheric condition

To validate the temperature of combustion zone, CFD simulation was attempted using finite volume package FLUENT 6.3 in which turbulence model (K

Epsilon) is considered. Species transport and reaction model solves a set of governing equations related to fluid flow such as continuity, momentum and energy equations as well as species transport phenomena. An axis-symmetric two dimensional model was generated using GAMBIT 2.4 tool and exported to FLUENT 6.3 solver. The volumetric reaction option was considered throughout the domain and different types of boundary conditions were employed to get the temperature distribution. Combustion zone temperature was increased with pressure rise. In all cases, the maximum temperature obtained was between 0.35m to 0.4m axis of reactor. The thermal vibrations were concentrated towards the axis in pressurized conditions.

VI. REFERENCES

[1] Pratik N Sheth, B. V. Babu, Experimental Studies on Downdraft Biomass Gasifier, birla institute of technology and science(bits), pilani – 333 031.

[2] Avdhesh Kr. Sharma,Experimental study on 75 kWth

downdraft (biomass) gasifier system, Renewable Energy 34 (2009) 1726–1733.

[3] Pengmei Lv,Zhenhong Yuan, Longlong Ma, Chuangzhi Wu, Yong Chen, Jingxu Zhu, Hydrogen- rich gas production from biomass air and oxygen/steam gasification in a downdraft gasifier, Renewable Energy 32 (2007) 2173–2185.

[4] Paulo R. Wanders, Carlos R. Altafini, Ronaldo M Barreto, 2004, Assessment of a small sawdust gasification unit. Biomass and Bioenergy 27 (2004) 467–476.

[5] K. Sivakumar, N.Krishna Mohan, Performance analysis of downdraft gasifier for agriwaste biomass materials, 2010, Indian journal of science and technology. Vol-3.

[6] S Sivakumar, K.Pitchandi, and E Natarajan, Design and Analysis Of Down Draft Biomass Gasifier using Computational Fluid Dynamics, Anna University, Guindy, Chennai-25.

[7] H V Sridhar, G Sridhar, S Dasappa, P J Paul, H S Mukunda, On the operation of a high pressure biomass gasifier with gas turbine, Biomass conference,2007,Germany.

[8] H.V.Sridhar et al, On the operation of a high pressure biomass gasifier with gas turbine, 15^th Eropean biomass conference & Exhibition(2007) Berlin, Germany.

[9] Jae Ik Na*, So Jin Park, Yong Koo Kim, Jae Goo Lee, Jae Ho Kim, Characteristics of oxygen-blown gasification for combustible waste in a fixed-bed gasifier, Applied Energy 75 (2003) 275–285.

600 650 700 750 800 850 900 950 1000

0.2 0.25 0.3 0.35 0.4 0.45 0.5

Temperature in K

Length of combustion chamber in m Experimental

Simulated