Modelling a calciner with high inlet oxygen concentration
for a calcium looping process
Jarno Parkkinen1, Kari Myöhänen1, Juan Carlos Abanades2, Borja Arias2, Timo Hyppänen1
1
Lappeenranta University of Technology, Lappeenranta, Finland
2
Spanish Research Council, CSIC-INCAR, Oviedo, Spain
Abstract
A calcium looping (CaL) process is a carbon capture technology which utilizes calcium oxide
(CaO) to remove carbon dioxide (CO2) from the flue gas of a power plant (Figure 1). The capture
process takes place in two interconnected circulating fluidized bed (CFB) reactors: carbonator and
calciner. In the carbonator, CaO reacts with CO2 from the flue gas forming calcium carbonate
(CaCO3). After carbonation, CaCO3 is transferred to the calciner where it calcines to regenerate
CaO and produce a CO2 rich flow, which is suitable for purification, compression, transport, and
storage. The CaL process can be retro-fitted to an existing power plant or integrated to be part of a
cement plant.
Figure 1. Simplified process scheme of CaL process with high O2 inlet to the calciner.
Like most capture technologies, CaL process has a high energy demand, which reduces power plant
efficiency. In CaL process, the main energy penalty comes from air separation unit (ASU), which is
needed to produce oxygen for combustion in the calciner. The combustion in calciner raises the
calcium material temperature from 650 °C, which is the carbonator operation temperature, to
around 900 °C, which is needed for calcination reaction. The required oxygen flow depends on the
required heat input in the calciner. In oxy-fired combustion, the flue gas recycling is used to control
the oxygen level. The energy penalty and the operating and capital costs of the unit can be reduced
by reducing the recirculation of flue gas to the calciner, which increases the concentration of O2 in
the oxidant flow to calciner. This results to a smaller total gas flow through the calciner, which
results to a smaller required heat input to heat the gas, smaller fan power, and smaller crosssectional area of the reactor with same superficial fluidization velocity. Moreover, with smaller CO2
recycle, the partial pressure of CO2 is lower at the bottom of the calciner, which decreases the
calcination temperature and improves the calcination efficiency.
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Oxygen fired CFB boilers have been widely studied and tested in small scale up to 30 MWth
thermal input. Large scale design concepts have been presented for up to 600 MWe scale. In an
oxy-CFB boiler, increasing the inlet oxygen concentration results to a higher combustion
temperature and increases the share of heat that needs to be recovered inside the CFB furnace. In
recently published large scale oxy-CFB designs, the maximum inlet O2 concentration has been set
below 50%. With much higher inlet O2, the challenges related to heat recovery in the furnace would
increase. In contrast, in a calciner, there are no similar heat transfer challenges, as the heat sources
due to combustion reactions are balanced by heat sinks due to CaCO3 calcination reaction. This
provides a possibility to apply considerably higher O2 concentrations.
In this study, a calciner with high inlet oxygen concentration has been studied with a three
dimensional, steady-state, CFB process model. First, a 3D modelling was carried out for the 1.7
MWth pilot unit calciner in La Pereda CaL pilot, which is the largest test facility for CaL process.
The data from pilot unit was used to validate the model. Good results from the pilot unit tests with
high inlet oxygen concentration motivated to design a 3D model for a 200 MWth commercial scale
calciner (Figure 2). In a large scale unit, the lower reactor geometry and placement of material inlets
are very important because in a CFB reactor, the major share of the reactions takes place at the
bottom part. Properly located inlets provide good mixing of both solids and gases in the calciner,
which is one key factor when fuel and oxygen demand are optimized and the hot spots inside the
calciner are prevented.
Figure 2. Initial 3D-model results of a 200 MWth calciner: oxygen concentration, temperature, and calcination rate.
Acknowledgement:
The work presented in this paper is being partially funded by the European Commission under the
RFCS “CaO2” project.
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