Gaseous Emissions during Oxy-Fuel Combustion of Sewage Sludge in a Circulating Fluidized Bed

Increasing production of sewage sludge in Poland has created some environmental problems related to its disposal. These problems have resulted in recent changes in legislation. In the light of the Minister of Economy Regulation of 16 July 2015, regarding the criteria and procedures for releasing wastes for landfilling, the thermal disposal of sewage sludge is important because of its calorific value, which is greater than 6 MJ/kg, and problems associated with its use. Consequently, a restrictive legislation was introduced on the 1^st January 2016 for sewage sludge storage in Poland. Sewage sludge incineration is an attractive option because it significantly reduces the volume of the waste material, minimizes odor, and thermally destroys the organic and toxic components of the off pads. Currently, 11 plants in the country use sewage sludge as fuel thus this technology must be further developed in Poland.

Furthermore, greenhouse gases emissions from coal-fired systems, particularly carbon dioxide,become an important issue in the European Union countries. Oxy-fuel combustion is one of the promising technologies for power generation with CO₂ capture. This technology can also significantly reduce emissions of NO_x and improve the thermal efficiency of the combustion process through the reduction of flue gas volume by about 80%. In the oxy-fuel combustion, fuel is burnt in a mixture of pure oxygen and recycled flue gas. Because nitrogen is eliminated from the oxidizing gas, the flue gas leaving the combustion chamber is highly enriched in CO₂ which means that the combustion process takes place in an O₂/CO₂ environment. Partial recycling of flue gas helps to control the flame temperature in the combustion chamber.

Results of sewage sludge oxy-fuel combustion tests carried out in a 12-kW circulating fluidized-bed (CFB) bench-scale combustor are presented in this paper. The electrically heated rectangular column, 680×75×35 mm, is the main component of the CFB test unit. The front of the column is made of transparent quartz window through which the combustion process can be directly observed. Silica sand (particles smaller than 400 μm) constitutes the inert bed. The tested material consisted of sewage sludge samples in the form of spherical granules. They originated from large-scale commercial sewage treatment plants operating in Poland. Granulated sludge samples were burnt in air (base case) and mixtures of O₂/CO₂ at 850°C. Concentrations of emitted pollutants were measured using Gasmet DX-400 FTIR analyzer.

The main objective of this study was to investigate pollutant emissions from the combustion process of sewage sludge under oxy-fuel conditions. The influence of the inlet O2/CO2 ratio on emissions of NO, NO₂, N₂O, SO₂, CO and HCl is reported and discussed in this paper. Compared with combustion in air, combustion in a mixture of 21% O₂ and 79% N₂ caused a reduction in NO emission, a slight increase in N₂O and SO₂ emissions and a dramatic increase in CO emission. An increase in the O₂ inlet concentration resulted in higher emissions of NO and SO₂ and lower emissions of N₂O and CO.