Emission Characteristics of Co-combustion of Sewage Sludge with Olive Cake and Lignite Coal in a Circulating Fluidized Bed

Abstract

In this study, a circulating fluidized bed (CFB) of 125 mm diameter and 1800 mm height was used to find the combustion characteristics of sewage sludge (SS) produced in Turkey. Sludge + olive cake, and sludge + lignite coal mixtures were burned separately. Various sludge-to-lignite coal and sludge-to-olive cake ratios (5/95, 10/90, 15/85, 20/80) were tried. On-line concentrations of major components (O₂, SO₂, CO₂, CO, NO _x , C _m H _n ) were measured in the flue gas, as well as temperature and pressure distributions along the bed. Combustion efficiencies of sludge + olive cake and sludge + lignite coal mixtures were calculated, and the optimum conditions for operating parameters were discussed. The results have shown that the combustion mainly takes place in the upper regions of the main column where the temperature reaches 900°C. SS + Coal burn in the CFB with an efficiency of 95.14% to 96.18%, which is considered to be quite good. When burning sludge mixed with olive cake, appreciable amounts of CO and unburned hydrocarbons are formed and the combustion efficiency drops to 92.93%. CO and C _m H _n emissions are lower when lignite coal is mixed with various amounts of SS than the emissions when the coal is burned alone. As the %SS is increased in the fuel mixture, the SO₂ emission decreases. NO _x emissions are slightly higher. When burning sludge mixed with olive cake, SO₂ and NO _x emissions are slightly higher. CO and C _m H _n emissions decrease sharply when SS is mixed with 5%wt. olive cake. With increasing sludge ratio these emissions increase due to the unburned hydrocarbons. As a result of this study, it is believed that SS can be burned effectively in a CFBC together with other fuels, especially with olive cake (OC). OC will be a good additive fuel for the combustion of lower quality fuels.

Keywords:

• Sewage sludge
• Olive cake
• Co-combustion of sewage sludge
• Circulating fluidized bed (CFB)
• Emissions

Acknowledgments

The financial support provided to this project by the Turkish Scientific and Technical Research Council-TUBITAK (Project Code: MISAG-195) and the British Council, Ankara is greatly appreciated.