1. Introduction
This work falls within the scope of ‘Energy and Environment. Sustainable Transition’ because it focuses on converting renewable biogas into methanol and synthetic fuels through integrated processes, contributing directly to industrial decarbonisation and the transition to more sustainable energy systems. The main objective is to develop a methanol production model in a pilot plant, integrated with a steam methane reforming (SMR) unit, fed with biogas from anaerobic digestion processes of organic waste, thus enabling the production of H₂ from renewable sources.
In addition, the integration of a synthesis gas purification stage is proposed, which will not only allow the stream to be adapted for methanol synthesis, but also enable its possible recovery in a Fischer–Tropsch plant for the production of liquid hydrocarbons. In this way, the proposed system is conceived as a flexible platform for converting biogas into fuels and chemical products, whose technical and economic viability can be evaluated using the model developed.
2. Methods
The analysis was carried out using Aspen HYSYS software (Aspen Technology, Inc.), widely used in the industry for chemical process simulation. The methodology included the following stages:
Process definition: all plants were designed with preliminary calculations of conversions, efficiencies and methanol yields, standardised for a pilot capacity of approximately 2 kg/h of CH3OH and 1.5 kg/h of petrol.
The methanol plant was modelled using a plug flow reactor under conditions determined to meet the following requirements:
3H2(g) + CO2(g) → H2O + CH3OH
The SMR was modelled as a tubular conversion reactor for the steam methane reforming reaction and two equilibrium reactors for the water gas shift reaction:
CH4(g) + H2O(g) → CO(g) + 3H2(g)
CO(g) + H2O(g) → CO2(g) + H2(g)
This scheme allows the H₂/CO ratio of the synthesis gas to be adjusted according to the requirements of the subsequent synthesis stages.
The Fischer–Tropsch plant was modelled based on representative reaction conditions for the synthesis of hydrocarbons from synthesis gas, considering the overall reactions for the formation of paraffins and olefins:
Paraffins : (2n+1) H2 + n CO → CnH2n+2 + n H2O
Olefins : 2n H2 + n CO → CnH2n + n H2O
The model allows the conversion of synthesis gas into a mixture of liquid hydrocarbons such as synthetic fuels to be represented, highlighting the role of the Fischer–Tropsch process as a complementary route to methanol synthesis for the valorisation of reformed biogas.
Integration of units and economic estimation: the necessary equipment was integrated to ensure continuity of operation and cushion fluctuations in biogas flow. A preliminary assessment of the most relevant costs of the system was also initiated, considering both capital expenditure (CAPEX) and operating expenditure (OPEX).
3. Results
The HYSYS simulation showed that integrating a methanol plant with a biogas-fed SMR unit is technically feasible, provided that suitable equipment and thermal and compression strategies are used to ensure operational stability. The energy balance shows that SMR is highly demanding in terms of thermal energy, although it has CH₄ to H₂ conversion efficiencies of over 70%. Its integration with the methanol plant makes it possible to easily achieve production rates of around 2 kg/h of CH₃OH.
Furthermore, the results indicate that the same synthesis gas purification system can be used both for methanol synthesis and to feed the Fischer–Tropsch plant, which increases the versatility of the process scheme. In this sense, the Fischer–Tropsch process appears to be a complementary alternative for transforming synthesis gas into liquid hydrocarbons, expanding the range of products that can be obtained from biogas as a raw material.
From an economic point of view and for both systems, the most relevant equipment in terms of CAPEX is the reformer, the conversion and plug flow reactors, and the purification system. In terms of OPEX, the costs associated with biogas supply and thermal energy input are noteworthy. However, this production method has an advantage in terms of the specific cost of hydrogen (€/kg H₂), due to the lower relative cost of the renewable raw material.
4. Conclusions
The study confirms the technical feasibility of integrating a pilot methanol plant of up to 2 kg/h with a methane reforming plant fuelled by biogas. Simulations show that this configuration can be cost-competitive and also offers high operational flexibility by allowing the synthesis gas to be converted into both methanol and synthetic fuels using Fischer–Tropsch technology.
