(336f) Sustainable Methanol Production Using a Multi-Time Energy Integration and CO2 Management Approach As an Energy Carrier for Synthetic Natural Gas Synthesis

Given the increasing concern about the environmental impact associated with the production of power, fuels, and chemicals, significant efforts are being made to explore sustainable alternatives to meet the energy needs. Biomass resources offer a promising solution for the energy transition, given their dynamic nature and the ability to be transformed into solid, liquid, or gaseous fuels. Biomass gasification, in particular, provides a versatile way to convert waste into energy. However, the adoption of such an approach must consider not only the mitigation of emissions but also the uncertainties associated with energy prices and carbon management strategies, especially in light of more stringent environmental regulations. Also, rigorous process synthesis and economic analysis are crucial for the decision making of the best pathways to boost bioeconomy.

On the other hand, methanol is a versatile chemical with significant potential in the energy sector. It can serve as a direct fuel or blended with gasoline and act as an intermediate molecule for various chemical syntheses, including hydrogen, methane, olefins, amines, acetic acid, dimethyl ether, and formaldehyde [1]. However, traditional industrial processes for methanol production rely on fossil sources, such as coal and natural gas, which have several drawbacks, including high energy and water consumption, pollutant emissions, nonrenewability, and future depletion [2]. To address these issues, alternative routes are being explored to decarbonize methanol's production supply chains, such as biomass thermochemical conversion via gasification [2]. Another advantage of methanol is that it can be stored as a liquid under ambient conditions, without the need for refrigeration or pressurization, which sets it apart from other energy carriers like hydrogen, liquefied natural gas, ammonia, and batteries [3].

This study proposes an integrated system for methanol production that combines different technologies and evaluates its potential as an energy carrier for synthetic natural gas (Figure 1). The production of methanol is achieved through the gasification of woody biomass, offering an alternative route to traditional methods. Additionally, the study considers the use of a water electrolysis system to produce hydrogen and generate syngas by combining it with the CO₂ stream from the syngas purification unit. The CO₂-H₂ mixture is sent to a reverse water gas shift reactor to adjust the composition before following to methanol synthesis and purification. The purified methanol can be stored and later converted to synthetic natural gas in a reformer, with the storage decision based on seasonal electricity and synthetic natural gas market costs.

The chemical processes synthesis, modeling and simulation are performed by using Aspen Plus® software [4]. The ultimate mass-based wood composition is set to 46.7%C, 6.02% H, 44.95% O, 0.17% N, 0.02% S and 2.14% ashes, whereas the mass-based proximate analysis is considered as 50% moisture (as-received), 14.32% fixed carbon, 83.54% volatiles, and ash in balance. The Battelle Columbus Laboratory indirect gasifier operating at atmospheric pressure and using steam as gasification medium is used for modeling the wood gasification system. This system avoids the dilution with nitrogen of the syngas produced, as the combustion and gasification processes occur in a separate double column system. After the produced syngas leaves the gasifier, a thermal catalytic cracking of the produced tar is performed. A fraction of the char produced in the pyrolysis step is combusted to supply the heat required by the endothermic drying, pyrolysis and reduction reactions. After the produced syngas leaves the gasifier, it is cooled down and scrubbed with water, in order to remove the impurities that may affect the downstream equipment. The syngas composition must be adjusted considering the H₂:CO ratio equal 2. Thus, it is necessary autothermal reforming, shift and CO₂ capture processes in order to purify and upgrade the syngas composition.

In the methanol synthesis process, purified syngas is compressed to 90 bar and heated by the reactor outlet stream in a feed-effluent heat-exchanger. The syngas is then fed to an isothermal reactor operating at 90 bar and 210 °C. The reactor outlet stream is a gaseous mixture of methanol, water, and unconverted reactants which is cooled and flashed twice. The first flash cools the mixture to 30°C and 45 bar, while the second flash cools it to 3.5 bar, allowing for the separation of the condensable products and non-condensable reactants. The condensed stream proceeds to a distillation column operating at atmospheric pressure, producing methanol with purity above 99% wt.

The Peng-Robinson equation of state with Boston-Mathias modifications is mostly used as the thermodynamic model. The NRTL-RK property method (a nonrandom two liquid activity-coefficient model and the Redlich-Kwong equation of state for the vapor phase) is used to model the distillation section of methanol, which is suitable for describing the phases present in the system. The methanol synthesis is modeled using the LHHW (Langmuir-Hinshelwood Hougen-Watson) kinetic model reformulated for implementation in Aspen Plus [5].

Meanwhile, the OSMOSE Lua platform handles both the determination of minimum energy requirements and the solution of the energy integration problem. This optimization problem minimizes the resources consumption (wood, power) and, thus, the operating cost of the chemical plant, and also the investment required to purchase these new technologies (a.k.a capex), while satisfying some constraints, such as the heat balance at each temperature interval, the balance of consumed and produced power, the feasibility of the solution and the existence and size of the utilities units. In summary, it deals with the tradeoff of buying the technology and the operating revenues that may arise from its installation. To account for seasonal variations in energy prices, a multi-time integration approach combining different technologies and energy inputs is used to identify optimal operating conditions and arrangements that minimize energy consumption. The exergy method is used along with other financial indicators to determine whether and in which scenarios the proposed integrated setup would be more attractive.

As a result, the biomass to methanol production via gasification (see green section in Figure 1), presented a relative exergy efficiency that quantifies the deviation from the minimum theoretical exergy consumption necessary to make up the main industrial product, i.e. methanol, equal to 49.66%. Moreover, this system is producing 61.78 t_CO2/h from the CO₂ capture unit. The integration of technologies such as biomass gasification and electrolysis along with liquid fuel storage, may help tackling the intermittency of the renewable energy resources and increasing the economic revenues of the integrated plant. The optimal CO₂ management and methanol storage may ensure a reliable operation and synthetic natural gas availability even during the strained periods of the electricity grid and market conditions.

References

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