Carbon dioxide capture methods for industrial sources.

Abstract

In order to reduce the rate of climate change, particularly global warming, it is imperative that

industries reduce their carbon dioxide (CO2) emissions.

A promising solution of CO2 emission reduction is Carbon dioxide Capture and Storage (CCS)

by sequestration, which involves isolating and extracting CO2 from the flue gases of various

industrial processes, and thereafter burying the CO2 underground.

The capture of CO2 proved to be the most challenging aspect of CCS. Thus, the objective of this

research was to identify the most promising solution to capture CO2 from industrial processes.

The study focussed on capturing CO2 emitted by coal power plants, coal-to-liquids (CTL) and

gas-to-liquids (GTL) industries, which are common CO2 emitters in South Africa.

This thesis consists firstly of an extensive literature review detailing the above mentioned

processes, the modes of CO2 capture, and the various CO2 capture methods that are currently

being investigated around the world, together with their benefits and drawbacks in terms of

energy penalty, CO2 loading, absorption rate, capture efficiency, investment costs, and operating

costs. Modelling, simulation, and pilot plant efforts are also described.

The study reviewed many CO2 capture techniques including solvent absorption, sorbent capture,

membrane usage, hydrate formation, and newly emerging capture techniques such as enzyme

based systems, ionic liquids, low temperature cryogenics, CO2 anti-sublimation, artificial

photosynthesis, integrated gasification steam cycle (IGSC), and chemical looping combustion

The technique of solvent absorption was found to be the most promising for South African

industries. Vapour-liquid-equilibrium (VLE) measurements of solvent absorption using amine

blends were undertaken, using blends of methyl-diethanol amine (MDEA), diethanol amine

(DEA) and water (H2O) with composition ratios of 25: 25: 50 wt% and 30: 20: 50 wt%

respectively, and with CO2 and N2 gases at CO2 partial pressures of 0.5 to 10.5 bar. Experiments

were conducted under system pressures of 5 to 15 bar and temperatures of 363.15 and 413.15 K,

using a static analytic apparatus. CO2 liquid loading results were analysed and discussed.

The experimental data were regressed in Matlab (R2009b) using the Posey-Tapperson-Rochelle

model and the Deshmukh-Mather model. The Matlab programmes are presented along with the

regressed binary interaction and model parameters. The accuracy of model predictions are

discussed.

Thereafter an Electrolyte-NRTL model regression and simulation of the absorption process was

conducted using Aspen Plus V 7.1. for flue gas compositions, solvent compositions,

temperature, and pressure conditions similar to that of process operating conditions. CO2

loading, design factors, CO2 recovery, and CO2 purity results were analysed and compared where appropriate, with experimental results. Finally a general preliminary energy efficiency

and cost analysis was conducted based on the simulation results.

The main conclusions reached are that the amine solvent blend containing 25:25:50 wt% of

MDEA:DEA:H2O, produced higher CO2 loadings for its respective system conditions than other

solvents studied and those found in literature. However, absorption of CO2 was found to be

highly dependent on system temperature and pressure.

The Deshmukh-Mather model provided higher accuracy than the Posey-Tapperson-Rochelle

model, producing CO2 loading predictions with a relative error not exceeding 0.04%, in 1.5 to 3

minutes using a dual core processor.

Aspen absorption simulations provided significantly lower CO2 loading results than those

experimentally obtained, due to the low contact time achieved and higher temperature

dependence in the proposed absorption process. Process improvements were highlighted and

implemented to increase CO2 recovery and purity. Energy penalty values were found to be

higher than those found in literature, but room for process and design improvement was

identified and recommendations were given. Investment cost estimates were found to be

justifiable and within reason. Limitations of the simulation were also identified and discussed.