NÚMEROS Y ÁLGEBRA

CCUS is expected to have an important contribution to the reduction of the carbon footprint of various industrial processes. In general, there are three different techniques for carbon capture from industrial processes: pre-, post- and oxy-combustion capture. Pre-combustion capture method is based on partial oxidation of the fuel through catalytic reforming or gasification for example. Post-combustion capture is an end-of-pipe solution: CO2 is captured from the flue gases from the industrial process. Downside is

that often the CO2 concentration in the flue gases is low, and thus large amount of energy

and major facilities are required for the capture. In oxy-combustion (also called oxy-fuel combustion) method oxygen or oxygen rich gas such as a mixture of oxygen and recycled flue gas (RFG) is utilized instead of air for combustion to avoid nitrogen contamination. The absence of nitrogen increases partial pressure of CO2 in the flue

gases and thus simplifies the capture process. Disadvantages are the investment and ancillary energy needed for cryogenic air separation for oxygen production. (Cormos et

al. 2018, p. 883; Boot-Handford et al. 2014)

For lime and cement production CCUS is a viable option as the production process is highly CO2 intensive and relatively simple with only one flue gas stream (Bui et al. 2018;

Leeson et al. 2017). The CO2 concentration in the flue gases from lime kiln varies from

22 % to 29 % (Nwaoha et al. 2018, p. 7102). In case of cement plant, the CO2

concentration in flue gases from the kiln varies from 14 % to 33 % (Bosoaga et al. 2009, p. 134). Carbon neutral fuels and energy in combination with carbon capture open up opportunities for even “below zero emissions” operation (Eriksson et al. 2014). Pre- combustion technologies do not capture process emissions from lime or cement production which reduces CO2 capture rate considerably. In addition, effects on the

product quality need to be considered when developing CCUS technologies for lime and cement production. Suitable CO2 capture technologies are solvent scrubbing, oxy-fuel

combustion, calcium looping and direct capture. (Hills et al. 2016)

Solvent scrubbing is an end-of-pipe technology and thus does not have direct effects on the lime or cement manufacture process nor the product (Hills et al. 2016). Amine are one option for solvent used in the capture process. The amine sorbent is contacted with the flue gases in an absorption column where CO2 is absorbed. The CO2 rich solvent is

then transferred to a separate column in which heat is required for CO2 desorption and

is high and there are other environmental side effects from amines. (Cormos et al. 2018, p. 884) Thus, amine sorbents are not likely to be used on industrial scale CCUS technologies and more modern sorbents with lower heat requirement for regeneration are being developed (Leeson et al. 2017).

Oxy-fuel combustion has been acknowledged to be one of the most promising technologies for lime and cement production. Combustion in pure oxygen or increased oxygen concentration enhances the combustion process and increases flame temperature. Thus, RFG is usually added to moderate the temperatures. (Boot-Handford

et al. 2014) Especially in case of lime high temperatures may be harmful and increase

oxide melt phase (Eriksson et al. 2014). In cement production higher temperatures are required for clinker formation. Based on laboratory tests performed by the European Cement Research Academy (ECRA) oxy-fuel combustion has no considerable effects on the quality of clinker (Bui et al. 2018). Oxy-fuel combustion equipment affects the whole lime or cement production plant and requires large amount of R&D and will be expensive if implemented as retrofit. Thus, in case of retrofitting an existing plant a more feasible option seems to be partial oxy-fuel combustion approach. In partial oxy-fuel combustion only preheaters and pre-calciner depending on whether lime or cement plant is in question are oxy-fueled. (Hills et al. 2016)

In calcium looping (CaL), CO2 is captured with CaO sorbent. Traditionally in power

sector, CaL system consists of two parallel circulating fluidized beds in one of which capture of CO2 through recarbonation takes place and in the other calcination to

regenerate the CaCO3 back to CaO. (Hills et al. 2016) CaL cycle could contribute to the

thermal energy efficiency of the plant as it has high temperature heat recovery potential (Cormos et al. 2018).

Direct separation of CO2 is not included in the three main CCUS technologies. However,

in lime and cement production direct separation is a significant possibility. Direct separation in lime and cement production can be performed by indirect heating of limestone to prevent mixing of CO2 from calcination process and diluting combustion

gases. The gases from the indirectly heated kiln are virtually pure CO2 and should be

directly appropriate for compression, transport and utilization. Impurities in the lime or cement raw material may cause challenges for direct utilization. (Hills et al. 2016) Direct separation technology is piloted in an ongoing project called Low Emissions Intensity Lime and Cement (LEILAC). LEILAC is funded by the consortium of eleven partner organizations and the European Union by the Horizon 2020 research and

innovation program. (LEILAC 2017) A schematic illustration of the pilot scale kiln used in experiments of LEILAC project is presented in Figure 10.

Figure 10. The basic concept of the LEILAC project, adapted from (LEILAC

2017)

Limestone is fed from top to the calciner tube which is indirectly heated by conductive and radiative heat transfer with flue gases from combustion of natural gas. CO2 released

in calcination flows upstream in the calciner tube and exit the tube from the top. As the flue gases from combustion are segregated the gases from the calciner tube should only consist of CO2. The capacity of the pilot scale plant is around 240 t/d of cement raw meal

or 190 t/d of limestone. In case of cement, this unit will only replace the pre-calciner. LEILAC project only captures the process emissions but does not reduce energy-related emission. However, direct separation could be combined with other carbon capture technologies. (LEILAC 2017; Hodgson et al. 2019) In consequence of the success of LEILAC project, LEILAC 2 project pilots indirect heating with multiple fuel sources, particularly electricity. Also, retrofitting of an existing cement plant with the new technology is demonstrated. A pilot plant will be built alongside an operational cement plant. (CORDIS 2020)

Most of the CCUS technologies in general are still on pilot scale or demonstration phase and unfortunately there has not been enough progress to achieve the international and national climate targets. Lack of proven models for commercialization of CCUS technologies, political attitude and public acceptance cause challenges for a wider deployment of CCUS. (Bui et al. 2018) General constrains for CCUS technologies are high transportation, energy and other additional costs compared to emitting the CO2 and

limited storage locations and their permanence (Rahman et al. 2017). In addition, development of CCUS technologies for industrial processes lags behind power sector. However, as many industrial processes do not have possible alternative raw materials or processes CCUS is important for mitigation of CO2 emission. (Leeson et al. 2017)

Due to limitations on CO2 storage, utilization seems a more tempting option. CO2 with

hydrogen (H2) can be used to produce hydrocarbon fuels. In future, fuels produced of

CO2 emitted from power and industrial sector may serve as an option for finite fossil

fuels. (Rahman et al. 2017) One objective of the project that this thesis is part of is to study production of synthetic fuels with two methods: Fischer-Tropsch synthesis and methanation of H2 and CO2 to synthetic natural gas (SNG). However, in this thesis

processes for production of synthetic fuels are not discussed in more detail.