title: Negative CO₂ Emissions in the Lime Production Using an Indirectly Heated Carbonate Looping Process
Creators: name: Greco-Coppi, Martin
name: Hofmann, Carina
name: Ströhle, Jochen
name: Epple, Bernd
Publisher: name: UNSPECIFIED
Year: 2024

description: Lime plants are responsible for the production of raw materials that are widely used in the agriculture and the industrial sector. Lime-related products are obtained from the calcination of limestone (mainly CaCO3) at high temperature (900-1200 °C). The calcination reaction is highly endothermic, and thus a heat input, e.g. from the combustion of fuels such as coal, coke, and secondary fuels, is required. CO2 is emitted as a result of the combustion. Additional CO2 is produced due to the chemical conversion of CaCO3 into CaO during the calcination. This so-called “process CO2”, which can only be avoided through CO2 capture, represent approximately 65% of the total CO2 emissions. Overall, the total CO2 emissions per ton of burnt lime vary between 1 to 2 tCO2/tlime.
In order to capture the CO2 emissions from lime plants, post-combustion technologies are to be integrated into the production process. Nonetheless, the majority of these technologies have very high energy requirements, which increase the costs of the final products and reduce the efficiency of the entire system considerably. One noteworthy post-combustion carbon capture technology is the carbonate looping process (CaL). The CaL has the potential to efficiently capture the CO2 from lime plants without considerably increasing the energy requirements of the entire process. The CO2 capture is achieved utilizing limestone as a sorbent, i.e. the raw material of the lime production facility, which makes CaL especially interesting for the application into lime plants. The sorbent binds CO2 from the kiln flue gases in a carbonator, and is regenerated with a temperature increase at a calciner. This technology has been successfully operated up to the pilot scale in Darmstadt, Germany (1 MWth), and in La Pedrera, Spain (1 MWth).
For the regeneration of the sorbent in the standard CaL fuel is burnt directly in the calciner. For this, technically pure oxygen is used, which requires an air separation unit (ASU). The ASU can be avoided by indirectly heating the calciner, and thus the energy penalty is further reduced. One excellent means to achieve this is through heat pipes, which transfer heat from an external combustor into the calciner via evaporation and condensation of a fluid. This indirectly heated carbonate looping process (IHCaL) present several advantages compared to the oxy-fired CaL: reduced energy requirement, improved sorbent activity, lower sorbent attrition rates, and high purity of the captured CO2. The IHCaL has been successfully operated for 400 h at the 300 kWth facility of the Technical University of Darmstadt. Additional test campaigns in Darmstadt will be carried out during 2022 to prove the operability of the IHCaL process under lime plant conditions at the pilot scale. 
At the Technical University of Darmstadt, novel concepts for the integration of the IHCaL process into the lime production were developed and evaluated through process simulation. The published results consider the utilization of dried lignite as fuel for both the lime kiln and the IHCaL combustor. Nevertheless, the utilization of renewable fuels of high biogenic content, applied to these concepts has not been discussed yet.
This work presents the results of the energy and mass balances of two IHCaL concepts for the lime production, utilizing high quality solid recovered fuel (SRF; LHV: 21.3 MJ/kg; 40% biogenic carbon fraction) to provide the thermal energy for the carbon capture. Furthermore, a heat recovery steam cycle is implemented in order to produce electrical power from the high temperature output flows (650 °C and 900 °) and the cooling heat from the carbonator (650 °C). For the comparison of the SRF concepts with the lignite concepts, key performance indicators are calculated, namely, net CO2 emissions—considering indirect CO2 emissions and biogenic content—, CO2 capture efficiency, avoided CO2 emissions, and specific primary energy consumption per CO2 avoided (SPECCA).