Decomposition of Tar in Pyrolysis Gas by Partial Oxidation and Thermal Cracking

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DECOMPOSITION OF TAR IN PYROLYSIS GAS BY PARTIAL OXIDATION AND THERMAL CRACKING. PART 2.

% P. Brandt and U. Henriksen ft L, eo Department of Energy Engineering, Building 403 Technical University of Denmark, DK - 2800 Lyngby . MAY 1 9 1999 Tel: +45 45 25 41 74 Fax: + 45 35 93/57 61 E-mail: [email protected] Q 0 *| “ |

ABSTRACT ^Thermal cracking and partial oxidation of pyrolysis gas has been investigated i a reactor at 800, 900 and 1000 °C. Experiments has been carried out with excess air ratio varied from 0 (thermal cracking) to 0.7. It was shown that it is possible to achieve a significant reduction in tar content in the gas by partial oxidation. The minimum tar content measured was 0.5 g pr. kg dry straw giving a 98-99 % reduction by 900 °C in the cracking reactor and a excess air ratio of 0.5. It was also shown that the temperature in the cracking reactor had no influence on the tar content at partial oxidation with a excess air ratio above 0.2 in the temperature area investigated. The content of H2, CO, CH4 and C02 in the produced gas were measured. It was shown that it is possible toreduce the tar content in the gas without reducing the volume of H2 and CO per kg dry straw. The volume of CH4 per kg dry straw decreases with increasing excess air ratio, and the volume of C02 per kg dry straw increases with increasing excess air ratio.

1. INTRODUCTION

Experimental investigations of the two-stage gasifier [1] has shown a low content of tar in the produced gas. Samples taken out different locations have indicated that the primary decomposition is due tothe partial oxidation rather than due to thermal cracking of the pyrolysis gas. In order to examine this phenomena in more detail an experimental test stand consisting of a continuous feed pyrolysis reactor and a cracking reactor was constructed. This work is at part of the on-going work concerning decomposition of tar in pyrolysis gas by partial oxidation and thermal cracking. The experimental test stand and the first part of the work is described in [2], A similar study of partial oxidation was carried out at RIS0 National Laboratory, Denmark [3],

2. APPARATUS

2.1 Experimental test stand The following is a short description of the experimental test stand. More details are given in [2], The test stand consists of a continuous feed pyrolysis reactor and a cracking reactor. The gas from the pyrolysis reactor flow into the cracking reactor where air and steam was added. The steam and air was added in varying quantities and at controlled temperatures. The mixing of air and gas at the

DISTRIBUTION OF THIS DOCUMENT IS UNLIMITED FOREIGN SALES PROHIBITED^' Paper presented at: Biomass for Energy and Industry. 10th European Conference and Technology Exibition Page 1 8-11 June 1998, Wurzburg, Germany. DISCLAIMER

Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. point where the air were added was good. The residence time in the cracking reactor was controlled by adding nitrogen to the air. Coarse-chopped straw were used as feed stock, and the feeding rate was 1.2 kg an hour. Se figure 1.

2.2 Tar measuring The tar content in the gas were measured by weight. After the cracking reactor a sample of tar were collected by cooling a volume of gas down to 15 °C in a condenser. The condensate were collected in the condenser and a flask. Aerosols in the cold gas were captured by a filter downstream of the condenser. After each experiment the water phase collected in the flask were removed and the tar-capturing system were washed with acetone. The tar-contaminated acetone were filtered to remove solids, and the tar content were determined by evaporating the acetone with dry air at room temperature as described in [2], The water soluble tar in the water phase were determined by non volatile organic carbon (NVOC) analysis.

2.3 Gas analysis Down stream of the tar measuring equipment the content of H2, CO, CH4, C02 and 02 in the clean gas were measured. N2 content were calculated as the rest.

Flare Flow meter

Gas analysis Feed stock Tar measurements

Cracking reactor Air, nitrogen Feed and steam system Pyrolysis gas

Feed Pyrolysis reactor mechanism Char container

Electrical heaters

Figure 1. Block diagram of the experimental test stand.

Paper presented at: Biomass for Energy and Industry. 10th European Conference and Technology Exibition Page 2 8-11 June 1998, Wurzburg, Germany. 3. EXPERIMENTAL INVESTIGATIONS

This work has covered thermal cracking and partial oxidation at the temperatures 800, 900 and 1000 °C in the cracking reactor. Experiments has been carried out with excess air ratio varied from 0 (thermal cracking) to 0.7. Two experiments were carried out with the temperature keptat pyrolysis conditions, 600 °C in the cracking reactor and no air supplied. The excess air ratio was calculated as air supplied divided by air necessary for full stoechiometric combustion of the volatile pyrolysis products. The feeding system introduced some pulsation in the flow of gas from the pyrolysis reactor to the cracking reactor giving a small variation in the excess air ratio under each experiment. The steam to feed stock mass-ratio was approximately 0.25, and the gas had a residence time in the cracking reactor of approximately 2 seconds. All experiments were carried out with a gas generated by slow pyrolysis of straw at 600 °C with a solid residence time of 23 minuets, giving a char yield of 0.31 kg / kg dry straw.

4. RESULTS AND DISCUSSION

4.1 Tar content Figure 2 shows the tar collected in the condenser and the aerosol filter as a function of temperature in cracking reactor and excess air ratio. It can be seen that except at small excess air ratios there was no difference between the runs at 800 °C and 900 °C. The temperature in the cracking reactor did not appear to have any influence at these temperatures. The same tendency was seen with partial oxidation of pyrolysis gas in a batch reactor described in [3], and by partial oxidation of naphthalene in a artificial biomass producer gas [4],

Partial oxidation

Excess air ratio 800 °C -Q— 900 °C ♦ 1000 "C

Figure 2. Tar collected in condenser and aerosol filter. Partial oxidation experiments.

Paper presented at: Biomass forEnergy and Industry. 10th European Conference and Technology Exibition Page 3 8-11 June 1998, Wurzburg, Germany. The carbon content in the water phase condensate collected in the flask was approximately 10 % of the tar collected in the condenser and the aerosol filter. At the two runs with pyrolysis conditions in the cracking reactor the amounts of tar collected in the condenser and the aerosol filter were measured to 38 and 42 gram per kg dry straw. In the experiments at 900 °C and excess air ratio of 0.5 the tar content was less than 0.5 gram per kg dry straw giving a 98-99 % reduction. Figure 3 shows the tar content in the gas after thermal cracking. By comparing figure 2 and 3 it can be seen that partial oxidation gives a many times bigger reduction in tar content than thermal cracking in the temperature region investigated.

Thermal cracking

1000 Temperature in cracking reactor [°C] Figure 3. Tar collected in condenser and aerosol filter. Thermal cracking experiments.

4.2 Gas composition Figures 4 to 7 show the content of H2, CO, CH4 and C02 in the gas after the tarmeasuring equipment. The results are given as normal litre ( 0 °C and 1 atm ) per kg dry straw. The content of hydrogen in the gas did not show a strong dependense on the air supplied, but showed an increase with an increase in temperature. Calculations of the reaction quotient of the water-gas shift reaction showed that the gas composition at all experiments were fare from equilibrium. The content of carbon monoxide in the gas showed a maximum at a excess air ration of 0.4 and no big difference between experiments at 800 °C and 900 °C. The content of methane in the gas was showing a very short increase and then decrease with an increase in excess air ratio. There were no difference between the pattern at 800 °C and 900 °C. By comparing these results with those shown in figure 2 it can be seen that it is possible to reduce the content of tar in the gas without a reduction in hydrogen and carbon monoxide but with a decrease in the content of methane in the gas. The content of carbon dioxide increase with a increase in excess air ratio.

Paper presented at : Biomass forEnergy and Industry. 10th European Conference and Technology Exibition Page 4 8-11 June 1998, Wurzburg, Germany. Paper Biomass 8-11

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Figure 7. Content of carbon dioxide in the produced gas.

5. FURTHER WORK

Future plans include investigations of the tar content in the gas by varying temperature in the cracking reactor, water content in the gas, and air supplied. Experiments over the range of the

Paper presented at : Biomass for Energy and Industry. 10th European Conference and Technology Exibition Page 6 8-11 June 1998, Wurzburg, Germany. following values is planned: temperature in cracking unit: 900-1100 °C, steam to feed stock mass ratio: 0-0.9 and excess air ratio: 0-0.6. All runs will be carried out with gas generated by slow pyrolysis of straw at 600 °C.

6. CONCLUSIONS

Thermal cracking and partial oxidation of pyrolysis gas has been investigated at 800, 900 and 1000 °C. It was shown that it is possible to achieve a significant reduction in tar content by partial oxidation. The minimum tar content measured was 0.5 g per kg dry straw giving a reduction of 98- 99 % by 900 °C in the cracking reactor and a excess air ratio of 0.5. The excess air ratio was calculated as air supplied divided by air necessary for full stoechiometric combustion of the volatile pyrolysis products. It was also shown that the temperature in the cracking reactor had a influence on the tar content only at small excess air ratios in the temperature region investigated. The content of H2, CO, CH4 and C02 in the produced gas were measured. It was shown that it is possible to reduce the tar content in the gas without reducing the volume of H2 and CO per kg dry straw. The volume of CH4 per kg dry straw decreases with increasing excess air ratio, and the volume of C02 per kg dry straw increases with increasing excess air ratio.

The work is being continued.

7. ACKNOWLEDGEMENTS

This work was supported by the Danish Ministry of Energy and ELKRAFT.

REFERENCES

[1] Henriksen, U. and Christensen, O. (1994). Gasification of straw in a two-stage 50 kW gasifier. Proceedings of the 8th European Conference on Biomass for Energy, Environment, Agriculture and Industry 3-5 October 1994. Vienna, Austria, p.1568.

[2] Brandt, P. and Henriksen, U. (1996). Decomposition of tar in pyrolysis gas by partial oxidation and thermal cracking. Proceedings of the 9th EuropeanBioenergy Conference 24-27 June 1996. Copenhagen, Denmark, p. 1336.

[3] Jensen, P.A., Larsen, E. and Jorgensen, K.H. (1996). Tar reduction by partial oxidation. Proceedings of the 9th European Bioenergy Conference 24-27 June 1996. Copenhagen, Denmark, p. 1371.

[4] Lammers, G., Beenackers, A.A.C.M. and Corella, J. (1997). Catalytic tar removal from biomass producer gas with secondary air.Tar removal with secondary air. Developments in thermochemical biomass conversion (ed. A.V. Bridgwater and D.G.B. Boocock ) Blackie Academic & Professional, London, p. 1179.

Paper presented at: Biomass for Energy and Industry. 10th European Conference and Technology Exibition Page 7 8-11 June 1998, Wurzburg, Germany.