The Italian Zecomix platform is a gasification pilot-scale plant integrated CCS technology, whose flexibility allows operating each unit alone. For quantitative analysis of the gaseous species an analytical system based on commercially available instruments and sampling lines at various points of plant was properly designed. A Fourier Transform Infrared Spectrometric (FTIR) with portable low resolution and gas chromatographic tools were used. The fluidized bed calciner/ carbonator unit was fed with naturally occurring dolomite (about 300 kg) from quarry of Biella (Italy) and tested for CO2 capture. Carbonation reaction was conducted at around 700°C under simulated reformed synthesis gas containing 30% of CO2. Based on GC and FTIR results a CO2 retention better than 70% was achieved. The application of grain model establishing the carbonation reaction was kinetically controlled within 50 seconds and the reaction rate constant was comparable to that obtained at laboratory-scale level by thermogravimetric analyzer.
The usage of solid fossil fuel such as coal for power generation is allowed with high efficiency and low pollutant emissions and storage of greenhouse gas (CO2) in an integrated gasification combined cycles plant (IGCC). The basic concept of this technology relies in exposing the coal gasification product (synthesis gas) to a complex cleaning and H2-enrichment coal gas processes. The latter takes place through sorption enhanced water shift re- action (SE-WGS), which combines the WGS reaction and CO2 capture on a solid sorbent in a single step. The H2-rich cleaned syngas is then burnt in a gas turbine, whereas the pure CO2 stream obtained during spent sorbent regeneration can be geologically stored or/and utilized in chemical synthesis.
At ENEA Research Center (Rome, Italy) is currently testing a pilot-scale plant denominated ZECOMIX (ac- ronym for Zero Emission CarbOn MIXed), based on an gasification system unit and CCS (carbon capture and storage) technology for the production of energy with almost zero emissions of CO2 [
The real parameters of the whole plant have to be available, which can be adjusted to maximise the selective reaction conversions and increase the chemical efficiency of the process. Parameters such as temperature, pres- sure, and flow rate are of typical interest of engineering process [
A broad range of tools for analysing gases, which can come from a process as yield yielded or reactant, are commercially available [
As a part of making progress in fundamental research in the field of CCS technologies, this paper describes the on-line analytical measurement systems projected for the ZECOMIX platform. The system was built based on commercially available analysers, namely, a GC from Agilent and a portable FTIR gas analyser from Gasmet Technologies Oy, Finland. The sampling lines were properly designed and realized. Sampling points located in various parts of the units were identified. As an example the results of testing the fluidized-bed calciner/carbon- ator unit through CO2 capture on naturally occurring dolomite are presented.
The bed/sorbent material consisted of dolomite from quarry of Biella (Italy), a locality in the foothills of Alps. The chemical composition was determined by thermogravimetric analysis (TGA/DSC1 Star System, Mettler- Toledo) [
GC. Agilent 6850 gas chromatograph-TCD detector equipped with two columns connected in series, namely, Molesieve 5A for permanent gases (H2, N2, O2, CO, and CH4) and Hayesep Q for CO2 was used. Quantification was made by GC Chemstation software (Agilent) according to the predefined method. Prochem software by S.R.A. Instruments (Milan, Italy) allowed to communicate with ABB Control and Data Acquisition Systems (SCADA), withdrawn gas sample from different sampling lines according to preset measurement sequence and compute reports created by Chemstation in final mole fraction. Remote control connection was made by VNC software. The GC was calibrated with reference gas (Air Liquid, France) supplied by high pressure cylinders before the start and after the end of carbonation test. The ratio between sample peak area and the bracketing ref- erence peaks is used to calculate gas volume%.
FTIR. Gasmet DX-4000 portable gas analyser, incorporating a low-resolution (4 cm−1) Fourier transform in- frared spectrometer and a ZrO2 sensor for accurate O2 measurements, was used. It was equipped with a tem- perature-controlled gas cell, heated transfer line, and portable heated probe. CALCMET software was used for the identification and quantification of compounds. The instrument was factory calibrated for the determination of species such as H2O (0% - 40% v/v), CO (400 mol ppm), CO2 (0% - 80% v/v), CH4 (200 mol ppm), and O2 with 2% error. IR spectra were also collected and stored on the hard disk of analyser, along with the concentra- tions information. This allowed checking possible spectral overlapping.
The sampling lines and analytical system was realised by SRA Instruments. It was designed to measure alter- nately synthetic gas and flue gas from six sampling points (V as the analogous chromatographic vial), located in different parts of the platform, and referred as VN (where N is a number indicating a specific sampling point) according to a predefined sequence. The schematic drawing of calciner/carbonator unit is illustrated in
The chamber for calcination/carbonator processes has an internal diameter of 1 m and 4.5 m in height, whereas the walls are coated by a refractory material. The chamber was connected to a primary cyclone. The refractory material was heated to 520˚C by combustion of methane with oxygen in excess through a burner. The exhaust combustion was measured for O2, CO2, CO, and CH4 through V4 sampling point by FTIR. Then the chamber was feed by dolomite at 20 kg/h for five hours and the temperature was brought around 850˚C for calcination reaction. The bubbling bed conditions were initiated by air, or alternatively N2, which contained 30% v/v CO2
. Chemical composition of dolomite Turin and particle mean diameter
MgCO3 | CaCO3 | LOI | |
---|---|---|---|
wt% | 43.2 | 53.9 | 2.9 |
dp° | 0.586 mm |
°Particle mean diameter.
Schematic drawing of calciner/carbonator unit of ZECOMIX plant
at a gas velocity (Ug) of 5.7 kg∙second−1. After 2 h calcination operation the temperature of fluidized bed was dropped to about 700˚C and CO2 capture was initiated. The flue gas leaving the carbonator passes through the heated inner tube of vial 3. The inlet and outlet lines (V3 and V4 sampling points) of carbonator were analysed occasionally by GC. To obtain parallel FTIR data the inlet of Gasmet DX-4000 analyser was connected through the heated sample probe to the V5 sampling point, which head off the same gas sample than V4.
The CO2 uptake by means of calcined dolomite, expressed by the equation below:
was followed over time by determining the CO2 and N2 concentrations through GC analysis. The overall chro- matographic analysis time took 4 minutes. So, the GC data were quite sparing. The CO2 and N2 content in the outlet stream at sampling point V4 over time is showed in
Picture of IP55 Cabinet with inside GC, PC, flowmeters, Peltier coolers, particulate filters
Typical CO2 and N2 trends at outlet carbonator during carbonation reaction under 30% v/v CO2 (N2 balance). Data ob- tained by on-line GC analysis
took place just after the switching of air bubbling gas to synthetic reformate gas (30% v/v CO2 in N2) as indi- cated by the suddenly decrease of CO2 concentration at the outlet of carbonator and simultaneously increase of the concentration of balance N2. At a certain time the CO2 concentration remained quite constant for almost 15 minutes and then went up again to its inlet value.
Due to the slowly response of GC any consideration on kinetic parameters may not be meet. In cases where analytical species present bands well isolated and spectral overlapping is avoided a quickly response can be ob- tained by FTIR analysis. This allows to monitoring in real time the Reaction (1). The CO2 concentration during carbonation reaction of calcined dolomite was followed over time at 7 seconds intervals.
In
Conversion vs time curve was derived from above-mentioned breakthrough curve. The conversion of CaO to CaCO3 X, is defined as follows:
where yt(CO2) is the number of CO2 reacting moles at time t and y0(CO2) the number of CO2 moles at the inlet carbonator, whereas yCaO is the number of initial moles of CaO, which is calculated multiplying the mass of dolomite (kg/h) added to carbonator chamber by the mass fraction of CaO (see
Continuously CO2 monitoring during car- bonation reaction under 30% v/v CO2 (N2 balance) by FTIR analysis
Conversion X over time for calcined dolomite in the carbonator unit (data obtained by FTIR analysis). The inset shows the conversion X of calcined dolomite in TG at first cycle under 30% v/v CO2 (N2 balance)
this stage kinetically controlled, and immediately slows down because of instauration of a diffusion regime due to covered CaO particles with a layer of solid-product CaCO3, which obstacles the access of CO2 gas onto CaO particle surface [
The shrinking core model of Szekely et al., [
In order to survey to what extent dolomite was calcined spent dolomite was collected from carbonator cham- ber and submitted to TG/DSC analysis. The sample was previously homogenized in an Agate mortar and sub- mitted to heating program of 10˚C∙min−1 up to 950˚C in 100 v/v% N2 atmosphere, kept isothermally for 5 mi- nutes at the final temperature in the same atmosphere and then the temperature was slowed down after switching gas to CO2. The TG/DSC curves are shown in
The endothermic calcination reaction under N2 atmosphere expressed by the following equation:
was approaching the theoretical value, which produces 100% CaO, whereas completely re-carbonation through subsequent exothermic Reaction (1) under CO2 atmosphere was observed. These findings suggested that the de- gree of calcination of dolomite inside calciner was about 70%.
The Italian Zecomix platform was tested through experiment of CO2 capture in the bed-fluidized calciner/car- bonator unit. The operating conditions were: bed material made of naturally occurring dolomite from Biella quarry; operation temperature around 700˚C and simulated reformate flue gas containing 30% v/v CO2 as
Grain model plots for dolomite under carbonator conditions and in TG analyser. The inset shows the carbona- tion reaction at initial stage
TG/DSC curves of spent dolomite in different atmos- pheres
fluidizing agent. The sampling lines and analysis system constituted by GC and FTIR was properly designed. Based on GC and FTIR results a CO2 retention better than 70% was achieved. The FTIR data allowed entering in insight of the kinetics. The conclusions established from the results obtained in pilot-scale carbonator confirmed that obtained at laboratory-scale level by TGA.
This work was funded by the Ministero dello Sviluppo Economico (Italy) within the Ricerca del Sistema (RdS) Elettrico Programme.