Due to the ongoing growth in global coal use, especially in the power sector, CO2 mitigation through carbon capture and storage is essential in order to meet international climate targets. The oxy-fuel process represents an option for carbon capture in fossil-fired power plants. Within this process, the fuel is combusted with pure oxygen instead of air in order to produce a CO2-rich flue gas stream, which can then be treated and stored. Applying this process to the firing technology of circulating fluidized bed (CFB) combustion can combine potential benefits in terms of CO2-capture efficiency with well-known unique CFB features. Within the scope of this work, the applicability of the in-situ desulfurization using raw limestone in an oxy-fuel-fired CFB is investigated. The change in reaction atmosphere has an impact on the sulfation reaction mechanisms and reaction rates, and hence influences the process operation.
In a first step, the reaction kinetics of the limestone sulfation are examined by batch experiments at laboratory scale. Taking into account the influence of increased CO2 and H2O-concentrations in oxy-fuel combustion, the sulfation behavior is investigated in atmospheres from air-firing to oxy-fuel-firing with varying temperatures.
These findings are then confirmed by the subsequent oxy-fuel CFB coal combustion tests at pilot scale including the application of in-situ desulfurization. In stable test runs with similar combustion settings as well as comparable limestone injection during both air- and oxy-fuel-firing, a change in the optimal temperature for sulfur capture is determined. In both cases, air- and oxy-fuel-firing, a similar high level of sulfur capture efficiency is reached, though under oxy-fuel conditions, the temperature in the combustion chamber had to be increased from 850 °C, the optimal temperature for air-fired CFBs, to 875 °C.
Operating the oxy-fuel CFB at these conditions with overstoichiometric limestone flow leads to inevitable fractions of calcined limestone in the circulating bed material and in the fly ash. In parts of the combustion system, where the solids are cooled down below the decomposition temperature under oxy-fuel atmosphere, a recarbonation of CaO can occur and stable agglomerates and depositions might form. Within this work, the deposition formation through recarbonation on immersed heat exchanger tubes in external fluidized bed heat exchangers are investigated in a BFB reactor at laboratory scale.
The results of the investigations carried out within this work provide an integrated insight into the behavior of limestone as a desulfurization sorbent to be used in oxy-fuel CFB systems. The applicability of this sulfur capture technology is verified for oxy-fuel-combustion, if required changes in the operation of the firing system are implemented.