Evaluation of Ash-Free Coal for Chemical Looping Combustion -
Part I: Thermogravimetric Single Cycle Study and the Reaction
Mechanism
Azar Shabani,
1
Moshfiqur Rahman,
1
Deepak Pudasainee,
1
Arunkumar Samanta,
1
Partha Sarkar
2
and Rajender Gupta
1
*
1. Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, T6G 2V4, Canada
2. Environment and Carbon Management Division, Alberta Innovates - Technology Futures, Edmonton, AB, T6N 1E4, Canada
In this study, performance of ash-free coal (AFC) was evaluated for chemical looping combustion (CLC) using CuO as an oxygen carrier during
reduction and oxidation processes in a thermogravimetric (TG) analyzer. TG experiments with CuO/AFC mixture with different ratios (10:1–50:1) at
various temperatures ranging from 450 to 900 8C were performed and the results were analyzed in greater detail for the best suitable ratio (R30).
The CLC reaction mechanism includes release of volatile matter until 450 8C and auto- decomposition of CuO at 790 8C. CuO reduction induced by
volatile matter from AFC occurred at temperatures below 450 8C. Char combustion induced by oxygen release from CuO occurred at about 800 8C
as auto-decomposition of CuO occurs at 790 8C. A slight loss in mass, observed between 450 8C and 790 8C, is attributed to solid-solid interaction of
char and CuO powder.
Advanced analytical techniques such as XRD, SEM, and ultimate analyses were employed to characterize the oxygen carrier and to understand the
possible interaction of the oxygen carrier with volatile matter and char. Overall, AFC as the solid fuel showed a promising oxidation/reduction
performance and has great potential to be used in the CLC process.
Keywords: chemical looping combustion (CLC), ash-free coal (AFC), copper oxide (CuO), reduction/oxidation and CLC mechanism
INTRODUCTION
C
oal is a major source of power generation worldwide. One of
the major concerns of the direct coal combustion is release
of ash-forming minerals. In order to address this, coal de-
mineralization has been carried out by means of physical and
chemical cleaning. One approach of chemical de-mineralization of
coal is solvent extraction of coaly matter—the product commonly
called ash-free coal (AFC).
[1]
AFC gets rid of all the operational
problems related to ash-forming minerals in utilizing coal in
different advanced technologies such as catalytic gasification,
[2,3]
direct carbon fuel cell.
[4]
AFC has an advantage in avoiding
interaction of ash and oxygen carrier in the chemical looping
combustion (CLC) process, one of the CO
2
capture technologies.
CLC is a novel technology for CO
2
capture which has been
studied widely. CLC technology, for clean energy production from
fossil fuels, could avoid the requirement of pure oxygen by using
oxygen carriers which deliver oxygen to the fuel as they circulate
between the fuel and the air reactors (Figure 1). It thus allows
combustion of fuel in oxygen without requiring an expensive air
separation unit. Predominantly, CLC produces mainly CO
2
and
H
2
O as flue gas and, consequently, sequestration-ready CO
2
with
little or no energy penalty.
The CLC process consists of two reactors: an air reactor and a
fuel reactor. In the air reactor, the oxygen carrier is oxidized by air.
Then, unused oxygen and nitrogen gases exit from the air reactor.
After oxidation in the air reactor, the oxygen carrier goes into the
fuel reactor and transfers oxygen to the fuel. Afterwards, the
reduced metal oxide is circulated back to the air reactor for another
round of oxidation. CO
2
and water vapour are released from the
fuel reactor. Once the water condenses, a high purity of CO
2
remains. The role of the oxygen carrier, which is mostly a metal
oxide in CLC, is to transfer oxygen to the fuel. Therefore, there is
no direct contact between air and fuel. As a result, CO
2
is prevented
from being diluted with nitrogen. In general, the main benefits
from the CLC process are as follows: it is a high efficiency system; it
generates CO
2
without any nitrogen dilution and reduced NO
x
as
well,
[5,6]
and no energy consumption for separating CO
2
and N
2
.
There are the two reactions that occur in the CLC reducer and
oxidizer reactors:
[7]
Oxidation in the air reactor
M
y
O
x1
þ 1=2 O
2
! M
y
O
x
ð1Þ
Reduction in the fuel reactor
2n þ m ð ÞM
y
O
x
þ C
n
H
2m
! 2n þ m ð ÞM
y
O
x1
þ mH
2
O þ nCO
2
ð2Þ
The oxidation reaction in the air reactor is always exothermic,
while the reduction reaction in the fuel reactor can be exothermic
or endothermic, which depends on the type of oxygen carrier used
in the CLC system. The total heat from the reduction and oxidation
reactions is equal to conventional combustion heat with direct
contact between air and fuel.
[8,9]
Most of the former studies on CLC
have been investigated with gaseous fuels; however, the use of
* Author to whom correspondence may be addressed.
E-mail address: rajender.gupta@ualberta.ca
Can. J. Chem. Eng. 9999:1–12, 2016
©
2016 Canadian Society for Chemical Engineering
DOI 10.1002/cjce.22721
Published online in Wiley Online Library
(wileyonlinelibrary.com).
VOLUME 9999, 2016 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 1