The CO2 emissions of cement industry accounts for 15% of the whole industrial CO2 emission in China. China is a big country for the production and consumption of cement, and the cement production of China has been ranked first for 24 years continuously in the world. Meanwhile, Chinese cement production and its CO2 emissions is growing year after year. It is calculated that 0.815 tons of CO2 emissions were produced directly from the production of every ton of cement clinker. Therefore, abatement of CO2 emission is becoming the focus of government attention and academic research. CCCR technology (a Ca-based sorbents Cycling Calcination/Carbonation Reaction technology) is a kind of new and effective method to capture CO2 in flue gas from industry.
Based on the cement industry, this paper studied the carbonation of a Ca-based absorbents and its stability in cycling carbonation reaction under high CO2 concentration (CCO2>30%), commonly found in cement industry. The orthogonal tests were carried out to determine the appropriate parameters for the CO2 capture process. Composition of Ca-based absorbents, the size of particle, calcining temperature, carbonating temperature, carbonating reaction time for the cycling carbonation rate of absorbents (XN) as well as the kinetics of limestone thermal decomposition and CaO carbonation were studied.
The results of orthogonal experiments show that, Ca-based sorbents can keep fine pore structure and able to resist sintering when contains a certain amount of dolomite. As a consequence, the XN of sorbents was improved. And we chose the calcination temperature of 850°C, carbonation temperature of 700°C and the CO2 concentration of CCO2=35% for the subsequent experiments.
The investigation were carried out under a high CO2 concentration condition (≥30%) to study the kinetics of limestone thermal decomposition and CaO carbonation in a stacked state and a simulated suspension state respectively. Under a stacked state or a simulated suspension state, limestone thermal decomposition process could be both modelled as a randomly nucleation, accompanying by particle size growth. The reaction order of limestone thermal decomposition varied from 2/5 to 3/4 under different CO2 concentrations. The higher the CO2 concentration was, the higher activation energy E and pre-exponential factor lnA could be. The activation energy varied from 958.28 to 2556.10 kJ/mol and the pre-exponential factor lnA changed from 103.67 to 242.44. In addition, the activation energy E rose exponentially with an increase of CO2 concentration in a stacked state, followed a relationship as.
On the basis of these studies,we found that the CO2 capture and sequestration (CCS) capability of limestone in flue gas was dramatically decline after several cycles due to the surface“sintering”. In order to improve the efficiency of recirculated limestone, we modified the limestone with four kinds of materials such as carbon materials, metal oxide and natural mineral (attapulgite and vermiculite).
A variety of carbon, such as carbon black, graphite, bamboo woods and coconut woods were studied. Our results show that coconut woods is the best carbon source and it has the performance of 13% increase of efficiency with 2 wt% adding amount.
we used some metal oxides such as MgO,SiO2 and Fe2O3 to modify the morphology of the limestone particles to increase the CO2 capture capability of limestone. The efficiency can be increased 7%`8% after adding 1wt% MgO particles.
The limestone modified with attapulgite, a kind of mineral with nano-fibrous structure, can reduce the sintering phenomenon between the limestone particles. So the carbonation rate during the CCS process can be improved effectively. The best performance of 13.11% increase of efficiency with 5wt% adding amount.
We used natural vermiculite which has unique piece structure and high temperature resistant properties to modify the limestone surface in order to delay the limestone particles sintering. The ability of CO2 capture, the morphology changes have been tested by TGA, XRD, FTIR and SEM. The efficiency can be increased 8.12% by adding 1wt% vermiculite.