Introduction. Characteristics of syngas from the pyrolysis and gasification of food waste has been investigated

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.

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.

[Some details are omitted]

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.

Дата добавления: 2014-10-31