Jochen Ströhle

Chemical-Looping Combustion of Hard Coal: Autothermal Operation of a 1 MWth Pilot Plant

Chemical-looping combustion (CLC) is an emerging carbon capture technology that is characterized by a low energy penalty, low carbon dioxide capture costs, and low environmental impact. To prevent the contact between fuel and air, an oxygen carrier is used to transport the oxygen needed for fuel conversion. In comparison to a classic oxyfuel process, no air separation unit is required to provide the oxygen needed to burn the fuel. The solid fuel, such as coal, is gasified in the fuel reactor (FR), and the products from gasification are oxidized by the oxygen carrier. There are promising results from the electrically heated 100 kWth unit at Chalmers University of Technology (Sweden) or the 1 MWth pilot at Technische Universitat Darmstadt (Germany) with partial chemical-looping conditions. The 1 MWth CLC pilot consists of two interconnected circulating fluidized bed reactors. It is possible to investigate this process without electrically heating due to refractory-lined reactors and coupling elements. This work presents the first results of autothermal operation of a metal oxide CLC unit worldwide using ilmenite as oxygen carrier and coarse hard coal as fuel. The FR was fluidized with steam. The results show that the oxygen demand of the FR required for a complete conversion of unconverted gases was in the range of 25%. At the same time, the carbon dioxide capture efficiency was low in the present configuration of the 1 MWth pilot. This means that unconverted char left the FR and burned in the air reactor (AR). The reason for this is that no carbon stripper unit was used during these investigations. A carbon stripper could significantly enhance the carbon dioxide capture efficiency.

Technical and Economical Assessment of the Indirectly Heated Carbonate Looping Process

Carbonate looping promises low energy penalties for postcombustion CO2-capture and is particularly suited for retrofitting existing power plants. To further improve the process, a new concept with an indirectly heated calciner using heat pipes was developed, offering even higher plant efficiencies and lower CO2 avoidance costs than the oxy-fired standard carbonate looping process. The concept of the indirectly heated carbonate looping (IHCL) process was tested at sufficient scale in a 300 kWth pilot plant at Technische Universitat Darmstadt. The paper presents a technical overview of the process and shows first test results of the pilot plant. Furthermore, the concept is economically evaluated and compared to other carbon capture processes.

Validation of a Detailed Reaction Mechanism for Sulfur Species in Coal Combustion

The understanding of the kinetics leading to the formation of gaseous sulfur species is of great importance because of their corrosive nature in coal-fired energy systems. In this study, reaction kinetics of important gaseous sulfur species in coal combustion are investigated using a detailed reaction mechanism with 10 sulfur species and 49 sulfur-related elementary reactions in combination with GRI-Mech 2.11 for hydrocarbon related reactions and a global two-step reaction mechanism for coal devolatilization and char oxidation. The reaction mechanism was applied to numerical simulations of an entrained-flow reactor, where lignite was burned with varying air ratio. The results of numerical simulations showed that H2S released during devolatilization is fast oxidized and is formed again under reducing conditions as coal conversion proceeds. Calculated concentrations of CO2, CO, and H2S show very good agreement with experimental data. SO2 concentrations under sub-stoichiometric conditions are overpredicted, most likely because not all sulfur is released or because part of the sulfur is bound by minerals in the fly ash in the experiments, which is not taken into account by the simulations.