The spatial proximity of CO₂-intensive processes at a site enables the joint and efficient utilisation of plants and infrastructure, as well as synergies with regard to the production and use of consuma-bles, waste heat, or other by-products. The integration of disparate CO₂ capture processes can yield specific benefits. The integration of exhaust gas pathways across multiple production lines has the potential to significantly reduce the requirement for specialized plant technology and enhance energy utilisation.
A common combination of production processes at a site, which results from the utilisation of lime-stone as a raw material, is the combination of cement clinker and lime production. In most cases, clinker production generates a significantly larger CO₂ stream than lime production due to the larger production volumes.
The lower absolute amount of CO₂ and the higher product-related CO₂ density of lime production compared to clinker production result in higher specific costs for CO₂ capture from lime production when separate separation plants are installed. However, economies of scale can be achieved by merging the waste gas streams and utilising a shared CO₂ capture or processing plant, thereby reducing the CO₂ avoidance costs, particularly in the context of lime production. Moreover, the con-solidation of smaller CO₂ sources facilitates the development of the CO₂ infrastructure, as the num-ber of emitters to be connected is reduced. However, it should be noted that the aforementioned syn-ergies are not confined solely to the domain of lime production. The combination of processes can be extended to encompass other combustion processes, thereby facilitating the efficient decarbonisa-tion of these processes.
A number of projects are being planned in the cement industry that utilise oxyfuel technology for the purpose of enriching the exhaust gas stream of a cement clinker production plant with CO₂. The CO₂-rich exhaust gas from such a plant can be concentrated to the required purity for storage in a cryogenic CO₂ processing unit (CPU) and cooled and compressed for transport. At a combined site consisting of an oxyfuel process for the production of cement clinker and a conventionally operated lime kiln or other incineration facilities with an end-of-pipe solution for CO₂ enrichment, various parts of the waste gas purification system or the CPU of the oxyfuel process can be used together. The selection of the technology for the end-of-pipe solution is a multifaceted process, with numerous combinations to be considered. The selection of membrane processes, PSA/TSA processes, or amine scrubbing, among others, leads to additional synergistic effects that can be utilised.
The complete separation of carbon dioxide (CO₂) from clinker production using amine scrubbing is often unsuccessful due to the high thermal energy requirements of amine scrubbing, which can typically only be met to a maximum of 50% by the waste heat from the clinker burning process. Conversely, this waste heat frequently suffices to meet the complete heat requirement of the amine scrubber for CO₂ separation from the waste gas of the lime kiln or a comparable firing process. The establishment of a heat displacement system, integrating the heat of the amine scrubbing process, would ensure the utilisation of this energy with maximum efficiency, while concurrently decarbonis-ing the process with minimal energy and equipment input. Alternatively or in addition to heat, other material or energy flows can be exchanged between the two processes or with the CPU to harness further synergistic effects.
In addition to enhanced energy efficiency, the integration of processes can be employed to optimise the capture rate. When viewed in isolation, many capture technologies have a maximum capture efficiency of 90-95%. However, the combination of different processes makes it possible to post-treat vented gas streams with a residual CO₂ load in the connected process. To illustrate this, consider the gas flow exiting the CPU, which can be directed to the amine scrubber with a residual load of CO₂ (CPU slipstream). This approach has been shown to enhance the separation efficiency of the system network significantly (figure 1).
