An Experimental Study of Carbon Dioxide Desorption from a
Calcium Oxide Based
Synthetic Sorbent Using Zonal Radio-Frequency Heating
Hydrogen (H2) is seen by many as a promising future energy carrier. As part of a larger study into the production
of H2 fuel cells, the main aim of this project is to test the efficiency of two different samples of Calcium Oxide
(CaO) in adsorbing Carbon Dioxide (CO2), using a zonal heating method which, in theory, should result in major
energy savings due to reduced heat losses to the environment.
Sorption Enhanced Reformation
Processes (SERP)
Overview
E. Pradhan, Dr. J. Fernandez, Prof. E. Rebrov – e.pradhan@warwick.ac.uk, J.Fernandez@warwick.ac.uk, E.Rebrov@warwick.ac.uk
Department of Engineering, University of Warwick, Coventry, CV4 7AL
Radio – Frequency (RF) Heating
Fuel Cells
Figure 1. Fuel cells are devices that
convert chemical energy from a fuel into
electricity through a chemical reaction of
positively charged H2 ions with oxygen
(O2). Fuel cells can eliminate pollution
caused by burning fossil fuels; for
hydrogen fuelled fuel cells, the only byproduct at point of use is water.
Figure 2. RF heating setup, with RF
generator connected to copper
coil via heating workhead. The use
of dielectric materials (electrical
insulators that can be polarized by
an applied electric field) means
heat is dissipated instead of
conducted through the material
enabling zonal heating. This also
allows much higher heating rates.
Sample 1
Sample 2
Capacity (mass %)
25
20
15
10
5
0
0
2
4
6
8
10
12
14
16
18
ΔHabs = -170 kJ mol􏰀-1
(4) (or adsorption/desorption)
Equations (4): The carbonation/calcination
Calcium
Oxide
equilibrium reaction. CaO has been investigated thoroughly as a
suitable sorbent for SER processes and is believed to be
thermodynamically the best candidate, among metal oxides, for CO2
capture in zero emission power generation systems. This experiment
tests two different samples of CaO powder.
The set up is shown in figures 1 and 2. CO2 and Nitrogen (N2) flow through the
gas inlet and into a mass spectrometer. A copper coil is placed around the
reactor, and is heated using a RF generator. With the intentions of carrying out
these same processes at much larger scales, industrial temperatures
(650°C - 850°C) are used. The research consists of twenty cycles worth of data.
Each cycle had two steps: adsorption/carbonation and desorption/calcination,
where the reversible reaction stated by equation 4 takes place at approximately
650°C and 850°C respectively, measured using an infrared camera .
Regenerating the sorbent (desorption/calcination step) results in releasing CO2
suitable for storage.
Figure 4. Expected
concentration of CO2
outlet in the mass
spectrometer for a given
CaO sample for one cycle
of the experiment. The
table shows how the CO2
signal measured by the
mass spectrometer
changes with respect to
the other variables in the
experiment.
Adsorption Cycle Summaries
30
enhancement enables lower reaction temperatures while maintaining
the conversion. To enhance the reaction, the CO2 produced should be
removed.
CO2 + CaO ⇔ CaCO3,
Experiment
Figure 3. Experimental setup of reactor
ΔH = 206.2 kJ mol􏰀-1
(1)
ΔH = -42.1 kJ mol􏰀-1
(2)
Equations (1) – (3): Steam reforming reversible reactions for methane.
ΔH = 165 kJ mol􏰀-1
CO2 can be removed from the equations stated above by using a
(3) liquids or gases). Sorption
sorbent (a material used to absorb or adsorb
CH4 + H2O ⇔ CO+3H2 ,
CO + H2O ⇔ CO2 + H2 ,
CH4 + 2H2O⇔ CO2 +4H2 ,
20
Cycles
Graphs 1 (above) & 2 (below)
Desorption Cycle Summaries
25
Sample 1
Sample 2
Results
Capacity (mass %)
20
15
10
5
0
0
2
4
6
8
10
Cycles
12
14
16
18
20
Rates of adsorption and desorption used to calculate the capacities (in
mass % i.e. the percentage mass of CO2 adsorbed per given percentage
mass of CaO) to calculate which of the two samples is more efficient as a
sorbent. Referring to graphs 1 and 2, sample 1 initially has a higher
capacity. However, the greater degradation of said capacity over twenty
cycles, in comparison to sample 2, leads to the idea that it has the lower
capacity, evident from adsorption cycle 15 onwards. Recorded
temperature data proved the effectiveness of RF heating: Temperatures
of 850°C were reached in a matter of five minutes with only 170A
supplied, much faster than conventional methods of heating.