CO2 Capture for Cement Technology

Production of cement is an energy intensive process and is the source of considerable CO2emissions. Itis estimated that the cement industry contributes around 8% of total global CO2emissions. CO2is oneof the major greenhouse gases. In the atmosphere, the CO2concentration has increased from 310 ppmvin 1960 to 390 ppmv in 2012, probably due to human activity.

A lot of research is being carried out forreducing CO2emissions from large stationary sources. Ofwhich, the carbonate looping process is anew process and has the potential to reduce CO2emissions with lower energy penalties. Most of thework performed recently has focused on CO2capture from fossil fuel-based power plants.

Inherently,this process is especially suitablefor cement plants, as CaO used for CO2capture is also a majoringredient for clinker production. Thus, a detailed investigation was carried outto study the applicationof the carbonate looping process to the cement industry. In order to study the application of thecarbonate looping process to cement industry, the project work is divided into three scales: 1) atparticle scale (TGA), 2) at reactor scale (Fluid-bed) and 3) at process scale (process modeling Pro/II).The results from TGA revealed that the CO2capture capacity of cement raw meal as a function of cyclenumber had a similar trend to that of limestone, i.e. the CO2capture capacity decreased with increasingcycle number.

However, the maximum CO2capture capacity of calcined cement raw meal (17%, firstcycle) was much lower compared to natural limestone (28%, first cycle), where calcination was carriedout under realistic conditions (950°C, CO2). After changing the calcination atmosphere from CO2to N2,the difference in the CO2capture capacity of the sorbents was large, but the capture capacitiesincreased for both limestone (58%) and raw meal (28%).

To investigate the influence of temperature,calcination was carried out at 850°C in N2. The results (limestone 65% and raw meal 63%) show thatthere was no significant difference in the CO2capture capacities under these conditions.ivTo reveal the reason behindthis difference in the CO2capture capacity of limestone and cement rawmeal, experiments were performed under realistic conditions to investigate the influence of the maincomponents (Al2O3, Fe2O3, SiO2) of cement raw meal on the major component i.e. limestone.

Theresults show that each component had a unique effect on the CO2capture capacity of limestone. BETsurface area measurements, SEM analysis and XRD analysis techniques were carried out on calcinedsamples to estimate the surface area of the raw meal (2 m2/g) compared to limestone (4 m2/g), tovisualize the surface morphology of calcined limestone in the raw meal, which indicated larger grainscompared to the grains of calcined natural limestone, and to investigate any interactions betweenlimestone and other components in the raw meal, which showed no significant interactions between thecomponents, respectively.In the fluidized bed reactor, cycle and continuouscarbonation experiments were carried out.

Cycleexperiments results on the trend in CO2capture capacity of sorbent (limestone and simulated raw meal)was similar to the TGA experimental results. Further, the fluidized bed cyclic experiment results showthat the CO2capture capacity of cement raw meal was similar to limestone, as a function of cyclenumber because the calcination conditions were mild (800°C in air).

The reaction rate constant wasestimated as a function of the conversion of bed. In the fluidized bed reactor reaction rate constant inthe initial fast reaction regime relevant for the carbonate looping process is 2 [m3/kmol.s] which dropswith conversion and this rate constant is comparable to the value estimated from the TGA, which is 3.5[m3/kmol.s].Continuous carbonation experiments were carried out to investigate the performance of carbonator as acirculating fluidized bed reactor.

A new experimental method was applied for accurate measurement ofthe particle recirculation rate which is the key parameter in a circulating fluidized bed reactor. Theexperimental results show that the most influencing parameter on the performance of carbonator is the v inlet Ca to C molar ratio. In this experiment, more than 80% of the inlet CO2 was captured by highly deactivated limestone, which had a maximum CO2 capture capacity of 11.5%, with an inlet Ca/C ratio of 13.

So, the performance of the carbonator can be defined by the inlet Ca/C ratio, which can be estimated if the maximum capture capacity of limestone is known. A circulating fluidized bed reactor model was proposed where the particle distribution profile along the reactor height was estimated from the experiments.

The reactor model was validated with experimental results, and it was used to simulate different operating conditions for the carbonator. Based on the model simulation results a particle recirculation of 2-5 kg/m2s is sufficient for 90% CO2 capture efficiency depending on active fraction, inlet CO2 concentration and composition of particle stream.

Based on the main experimental results, i.e. the CO2 capture capacity of raw meal as a function of cycle number and the main parameter that controls the performance of the carbonator, a process model integrating the carbonate looping process with the cement pyro-process was simulated. The process simulation results indicate that the CO2 emission was only 0.07 kg/ kg cl, with an energy penalty of 2 MJ/kg CO2 captured, whereas in a normal cement plant, it is 0.9 kg/ kg cl.

However the thermal energy demand in the integrated plant increases from 3.9 MJ/ kg cl to 5.6 MJ/ kg cl. But on the other side this additional energy spent can be recovered as a high quality heat to generate electricity. The potential to generate electricity depends on the scale of the plant, the bigger the production capacity of cement plant the better, with capacity higher than 3400 tons of clinker/day is required to produce captive electricity to meet the demand both from the cement plant operations and from the CO2 capture system operations.