Combustion characteristics of waste sludge at air and oxy-fuel combustion conditions in a circulating fluidized bed reactor

Fluidized bed combustion plants designed by researchers are known to generate lots of carbon dioxide, a greenhouse gas. This has led to development of carbon capture and storage (CCS) technologies to reduce emissions of carbon dioxide from fossil fuel combustion. Different types of carbon dioxide capture technologies include pre-combustion, post combustion and oxy-fuel combustion technologies. Oxy-fuel combustion technologies use oxygen combination greater than 95% purity level where recycled flue gas is utilized for combustion of fuel. The recycled flue gas is mainly carbon dioxide concentrated over 90% which is ready for sequestration without stripping of Carbon dioxide from the gas stream (Buhre et al. Prog Energy Combust Sci. 2005). Hence, oxy-fuel combustion technology for newly constructed and retrofitted waste sewage sludge-fluidized bed combustion plants would be ideal to generate high purity carbon dioxide in the flue gas.

Research team led by professor Yong-Chil Seo from Department of Environmental Engineering at Yonsei University studied the concept of circulating fluidized bed (CFB) Technology with oxy-fuel combustion technology as means of waste to energy technology where carbon capture has been applied to waste sewage sludge combustion. This is the first attempt to apply oxy-combustion technology to waste to energy for carbon dioxide reduction.

The new study published in journal, Fuel, used cold-bed tests in a circulating fluidized bed model to simulate the combustion of waste sewage sludge using the 30 KW_th oxy-fuel pilot test bed.

Pilot test bed projects done by researchers are mainly based on using coal-fired power plants due to its abundance of fuel to be utilized for energy production. Waste feed stock has been a limited use of fuel for oxy-fuel combustion. However, since Circulating Fluidized Bed Combustion (CFBC) technology has lots of advantages in application of heat recovery facility, waste sludge should be retrofitted or newly commercialized to circulating fluidized bed combustion facilities.

In order to carry out this concept, Jang et al. (2016) simulated a test for operating a 30 KW_th circulating fluidized bed oxy-fuel pilot test bed with a circulating fluidized cold bed. Heating value of the waste sewage was analyzed by using an AC-350 calorimeter, analysis of gaseous components was conducted by a portable gas analyzer while mass balance and carbon contents of bottom ash and fly ash were measured. Pilot test was conducted in a Circulating Fluidized Bed combustion system consisting of a riser, cyclone, down-comer and loop-seal with experiment test way carried out using 30 KW_th circulating fluidized bed combustor operating with air and oxy-fuel conditions at optimized feeding rate of waste sewage sludge of 13kg/h and oxygen injection rate between 21% and 40%.

According to results obtained from cold bed test, minimum fluidization velocity (U_mf) was determined as 0.120m/s. Fast fluidization was observed to initiate from 2.5m/s of air superficial velocity when the solid circulation rate was above 10 Kg/m³.

Test results for 30 KW_th Circulating Fluidized Bed oxy-fuel pilot test bed showed the pressure profiles showed higher pressure drop of air combustion than oxygen injection rate of 21 to 40% which is mainly due to loss of dynamic energy from pathway of fluidization air related to parameters such as kinematic viscosity, density, length of tube, and flow rate.

Temperature profiles of air combustion were relatively higher than that of 21% oxy-fuel combustion. However, temperature trends of oxy-fuel combustion above 25% was  higher than that of air 21% oxy-fuel combustion which showed that temperature trends gradually increases as oxygen injection rate increases.

Species of aluminum, potassium and calcium were seen to be dominated in fly ash from air and oxy-fuel waste sewage sludge at oxygen injection rate range of 21% to 40%. Concentrations from bottom ash were more decreased in range of 21% to 25% of oxygen fuel condition than air and oxy-fuel condition above 30%. Traces of zinc, copper, chromium and nickel was also found in both ashes.

Oxy-fuel combustion at the range of 21% to 25% would be more beneficial to carbon capture and storage technologies and to operate for longer time period other than other cases in waste sewage sludge.

Acknowledgement

This work was supported by the BK21 Plus in Environmental Engineering of Yonsei University. This work was also supported by the Korea Institute of Energy Technology Evaluation and Planning(KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20164030201250).