storage of renewable electricity in methanol
Martin Bos is a PhD student in the research group Sustainable Process Technology. His supervisors are dr.ir. D.W.F. Brilman and prof.dr. S.R.A. Kersten from the faculty of Science and Technology.
Since the start of the industrial era, the CO2 concentration in the air has risen from 250ppm to more than 400ppm nowadays. A large part of the increase can be contributed to use of fossil fuels for energy production. To reduce CO2 emissions, more and more capacity of renewable energy sources such as, wind power, solar PV and hydro-power are installed. However, most of these sources result in intermittent supply of electricity leading to a mismatch in supply and demand. To level supply and demand, storage of electricity is required. In this thesis, the storage of renewable electricity into methanol is studied. The goal of thesis is to develop technology for direct air capture of and improve efficiency of -- small scale -- CO2 to methanol technology. Technologies for conversion of H2O into H2 -- required for the synthesis of methanol -- are outside the scope of this project.
In Chapter 1 it is shown that for long term and large scale storage (i.e. seasonal storage), storage in chemicals is required. Methanol is liquid at ambient conditions and therefore an excellent storage medium, as it is easy to store and transport. Moreover, it can be produced from the abundant available chemicals CO2 and H2O. Furthermore, methanol is a very flexible product as it can be converted back to electricity, used as a gasoline substituent, converted to a diesel replacement and used as feedstock in the chemical industry.
To produce the CO2 required for the production of methanol, the transport phenomena for CO2 adsorption in solid amine sorbents are studied in Chapter 2. The adsorption kinetics are determine to be able to optimize CO2 air capture systems. For this, a new experimental method is developed to exclude heat and mass transfer limitations during kinetic adsorption experiments. Hereto, a novel contactor was designed and good process control, working with pure CO2 and small particle diameters enabled the measurement of intrinsic kinetics. A mathematical model describing convection, diffusion and reaction rate inside a particle confirmed the absence of mass and heat transfer limitations in the experiments. During normal adsorption conditions however, the uptake rate of CO2 will be strongly inhibited by diffusional resistances inside the particle.
Linear driving force and Toth-isotherm reaction rate equations are evaluated for the CO2 adsorption process studied. The results show that the experimental particle loading with time could not be described by the linear driving force models. On the other hand, the Toth reaction rate equation, consistent with the Toth isotherm to describe the adsorption equilibrium, showed a very good fit to the experimental data. This shows that a rate based isotherm equation is necessary for consistent prediction of both, adsorption rate and equilibrium loading.
In Chapter 3 and 4 the desorption of CO2 from the adsorbent is studied. While Chapter 3 focuses on the regeneration of sorbents, Chapter 4 focuses on the production of high purity CO2 from air. In Chapter 3 the regeneration conditions of a solid amine sorbent are evaluated by experiments and equilibrium modelling. It was found that when using an inert purge flow the desorption rate is strongly influenced by equilibrium between the gas and adsorbed phase. Because of the strong dependency of the isotherm on temperature, heat transfer is found to be an important design parameter. With elevated temperature (>80°C) both the working capacity and the productivity increase significantly. Therefore, most important design considerations are heat transfer and the trade-off between sorbent working capacity and energy consumption for sorbent heating.
The effects of water co-adsorption and steam purge on the CO2 working capacity and energy requirement for CO2 desorption are reported in Chapter 4. Working capacities are studied by fixed bed operation for changing temperature, pressure and amount of steam purge. Results show that for pressure-temperature swing adsorption a temperature above 100°C and a pressure below 200 mbar as desorption conditions are required to maximize CO2 working capacity and reduce energy requirement for desorption. Co-adsorption of water reduces energy requirement due to an increased CO2 working capacity. Application of a steam purge increases the CO2 working capacity and hence reduces sorbent inventory required. However, the net energy requirement per kilogram CO2 does not decrease due to the latent heat of water. Concluding, steam purge regeneration for air capture does not reduce OpEx but might reduce CapEx.
A novel reactor concept for the conversion of CO2 and H2 to methanol is developed in Chapter 5. Conversion limitations because of thermodynamic equilibrium are bypassed via in situ condensation of a water/methanol mixture. Two temperatures zones inside the reactor ensure optimal catalyst temperature, automatic gas circulation by natural convection and full conversion by condensation at a lower temperature in a separate zone. Experimental work confirmed full carbon conversion (>99.5%) and high methanol selectivity (>99.5% on carbon basis). Because of full gas conversion there is no need for an external recycle of unconverted reactants. Moreover, proof of concept for operation under natural convection conditions was shown.
A more detailed analysis of the condensing reactor for conversion of CO2 to methanol is given in Chapter 6. The reactor is characterized under forced convective conditions, both experimentally and by modelling. The goal of the study is to optimize the operation conditions and identify limitations of the reactor concept. Experimental results show that the productivity is limited by reaction equilibrium and mass transport at high temperature (> 250°C), while reaction kinetics limit productivity at low temperature (> 220°C).
Further analysis of the LOGIC concept is performed by an adiabatic 1D-reactor model in combination with an equilibrium flash condenser model. To enable autothermal operation without excessive heat exchange area, it was found that a condenser temperature below 70°C is required. Most important design parameter is found to be the conversion per pass over the catalyst bed. Increasing dimensions of the catalyst section will increase the conversion per pass, unless equilibrium is reached. On the other hand, heat exchanger and condenser area are reduced because of a lower recycle ratio. With the model developed, overall reactor performance can be optimized by finding the most optimal combination of reaction and condenser conditions.
In Chapter 7 a 100MW wind power to methanol process has been evaluated to determine the capital requirement and power to methanol efficiency. Power available for electrolysis determines the amount of hydrogen produced. The stoichiometric amount, for the methanol synthesis, of CO2 is produced using direct air capture. Capital cost for all process steps is estimated using short-cut equipment sizing and economics. Power to methanol efficiency was determined to be around 50%. The cost of methanol is around 800€ ton-1 including wind turbine capital cost. Excluding 300M€investment cost for 100MW of wind turbines, total plant capital cost is around 200M€. About 45% of the capital cost is reserved for electrolysers, 50% for the CO2 air capture installation, and 5% for the methanol synthesis system. The conceptual design and evaluation shows that renewable Methanol from CO2 from air, water and renewable electricity is becoming a realistic option at reasonable costs of 750-800 € ton-1.
The production of renewable methanol using direct air capture and electrolysis is currently not economical viable. As discussed in Chapter 7 the cost of wind energy, electrolysis, and air capture are expected to go down in the future, improving process economics. As discussed in Chapter 8, government legislation might be an important driver for the process. For example, the European Renewable Energy Directive II) requires 14% of renewable energy to be used in transportation by 2030. Moreover, the European Commission proposes a climate-neutral Europe in 2050, thereby renewable methanol could fulfil the role of sustainable carbon source for the chemical industry.