Dynamic modelling and simulation of catalytic carbon dioxide methanation with green hydrogen
Nguyen, Phat (2024)
Diplomityö
2024
School of Engineering Science, Kemiantekniikka
Kaikki oikeudet pidätetään.
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi-fe2024110589150
https://urn.fi/URN:NBN:fi-fe2024110589150
Tiivistelmä
Power-to-gas is a method for chemically storing discontinuous green power. Renewable electricity is able to generate “green” hydrogen (H₂), which can be subsequently combined with carbon dioxide (CO₂) in catalytically cooled multi-tubular reactor(s) to synthesize methane (CH₄). However, the dynamic behavior of the process with variating inlet flow rates caused by the intermittency of renewable electricity is the biggest challenge of this technology. Therefore, the process’ dynamic behavior should be investigated with variating inlet flow rates to design a control structure and to operate the process. In the first step, a steady-state model was built in Aspen Plus according to the capacity of a 35 MW electrolysis unit. After that, the model was exported to Aspen Plus Dynamics to study its dynamic behavior. Different H₂ feed flow rates were applied to investigate the composition of the product stream and the reactor. In addition, model’s limitations were also determined by testing different ramping rates and the minimum partial loading level of feed gases that can be achieved in the process model.
In the steady-state model, a methanation reactor was designed and simulated in Aspen Plus and Aspen Plus Dynamics using characteristics of the Koschany et al. kinetic model at 230 ℃ and 10 bar. Heat transfer coefficients were also calculated and applied to achieve a realistic model. As a consequence of the reactor type, the high stoichiometric ratio, and the fast and exothermic reaction, a hot spot of 768 ℃ appeared at the feed zone of the reactor. In the context of the absence of a recycle stream, the hot spot was 782℃, which is higher than with recycle. The composition of the product stream was 28.33% of H₂ mol-% and 70.79% of CH₄ mol-%. With the control system set up in Aspen Plus Dynamics, the controllers showed a quick response with different H₂ feed flow rate variations. Different H₂ loads were tested: +10%, −10%, and –30% of the maximum flow rate in steady-state simulation. When the feed flow changed from the maximum load, the peak temperature reached 752 ℃, 738 ℃, and 722 ℃ in cases +10%, −10%, and –30% of the full load respectively. In all tests, the product composition had no significant fluctuations, and H₂ and CH₄ molar fractions were stable and the same as at full-load. The model’s limitations were determined; the maximum achieved ramping rate was ±50% change in 5 minutes, and the minimum H₂ feed flow rate was 138 kmol/h corresponding to ~ 44.8% of the full load. Notably, in all tests, the quality of the product stream always remained in the favorable range regarding the CO₂ concentration.
In the steady-state model, a methanation reactor was designed and simulated in Aspen Plus and Aspen Plus Dynamics using characteristics of the Koschany et al. kinetic model at 230 ℃ and 10 bar. Heat transfer coefficients were also calculated and applied to achieve a realistic model. As a consequence of the reactor type, the high stoichiometric ratio, and the fast and exothermic reaction, a hot spot of 768 ℃ appeared at the feed zone of the reactor. In the context of the absence of a recycle stream, the hot spot was 782℃, which is higher than with recycle. The composition of the product stream was 28.33% of H₂ mol-% and 70.79% of CH₄ mol-%. With the control system set up in Aspen Plus Dynamics, the controllers showed a quick response with different H₂ feed flow rate variations. Different H₂ loads were tested: +10%, −10%, and –30% of the maximum flow rate in steady-state simulation. When the feed flow changed from the maximum load, the peak temperature reached 752 ℃, 738 ℃, and 722 ℃ in cases +10%, −10%, and –30% of the full load respectively. In all tests, the product composition had no significant fluctuations, and H₂ and CH₄ molar fractions were stable and the same as at full-load. The model’s limitations were determined; the maximum achieved ramping rate was ±50% change in 5 minutes, and the minimum H₂ feed flow rate was 138 kmol/h corresponding to ~ 44.8% of the full load. Notably, in all tests, the quality of the product stream always remained in the favorable range regarding the CO₂ concentration.