Oxy-Fuel Combustion of Solid Recovered Fuels in the Fluidized Bed Calciner of a 1 MWth Calcium Looping Unit

Carbon capture and storage (CCS) technologies represent an effective mean for climate change mitigation. Furthermore, the utilization of waste derived fuels, such as municipal solid waste (MSW) or solid recovered fuel (SRF) in CCS-equipped power or industrial systems might lead to the removal of CO₂ from the atmosphere, due to the biogenic fuel fractions. The calcium looping (CaL) process is a promising approach for CO₂ capture from various power or industrial systems. It is a post-combustion CO₂ capture technology using natural limestone based sorbents. The CO₂ in the flue gas stream is absorbed by calcium oxide (CaO) according to the reversible carbonation reaction in the carbonator. The formed calcium carbonate (CaCO₃) is transferred to the oxy-fired calciner to be regenerated subsequently. Whereas the carbonator operates at 650 °C, the temperature in the calciner needs to be raised to approximately 900 °C, in order to achieve a complete regeneration of the solid stream. The required heat is supplied by oxy-fuel combustion of additional fuel in the calciner. Within the past years, the CaL process has been tested thoroughly during thousands of operational hours in numerous test rigs worldwide. However, in most experimental investigations coal is the dominant fuel for the heat supply in the calciner.

Within the framework of this study, experimental results achieved during two consecutive test campaigns, each two weeks long, conducted at the 1 MW_th CaL Unit at Technische Universität Darmstadt are presented. During the test periods, two different types of SRF were used for the heat supply in the circulating fluidized bed (CFB) calciner. The two fuel candidates are derived from a German waste treatment facility, and are typically used in cement plants and dedicated combustion facilities. They show different properties such as maximal particle size (d_{max,SRF I} < 30 mm, d_{max,SRF II} <50 mm), lower heating value (LHV_{SRF I} > 16 MJ/kg, LHV_{SRF II} > 21 MJ/kg) and volatile content (SRF I: 60-70 m/m%_ar, SRF II: 70-80 m/m%_ar). The flue gas to be decarbonized in the CFB carbonator was based on the combustion of lignite in a combustion chamber on side. The main objective of these campaigns was the demonstration the steady state CO₂ capture by means of CaL technology while firing SRF in the calciner. Special focus has been given to assess the influence of the chlorine present in the fuel on the overall process performance. A continuously FTIR measurement at the calciner off gas and frequent solid stream sampling at the inlet and the outlet of the calciner cover the chlorine mass balance. In addition to the type of fuel for the calciner, the influence of several crucial CaL process parameters such as the operation temperatures of carbonator and calciner, the global solid circulation rate and the calcination conditions is discussed.

This work gives a deep insight to the experimental results achieved. In addition to pressure and temperature profiles of both CFB reactors, the particle size distribution of the circulating solid streams are presented. It was proven, that stable operation of the coupled CFB unit with CO₂ capture rates over 80 % in the carbonator are feasible while firing SRF in the calciner. The total CO₂ capture rate of the system accumulates to more than 90 %. Furthermore, the combustion of SRF and the calcination of the sorbent under air and oxy-fuel atmosphere are compared and assessed.