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Computer model for assessment of circulating fluidised bed gasification of sewage sludge/coal mixtures

The work carried out in IChPW comprises two comprehensive parts. The first part included experimental investigation of the pyrolysis process (as the first stage of gasification) of sewage sludge and/or mixture sewage sludge/coal leading to the enhanced fundamental understanding of the process. The second part was focused on the development of mathematical model of sewage sludge gasification in the circulating fluidised bed reactor.

A bench scale investigation of pyrolysis process was carried out. The experiments were performed using 5 different types of feedstock (coal, raw and dry sewage sludge and sewage sludge/coal mixtures) at 5 temperatures of the process (520-950 degrees Celsius). The results of the test were discussed and used for the determination influence of process temperature and composition of the feedstock on pyrolysis products yields (including char, gas and tar), chemical structure transformation of organic substances during thermal processing of the selected feedstock, changes of the char structure, particular internal surface development.

Experimental data were used for modelling of sewage sludge gasification. The products distribution of the pyrolysis and its chemical composition and structure strongly depend on process parameters, particularly on temperature, and physico-chemical properties of feedstock. Generally, temperature increase affects the decrease of char and increases the gas yield. Moreover at higher temperatures the ash content in a char increases and volatile matter decreases.

The pyrolysis with the temperature increase of dry and wet sewage sludge leads to higher yields of tars; water chemically formed in comparison to coal pyrolysis. But the tars yields from wet sludge pyrolysis are higher than from dry sewage sludge. It is due to different chemical structure of dry and wet sewage sludge; pre-drying process effected decomposition of peptidoglicane - main building material of bacteria cellular walls. Pyrolysis temperature of the order of 700 degrees Celsius is critical for decomposition of sewage sludge both wet and dry and its mixture with coal, particularly on char structural parameters and chemical composition.

Considering structure of resulting sludge char it can be stated that the properties of it strongly depend on pyrolysis process parameters, especially temperature. The influence of temperature under 700 degrees Celsius on sludge char structural parameters is insignificant. Temperatures higher than 900 degrees Celsius enable production of materials having high specific surfaces (about 100m2/g). Condensed water from sludge raw gases comprises chlorine, nitrite, nitrate, phosphoric and sulphate ions in higher amount than raw coal gas, and pyrolysis gases from mixture of coal and biomass.

It may affect higher corrosion in industrial installations. A computer model for assessment of circulating fluidised bed gasification of sewage sludge/coal mixtures was developed to provide improved predictive behavioural modelling capabilities. For sewage sludge gasification process, developed and demonstrated in the Institute circulating fluidised bed reactor was proposed.

The modelling work comprises development of aerodynamics, particle size population balance, static and kinetic models of sewage sludge-coal mixture gasification process. The models were used to predict the calorific value of the product gas, solid to gas conversion efficiency, gas and char yields and parameters such air demand etc.

For power generation applications, the most promising options were gasification of thermally dried sludge sewage and mechanically dried sewage sludge/coal mixture (the sludge content in the fuel mixtures should not exceed 25%). For the former, the product gas LHV was predicted to be 6.1MJ/kg at an optimum temperature of 710 degrees Celsius (kinetic model).

The third option considered, gasification of dry sludge/coal mixture, was not considered to be economically viable due to the sludge drying requirements and use of the additional fuel (coal). The pre-feasibility study defined the technical and organisational conditions for the construction of a 1.7t/h sewage sludge gasification plant and confirmed the economic viability of technology.

The data of sewage sludge in Poland have been also collected. Data concern year 2000 and contain information about main sources and wastewater treatment methods, sewage sludge production, characteristic and methods of utilisation. The information by different regions in Poland ware also presented. The final product of the work is computer code for the process and design development.

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Institute for Chemical Processing of Coal
Zamkowa 1 St.
41-803 ZABRE
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