Chemical looping steam reforming (CLSR) and sorption enhanced chemical looping steam reforming (SE-CLSR) were utilised as process intensification measures to optimise the steam reforming process of acetic acid as a base-case for the enhanced steam reforming of pyrolysis oils. Both processes were carried out in a packed bed reactor, using two conventional nickel-based catalysts (‘A’ with alumina support and ‘B’ with calcium aluminate support) with the view of ascertaining optimal conditions for sustained steam reforming, and observing changes in morphology and characteristics in the materials utilised.
An experimental review, carried out on the steam reforming of acetic acid indicates, an acetic acid and water pre-heat temperature < 70°C and > 120°C respectively is ideal for sustained hydrogen production, at weight hourly space velocity (WHSV) set between 2.3 hr-1 to 2.5 hr-1, steam reforming temperature (TSR) set to 600°C or 650°C and steam to carbon ratio (S/C) set to 3 for the reactor set up and configuration utilised. Sustained steam reforming was observed at all TSR investigated (550°C – 700°C), at all catalyst sizes compared, and all WHSV utilised (2.1 hr-1 to 2.8 hr-1), with acetic acid conversion efficiency >83% realised and a hydrogen yield efficiency >85% realised for all experimental runs when compared to equilibrium values.
Characterisation of the utilised catalyst through SEM images indicates the formation of filamentous carbon on the catalyst surface. Two peaks of CO2 obtained from the CO2 chemigram in TGA-FTIR analysis indicates two types of carbon are formed, with amorphous carbon and a polycrystalline pseudo-graphitic carbon observed through TEM images and diffraction patterns.
CLSR has been promoted as a viable measure of improving the efficiency of the steam reforming process. It is a cyclic process that incorporates an oxidation step for material regeneration alongside steam reforming. In this study, it was inferred that the proficiency of the subsequent steam reforming fuel-steam feed run is dependent on the operating conditions and type of materials utilised in the oxidation step.
Efficient auto-reduction and sustained CLSR close to equilibrium values were observed for both catalysts (S/C of 3, WHSV: 2.36 hr-1 and 2.5 hr-1, TOX: 600°C - 800°C, TSR: 600 °C and 650°C) except when oxidation was carried out at 600°C for catalyst A. This exemplifies the advantages of an alkali-based support for reforming catalysts such as the one in catalyst B in reducing thermal decomposition of the fuel and sintering. Study on TOX utilised indicates, an increase in TOX would lead to more efficient carbon and nickel oxidation. Care must be taken however because it also leads to a potential increase of sintering of the catalyst.
SEM-EDX analysis and CHN analysis in this study indicates, there is a close similarity between the oxidised catalyst and fresh as received catalyst B particularly when oxidation is carried out at 800°C. This indicates complete burning of carbon and catalyst oxidation during the oxidation phase utilised for the CLSR experiments. The complete oxidation observed ensured sustained steam reforming over 10 cycles of CLSR investigated in this study (TSR at 650°C, TOX: 800°C, WHSV set to 2.5 hr -1and S/C of 3) for catalyst B.
The sustained reforming, across all 10 cycles of CLSR, showed high consistency with >95% of acetic acid converted leading to a hydrogen yield efficiency >88% observed across all 10 cycles, when compared with equilibrium values. A carbon balance of the overall chemical looping reforming process infers most of the carbon share (ca 90%) in the process is utilised for effective steam reforming and hydrogen production.
SE-CLSR entails the addition of a sorbent into the chemical looping reforming material bed. This has been promoted to lead to an increase in hydrogen concentration and yield, due to the shift in equilibrium towards hydrogen production caused by the in-situ carbon-capture during the reforming process. Two stages occur, as in the case of CLSR. Calcining of the sorbent occurs in the oxidation stage alongside other processes that occur in the oxidation step in CLSR. In-situ carbon capture or carbonation of the calcined sorbent occurs alongside other reactions in the reforming stage.
Three phases were identified for the SE-CLSR reforming stage;
The pre-breakthrough phase, where complete sorption activity occurs and >99% of all carbon products produced is captured.
The breakthrough phase where a break of CO2 is observed indicating partial sorption activity.
The post-breakthrough phase where no sorption activity is occurring.
It was inferred that the mass of the sorbent added to the material bed has an influence on the carbonation duration (pre-breakthrough + breakthrough Phase). An increase in the sorbent/ catalyst ratio increases the duration of carbonation and number of moles of carbon captured.
20 cycles of SE-CLSR with TSR at 650°C and TOX set to 850°C using catalyst B with S/C set to 3 and WHSV set to 1.18 was conducted. Sustained and consistent reforming (>80 % of acetic acid converted) was observed across all 20 SE-CLSR cycles; this led to > 78% hydrogen yield efficiency across all 20 SE-CLSR cycles when compared to equilibrium values.
A close observation of the carbonation duration indicates a reduction in the pre-break duration for the first 10 cycles. This led to a decrease in sorbent conversion and efficiency. This was attributed to a change in kinetics and CO2 capture capacity in the pre-breakthrough phase, caused by sintering in the materials and reduction of open porosity.
At the end of the 20th cycle, the conversion efficiency had dropped to about 50% of the conversion efficiency at the end of the first cycle. This prompts the need for reactivation techniques to reduce the decay of the CaO sorbent across redox cycling.
Steam hydration of the material has been promoted to improve the reactivity of CaO based sorbents and was carried out at 250°C with the view to observe any changes in the conversion of the sorbent. It was observed that on comparison to the SE-CLSR experiments with no hydration at 250°C, there was a higher hydrogen yield and concentration. This was attributed to a higher rate of sorption enhancement as observed in the increased carbonation rate in the pre-breakthrough phase for the experiments with hydration set at 250°C. There is also increased sorption enhanced auto-reduction observed and an improved sorbent conversion outlook over the cycles investigated.
