Fixed Bed Methanation Research Articles

Modeling and simulation of CO methanation process for renewable electricity storage

In this paper, a new approach of converting renewable electricity into methane via syngas (a mixture of CO and H2) and CO methanation is presented. Surplus of electricity is used to electrolyze H2O and CO2 to H2 and CO by using a SOEC (Solid Oxide Electrolysis Cell). Syngas produced is then converted into methane. When high consumption peaks appear, methane is used to produce electricity. The main conversion step in this process is CO methanation. A modeling of catalytic fixed bed methanation reactor and a design of methanation unit composed of multistage adiabatic reactors are carried out using Aspen plus™ software. The model was validated by comparing the simulated results of gas composition (CH4, CO, CO2 and H2) with industrial data. In addition, the effects of recycle ratio on adiabatic reactor stages, outlet temperature, and H2 and CO conversions are carefully investigated. It is found that for storing 10 MW of renewable electricity, methanation unit is composed of three adiabatic reactors with recycle loop and intermediate cooling at 553 K and 1.5 MPa. The methanation unit generates 3778.6 kg/h of steam at 523.2 K and 1 MPa (13.67 MW).

Design, control and comparison of fixed-bed methanation reactor systems for the production of substitute natural gas

The design and control of processes for the methanation of synthesis gas to produce substitute natural gas (SNG) using fixed-bed reactors is investigated. Three different strategies for controlling the reactor temperature rise are considered: recycle of a portion of the reactor effluent, introduction of additional water into the reactor feed, and non-adiabatic reactor operation with catalyst dilution. The results show that the process with a non-adiabatic reactor has the lowest cost and produces the greatest amount of high-pressure steam. However the efficacy of catalyst dilution for preventing reaction run-away has not been tested experimentally. Among the remaining options, partial recycle of reactor effluent is preferred because it can produce a greater amount of high-pressure steam than the process with additional water. Control studies indicate that all processes can be controlled and can tolerate production rate changes, however none can tolerate a large change in the feed composition due to inherent stoichiometric limitations.

Dilute gas methanation of synthesis gas from biomass gasification

Synthetic natural gas (SNG) based on biomass is a secondary energy carrier with a lot of advantages. The methane synthesis reaction conditions are mild and the process is uncomplex. The whole natural gas infrastructure can be used for storage, distribution and application. In addition, methane is much easier to handle than hydrogen. In the SNG process synthesis gas will be converted to mainly methane and after a processing unit it could be fed into the natural gas grid. Air gasification technology is state-of-the-art and a low cost gasification technology. The synthesis gas out of an air gasifier is highly diluted with nitrogen and thus just used for combined heat and power production (CHP). The current SNG concepts need synthesis gas from a much more complex gasification with oxygen or steam. Because no literature could be found which analyzes the effect of nitrogen in the methanation unit, this paper will show first results in this field. Synthesis gas with different nitrogen content was tested in a fixed bed methanation reactor at different temperature levels, using a commercial 20wt.% Nickel catalyst. The nitrogen content has the positive effect to limit the temperature increase by the strong exothermic reaction. Full carbon monoxide conversion and high methane selectivity was observed. It can be shown, that the methane selectivity is independent up to 50Vol.% of nitrogen in the feed gas. An occurred side reaction is the ammonia production. The highest ammonia concentration, 614ppm, was observed at the lowest nitrogen content. The same experiment shows the highest methane selectivity of 90% and methane content of 30Vol.%. With increasing dilution, the ammonia formation falls off. The shown influence of nitrogen on the methanation reaction should point out the chances and risks for methanation of a diluted synthesis gas derived from biomass gasification, especially in small sizes, like decentralized air-gasification plants.

Modeling of fixed bed methanation reactor for syngas production: Operating window and performance characteristics

The present work focuses on the development of phenomenological model for the bio-syngas to methane conversion process. One dimensional heterogeneous and pseudo-homogeneous model were simulated for a typical pilot plant scale fixed bed methanator processing 55mol/h of CO (total molar flow rate of 310mol/h) with inlet composition of H2/CO=3, CO2/CO=1, CH4/CO=0.5 at 550K and 1atm. Performance of the fixed bed reactor at different operating conditions like CO2/CO ratio, H2/CO ratio, effect of H2O in the feed was studied. It was found that for feeds that were not pre-enriched with hydrogen, presence of water and water gas shift activity was found to decrease the catalyst inventory substantially. CO2 in the inlet feed stream would help to decrease the temperature due to dilution effect and more importantly, can be chosen to maximize methane yield per mole of CO converted. Further, the model was simulated to predict the performance characteristics of reactor with a mixture containing two types of catalyst, one of them being specifically added to increase H2/CO ratio in feed through water gas shift reaction. The work also laid the importance of incorporating pore diffusion and external mass transfer locally in the computation of actual catalyst inventory and reactor volume. The work was useful in selection of operating window and assessing the various viable options for an industrial reactor. The model developed will serve in selection of operability window for commercialization of substitute natural gas synthesis (SNG) process.

Enhanced fluidized bed methanation over a Ni/Al2O3 catalyst for production of synthetic natural gas

The fluidization behavior and CO methanation performance over a typical Ni/Al2O3 catalyst for synthetic natural gas production was systematically investigated in a fluidized bed reactor. The results indicated that the methanation performance of the catalyst depends, to a large extent, on the fluidization quality in the fluidized-bed methanation reactor. The pure Ni/Al2O3 catalyst with the particle sizes of 10–100μm failed to fluid, but the fluidization can be significantly improved by adding Al2O3 particles. The methanation reaction in the fluidized bed reactor was also substantially improved by adding Al2O3 particles, and the yield and selectivity of CH4 in the fluidized bed with Al2O3 addition were much higher than those of without Al2O3 addition. The CO methanation reaction was controlled by two different mechanisms, i.e. the surface reaction controlling in lower temperature range (250–330°C) and external diffusion controlling in higher temperature range (>330°C). Stability test demonstrated that the fluidized bed methanation reactor showed higher CO conversion, CH4 selectivity and yield, and more excellent carbon resistance than the fixed bed methanation reactor under high temperature, which is beneficial for reclaiming the high temperature steam.

Robust inferential control for a packed-bed reactor

In this paper, two different inferential control schemes are applied to an experimental fixed bed methanation reactor. The first scheme, proposed initially by Brosilow, is designed based on Kalman filter estimation. The second less traditional design uses an estimator computed from the partial least squares regression method (PLS). The second approach was found to give superior performance when the nonlinear system under study is operated in a wide range of operating points. Due to the nonlinearity of the system it is essential to address the issue of robustness of the proposed schemes. This is formally done in this work using structured singular value theory. For the robustness analysis it is crucial to develop a realistic but not overly conservative uncertainty description. Since the PLS estimator uses a large number of measurements, a robust design based on the uncertainty associated with each one of the measurements, a robust design based on the uncertainty associated with each one of the measurements would be very conservative. To overcome this problem, a lumped uncertainty description is proposed which is identified directly form experiments.

Control system design for a fixed-bed methanation reactor

A systematic approach is applied to the design of robust controllers for a fixed-bed, non-adiabatic, laboratory methanation reactor, a unit inherently difficult to control. The fundamental limitations to the achievable closed-loop performance are established by both physical arguments and dynamic resilience considerations. The Internal Model Control (IMC) structure provides a suitable framework for this analysis. IMC controllers are designed and their performance in the face of model uncertainty is tested both by simulation and by means of the Structured Singular Value (μ) analysis technique. The paper provides a basic insight into the characterization and solution of control problems that are particular to fixed-bed reactor systems and constitutes the first account of the application of several recent advances in control theory to a complex chemical process.

Transient behaviour of an adiabatic fixed-bed methanator—I: Experiments with binary feeds of CO or CO 2 in hydrogen

The transient behaviour of an adiabatic fixed-bed catalytic reactor has been studied experimentally as well as theoretically using the hydrogenation of small amounts of CO and CO 2 to methane as the test reactions. The axial temperature profile in a 0·5-litre reactor containing a Ni/Al 2O 3 catalyst was measured as a function of time after applying changes of the carbon oxide concentration in the feed and of the feed temperature. Besides, a quasi-homogeneous model was developed to simulate the transient behaviour of the reactor; its partial differential equations were solved numerically using the Crank-Nicholson algorithm. The agreement between measured and calculated reactor responses was quite good at carbon oxide concentrations up to 2·2 vol %, an inlet temperatures below 250°C and in the space velocity range of 5000–25000 h −1, under which conditions the reaction is so slow that mass and heat transfer limitations do not occur.

Transient behaviour of an adiabatic fixed-bed methanator—II: Methanation of mixtures of carbon monoxide and carbon dioxide

The transient behaviour of an adiabatic fixed-bed methanator has been studied using the hydrogenation of mixtures of CO and CO 2 at concentrations up to 2·7 vol.% carbon oxide in hydrogen as the test reactions. Responses to disturbances in feed conditions were studied by measuring the axial temperature profile as a function of time. The results show that the dynamic behaviour of the reactor is complicated by the inhibition by CO of the methanation of CO 2. The agreement between theory and experiment was again quite satisfactory: the quasi-homogeneous plug flow model which applied to experiments using binary mixtures of hydrogen and a carbon oxide applies to the data obtained with mixtures of CO, CO 2 and hydrogen, provided that the successive hydrogenation of CO and CO 2 is taken into account. However, it is improbable that the quasi-homogeneous model can be applied to industrial methanation, when the higher temperatures and consequent faster rates of methanation are likely to cause heat and mass transfer limitations. Nevertheless, there is no doubt that response times of but a few seconds must be expected in industrial methanation.