Elsevier

Waste Management

Volume 46, December 2015, Pages 459-471
Waste Management

Mechanisms and kinetics of granulated sewage sludge combustion

https://doi.org/10.1016/j.wasman.2015.08.015Get rights and content

Highlights

Abstract

This paper investigates sewage sludge disposal methods with particular emphasis on combustion as the priority disposal method. Sewage sludge incineration is an attractive option because it minimizes odour, significantly reduces the volume of the starting material and thermally destroys organic and toxic components of the off pads. Additionally, it is possible that ashes could be used. Currently, as many as 11 plants use sewage sludge as fuel in Poland; thus, this technology must be further developed in Poland while considering the benefits of co-combustion with other fuels.

This paper presents the results of experimental studies aimed at determining the mechanisms (defining the fuel combustion region by studying the effects of process parameters, including the size of the fuel sample, temperature in the combustion chamber and air velocity, on combustion) and kinetics (measurement of fuel temperature and mass changes) of fuel combustion in an air stream under different thermal conditions and flow rates. The combustion of the sludge samples during air flow between temperatures of 800 and 900 °C is a kinetic–diffusion process. This process determines the sample size, temperature of its environment, and air velocity. The adopted process parameters, the time and ignition temperature of the fuel by volatiles, combustion time of the volatiles, time to reach the maximum temperature of the fuel surface, maximum temperature of the fuel surface, char combustion time, and the total process time, had significant impacts.

Introduction

The development of civilization has resulted in greater waste production. Particularly, increases in municipal sewage sludge production are important because their properties may allow for its use as a valuable energy-efficient fuel. From ecological, economic, and technical viewpoints, sewage sludge is difficult to dispose of, primarily due to bacterial contamination and the possible presence of heavy metals. It should be emphasized that these substances can harm the environment, reducing the possibility of naturally developing or storing them, which are the current disposal methods.

Due to problems regarding the use and application of this waste, which are associated with increasingly restrictive legislation regarding the need for sewage sludge storage in Poland beginning January 1, 2016, and because the waste has a gross calorific value greater than 6 MJ/kg (Regulation of the Economy Minister, 2013), its thermal disposal has become extremely important. A sewage network development plan should be completed for Poland by the end of 2015. This plan will increase the amount of sewage sludge that must be disposed of. The appropriate choice of method for sludge disposal is dictated by its properties. In this paper, an analysis of sewage sludge properties and potential development is presented while stressing the benefits of its thermal use. Additionally, the large amounts of sewage sludge incineration and co-combustion in power plants provide several tangible benefits, such as reductions in the amounts of input material (dried sewage sludge) and the production of electricity or heat. The addition of these wastes also limits the amount of conventional fuel used. Moreover, according to law, sewage sludge is considered as neutral biomass CO2.

Thus, conducting scientific research to identify sewage sludge properties and combustion processes is urgently needed.

Section snippets

Review of the literature regarding sewage sludge characteristics and combustion

Choosing an appropriate method for sludge disposal should depend on the properties of the sludge. The physical, chemical, sanitary, and technological properties of sewage sludge are important and change depending on the type and method of sewage treatment. Because of its unusual properties, sewage sludge is different from coal and biomass. Sewage sludge combustion is different because it includes incineration drying, devolatilization and the combustion of volatiles and char, which contain an

Experimental studies

This paper presents the results of experimental studies aimed at determining the mechanisms and kinetics of fuel combustion in an air stream depending on the thermal conditions and flow (temperature range of 800–900 °C, air flow velocity range of 2.0–3.5 m/s, and fuel sample diameters range from 5 to 10 mm) using a bench test with a total capacity of 8.2 kW (Fig. 4). One interesting element of this research is the need to visualize the sewage sludge combustion process when using a combustion

Conclusions

The Polish energy sector relies on hard and brown coals. However, other biomass or waste fuel solutions are needed, including sewage sludge.

The experimental studies performed for this study resulted in the following conclusions.

  • 1.

    The statistical analyses of the test results indicated a correlation between the parameters that determined the course of the fuel combustion in the air stream and the designation of the function approximation that describe the process.

  • 2.

    The process parameter study (the

Acknowledgement

This work was financially supported by the National Science Centre (Poland) based on decision No. DEC-2012/07/B/ST8/03730. This support is gratefully acknowledged.

References (36)

  • J.B. Bień

    Sewage Sludge – Theory and Practice

    (2007)
  • J.B. Bień et al.

    Thermal Processes in Disposal of Sewage Sludge

    (2008)
  • D. Boruszko

    Processing and Disposal of Sewage Sludge: Laboratory Exercises

    (2001)
  • Brochure of Polish Cement Association, 05.2015. Co-combustion of alternative fuels in the cement industry (in...
  • Z. Heidrich et al.

    Wastewater Treatment Plant. Design Examples of Calculations

    (2010)
  • Kijo-Kleczkowska, A., Kosowska-Golachowska, M., Gajewski, W., Środa, K., Musiał, T., Wolski, K., 2014. Incineration of...
  • W. Kordylewski

    Combustion and Fuel

    (2005)
  • J. Malej

    Waste and Sewage Sludge. Characterization, Treatment, Management

    (2004)
  • Cited by (0)

    View full text