Pyrolysis and gasification of food waste: syngas characteristics and char gasification kinetics

I.I. Ahmed, A.K. Gupta

The Combustion Laboratory, University of Maryland, Department of Mechanical Engineering, College Park, MD 20742, United States

Keywords: Food waste gasification; Char gasification kinetics; Catalytic effect of ash; Compensation effect

Abstract

Characteristics of syngas from the pyrolysis and gasification of food waste has been investigated. Characteristic differences in syngas properties and overall yields from pyrolysis and gasification were determined at two distinct high temperatures of 800 and 900 ^oC . Pyrolysis and gasification behavior were evaluated in terms of syngas flow rate, hydrogen flow rate, output power, total syngas yield, total hydrogen yield, total energy yield, and apparent thermal efficiency. Gasification was more beneficial than pyrolysis based on investigated criteria, but longer time was needed to finish the gasification process. Longer time of gasification is attributed to slow reactions between the residual char and gasifying agent. Consequently, the char gasification kinetics was investigated. Inorganic constituents of food char were found to have a catalytic effect. Char reactivity increased with increased degree of conversion. In the conversion range from 0.1 to 0.9 the increase in reactivity was accompanied by an increase in prexponential factor, which suggested an increase in gasifying agent adsorption rate to char surface. However, in the conversion range from 0.93 to 0.98 the increase in reactivity was accompanied by a decrease in activation energy. A compensation effect was observed in this range of conversion of 0.93–0.98.

Introduction

Dumping food waste in a landfill causes environmental problems. By volume, the dumped landfill waste causes the largest contribution to methane gas production [1]. It causes odor as it decomposes to cause public annoyance in addition to forming germs, and attracting flies and vermin. Another serious problem of food wastes is the generation of landfill leachate. Landfill leachate is liquid that leaks from the landfill and enters the environment. Once it enters the environment the leachate is at risk for mixing groundwater near the site which then transports to some distances. Furthermore it has the potential to add biological oxygen demand (BOD) to the groundwater. BOD measures the rate of oxygen uptake by micro-organisms in a sample of water at a temperature of 20 ^oC and over an elapsed period of five days in the dark.

Food wastes have high energy content. Consequently, it offers a good potential for feed stock for gasification in power plants. Food waste gasification helps to solve two major problems at the same time. Gasification of food waste reduces landfill problems and efficiency. The results show that food wastes offers a good potential for thermal treatment of the waste with the specific aim of power generation. The average proximate analysis of food wastes is 80% volatile matter, 15% fixed carbon, and 5% ash. The volatile matter can be easily destructed in a relatively short period of time, extending from 8 to 12 min at reactor temperatures from 700 to 1000 ^oC . Energy recovery from volatile components in food wastes can be recovered using a simple pyrolysis process. However, in order to consume the residual fixed carbon after the pyrolysis, the sample must undergo a gasification process. Gasification of a food waste sample includes a pyrolysis part and a char gasification part. Char gasification reactions are slower than that of pyrolysis and consequently, is the rate limiting step in the overall gasification process.

The ash present in the sample does not react with the gasifying agent. The ash can be collected after cooling and cleaning the syngas, and then recycled for its further use in industrial processes.

Since the char gasification process is the rate limiting step, it is important to quantify the kinetic parameters of char gasification. Char gasification has been investigated by a large number of researchers. Some of the important parameters investigated include the origin of the char sample, gasifying agent, total pressure, variation of partial pressure of gasifying agents, geometric changes of the sample during gasification, and catalyzed char gasification. One of the most important parameters which have been investigated is the catalytic effect of ash content on char gasification.

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Consequently, for a desired feed rate of feedstock into the reactor and for known gasifier operational conditions an accurate reactivity expression will lead to a close estimate of the gasifier size and configuration. If a constant reactivity value is used in reacting flow simulations for feedstock having time dependant reactivity, misleading information on char particles residence time will be obtained.

This will consequently result in a departure gasifier size from the true design size and configuration. For example, if a constant reactivity value is used for chars having ash catalytic effect, such as the case examined here, the designed gasifier size will be over estimated since the reactivity of char was fond to increase with the degree of conversion.

Background

Tancredi et al. [2] investigated the catalytic effect of ash on char gasification for eucalyptus wood chars. The ash content in char was of the order of 1.45% on mass basis. The reactivity of the char increases monotonically with conversion. At low and intermediate conversion, it can be attributed to the increase in surface area as gasification proceeds. At high conversion levels a steeper increase in reactivity has been observed, which cannot be explained by the development of surface area. This region of the reactivity/conversion curves can be better explained as the result of an increase in catalytic effect of the metallic constituents (mainly Na and K) present as inorganic matter in the chars. Here CO₂ was used as the gasifying agent. Activation energies determined were found to vary within a narrow range of 230–257 kJ/mol. Arrhenius plots showed parallel lines for different degrees of conversion. Parallel line of Arrhenius plot indicates similar activation energies. The increase in reactivity was mainly due to an increase in pre-exponential factor. In a similar study by Montesinos et al. [3], steam gasification and CO₂ gasification of grape fruit skin char were investigated. They also observed an increase in reactivity at high values of conversion. However, a different trend of activation energies values was observed; in the case of CO₂ gasification, as the conversion increased, a decrease in activation energy was observed.

On the other hand an increase in activation energy was observed in case of steam gasification. This increase in activation energy was also, observed by Marsh et al. [4]. The decrease in activation energy values in the case of CO₂ gasification was accompanied by a decrease in pre-exponential factor as well. This behavior is called the compensation effect [5]. Montesinos et al. obtained a value of isokinetic temperature of 1150 K. The isokinetic temperature is the temperature at which all reactivities are equal for different conversions. An isokinetic temperature of 1449 K was obtained by Dhupe et al. [6] for CO₂ gasification using catalyzed sodium lignosulfonate. Feistel et al. [7] found this temperature to be 1425 K, obtained using potassium-catalyzed steam gasification.

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Food wastes, especially which have high percentage of vegetable oil and animal fat, provide a good potential for production of liquid fuels though transesterification. Transesterification is the process of exchanging the organic group R00 of an ester with the organic group R0 of an alcohol. The process is widely used to produce biodiesel fuels from vegetable oils and animal fats. The process is often catalyzed by an acid or a base.

Other than acid or base catalysts, enzyme or heterogeneous catalysts might be used as well. Among the mentioned catalysts, alkali catalysts are more effective. However, if the oil has high free fatty acid (FFA) content, higher than 3% (approximately), acid catalyzed transesterification is used rather than a base catalyst [10,11].

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Experimental

Fig. 1 shows a photograph of the laboratory scale experimental facility used to examine the pyrolysis and gasification of food wastes. Steam is generated from the stoichiometric combustion of hydrogen and oxygen. Steam generated is then introduced into the superheater section to form the gasifying agent at the desired condition. The temperature of the gasifying agent heater is kept at the same temperature as that of the main reactor in which sample material was allowed to undergo gasification. Steam is then introduced into the main reaction chamber that contained the hydrocarbon sample. The syngas flowing out from the main reaction chamber is sub-divided into two paths; one passes to the sampling line while the other is passed through the exhaust system.

Fig. 1. A photograph of the experimental facility.

The bypass line has a non-return valve and a flow meter to assure the desired unidirectional flow out from the reactor. The syngas sample is then introduced to a condenser followed by a low pressure filter and a moister absorber (anhydrous calcium sulfate). This procedure assured that the sample is dry prior to its introduction into a gas analyzer. The filtered and dried syngas is then analyzed using a GC or a mass spectrometer.

[The description of the experiment is omitted]

Results and discussion

The characteristic of syngas from food waste pyrolysis and gasification have been investigated. Food waste is a char-based sample. Results from char-based samples (samples containing volatile matter and char, such as paper [13], cardboard [14,15], woodchips and food waste) follow, qualitatively similar trend. Syngas is characterized by a high flow rate initially, due to pyrolysis, and then followed by a small flow rate which lasts for longer period, which is due to char gasification.

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Conclusions

Gasification yielded enhanced production of syngas, hydrogen and energy as that obtained from pyrolysis. However the time required for gasification is more as compared to pyrolysis. As compared to paper gasification at the same conditions, food waste needed more time to complete the gasification process. Inorganic constituents in food char were found to have a catalytic effect. Char reactivity increased with degree of conversion. In the conversion range from 0.1 to 0.9 the increase in reactivity was accompanied by an increase in pre-exponential factor, suggesting an increase in gasifying agent adsorption rate to char surface. However, in the conversion range from 0.93 to 0.98 the increase in reactivity was accompanied by a decrease in activation energy. A compensation effect was observed in this range of conversion, from 0.93 to 0.98. Isokinetic temperature obtained from Arrhenius plots for X from 0.93 to 0.98 was 1001 ^oC.

Acknowledgment

This research was supported by the ONR and is support is gratefully acknowledged.

References

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BioMedical Engineering OnLine 2010, 9:84 (http://www.biomedical-engineering-online.com)