Power-to-methanol: The role of process flexibility in the integration of variable renewable energy into chemical production

https://doi.org/10.1016/j.enconman.2020.113673Get rights and content

Highlights

  • Flexibility is key to tackling the variability of renewable energy sources.

  • Flexible chemical process is able to reduce the methanol production cost by 20–35%

  • Interplay between subsystems offers space for overall optimisation.

  • Process flexibility and process integration may compete in advanced designs.

Abstract

Chemical process electrification and renewable energy integration facilitate one another along the pathway towards a greener industry. However, integrating intermittent and variable renewable power into large-scale chemical processes, which conventionally are preferred to operate at a steady-state with a constant load, could lead to prohibitive costs if intermittency is addressed solely by energy storage. Here, we consider the concept of a flexible chemical process which can operate with a variable load throughout the year while meeting a specified annual production target. Using methanol production via carbon dioxide hydrogenation as a case study and by means of process conceptual design and optimisation, we investigate how the over-sizing of flexible process units and the introduction of intermediate storage in the chemical process offer the possibility to improve the overall performance of systems. The impact of the characteristics of renewable power is also explored by performing the analysis using meteorological data from two locations dominated respectively by wind and solar energy. This study shows clear potential benefits of process flexibility when the renewable energy supply is highly variable and is to achieve a high level of penetration. For a 100% renewable production, the introduction of flexibility reduces the levelised cost of methanol by approximately 21 and 34% for the two case study locations, respectively. The cost attribution reveals further insights into the origin of the economic advantages through examining the comparative costs of chemical production, energy generation, intermediate product storage and renewable energy storage. The learning from this work suggests that incorporating process flexibility through a holistically optimised design of energy storage and chemical production has the potential to offer an economically viable route to large-scale green chemical production through renewables-enabled electrification.

Introduction

The transition in the major energy sources to fuel the economy has occurred several times in history: from animal power and biomass through coal to oil and gas, now moving towards a range of renewables in their modern versions. For chemical industry, the ongoing energy transition potentially impacts on both its energy supply and feedstock. In particular, electrification of chemical production with renewables presents a great opportunity to drive the industry away from its unsustainable dependence on fossil fuels [1], which can be viewed as part of the widespread electrification in the economy to harness the synergies from coordinating the deployment of renewable power generation and the demand sectors [2]. It is envisioned that the transition of electrification and the use of renewable power, which promote each other, will lead to a “green” industry and become the key in meeting environmental targets [3].

Although chemical industry electrification with renewable energy is promising, a high penetration of variable renewable sources in power systems poses significant operational challenges. Unlike other energy uses where demand side flexibility could be brought in to address the variability and intermittency of renewable energy [4], conventional chemical processes typically operate continuously at a steady-state, imposing a constant (and rigid) energy demand. Such a rigid demand on the other hand cannot be adequately met by the management of the generation side for mitigating renewable energy variability, such as diversifying renewable sources [5] and excess renewable energy provision [6]. To date, the limitation in renewable chemical production is largely addressed by energy storage, particularly in the form of compressed hydrogen (H2) to level renewable power output, which has been demonstrated in both “green” ammonia [7] and methanol production [8]. Although H2 plays a critical role in renewable energy storage, its technical barriers [9] are significant – expensive storage tanks, high pressure storage and potential safety issues – making it less favourable for long-term storage and large-scale applications. In light of this, it is urgent to explore new concept and techniques specifically for chemical production powered by renewables.

To tackle the burden of energy storage for integrating renewables into chemical industry, a separate line of thinking is on tapping into the flexibility in chemical processes, which has not been much emphasised in conventional steady-state production. A flexible chemical process allows the demand side to adjust according to the variable pattern of renewable energy, thus incorporating demand side management to balance power generation and end-use load. In fact, the reason why a H2-based energy storage system (ESS) has been widely used in balancing renewable power generation [10] can be attributed to its flexible operation, i.e., water electrolysis can generally run at flexible loads to accommodate the variability of renewable sources. Unlike the electrolysers, chemical synthesis reactors and other complex chemical processes require careful management in order to accommodate the capacity of being flexible.

Several recent studies have recognised the importance of the operational flexibility of chemical reactors, with the anticipation of increasing integration with renewable sources [11]. It should be pointed out that disparity exists in the level of flexibility rendered by different chemical process units. For example, a wide range of feasible load was reported of a fixed-bed methanation reactor with the lower and upper bounds of the superficial velocity being 10 times apart [12]. In contrast, cryogenic distillation for air separation was considered to be much less flexible, with the feasible load being no lower than 60% of its full capacity [13]. A chemical plant typically comprises multiple types of units for distinct tasks such as chemical conversion and separation; the difference in their flexibility makes it very challenging to vary the load of the chemical plant as a whole. Instead, these units may have to operate at different load levels within the limits they can individually accommodate. This would necessitate storage of intermediate products (exchanged between upstream and downstream processing) and possibly energy (such as heat due to energy integration between process units) to align the operation of multiple units. Furthermore, the use of such storage would need to be optimally coordinated with the use of the renewable energy storage, hence the need for the optimal coordination between flexible energy generation and flexible chemical production. This complexity has not been understood in existing power-to-chemical studies that consider some degree of process flexibility [14] (typically with the involved chemical reactor). Despite the added complexity, this strategy may lead to an enhanced economic feasibility of renewables powered chemical production, provided that the benefits of the flexibility-enabling storage introduced within the chemical plant outweighs its cost. Should such a system be proven to be more advantageous than integrating variable renewable energy into chemical production that operates at a constant load mediated solely by (expensive) energy storage, it could become one of the new paradigms for chemical process design in the era of renewables.

Renewable methanol as a commodity [15] or a vector to synthetic hydrocarbons [16] has received increasing attention in the recent literature, often with an emphasis on carbon dioxide (CO2) valorisation [17]. In this work, we have undertaken a detailed model-based evaluation of the novel strategy with power-to-methanol production. We have chosen methanol for a detailed case study not only because of its own importance as a platform chemical that can be further converted to a wide range of other chemicals and materials, but also because it presents a good range of challenges in terms of exploiting process flexibility to cope with the variable supply of renewables: (1) it involves reaction and separation process sections which are both fairly complex; (2) its use of CO2 as a feed and its production of crude methanol as an intermediate product introduce the potential need for storage of both gaseous and liquid materials; (3) the process also offers an opportunity to investigate the implication of process flexibility on heat integration across multiple units which is commonly encountered in a sophisticated modern chemical plant. These features are representative of future renewables-powered productions of carbon-based chemicals and fuels.

Herein, we consider a methanol plant which acquires CO2 by carbon capture from a point source and H2 from electrolysis, with a methanol synthesis loop centred on a catalytic reactor and a distillation-based step for product purification. The plant investigated in this work is fully electrified, supplied by a wind and/or solar power generation facility which is optionally complemented by dispatchable power. In this system, the variability of renewable energy supply is tackled by a combination of a H2-based ESS and process flexibility enabled by the storage of process materials and energy (heat). A detailed optimisation model was constructed to establish the economically best design of the combination, in terms of the sizes of system components, power mix, and the arrangement of energy and material flows between different system units. The optimal design was contrasted with the non-flexible conventional production under different settings of the renewables’ profile, the price of dispatchable power and the level of attainable process flexibility. In this analysis, both the cost for methanol production and level of renewables penetration (as an environmental consideration) were assessed. To our knowledge, this is the first detailed modelling work on power-to-methanol to investigate holistically the interplay between H2 and methanol processes and the complementary roles of multiple types of storage within the whole system. As a new effort in the emerging area of combining process electrification with CO2 utilisation, the intention of this work is to reveal the key mechanisms by which process flexibility can potentially improve the economic and environmental performances of chemical production powered by variable renewable sources.

Section snippets

Concept overview

A fully electrified methanol process (adopted from our previous study [18]) with an annual production rate of approximately 400,000 tonnes was considered for two geographical locations, namely Norderney (Germany) and Kramer Junction (US), which have excellent wind and solar power sources, respectively, with profiles quantified by year-round hourly data. As shown in Fig. 1, the system that converts power (from both variable renewables and the dispatchable backup) to methanol consists of four

Results

The optimisation results are shown to be strongly affected by two factors, i.e., the dispatchable energy price and the degree of process flexibility, which are of special interest from both policy making and engineering perspectives. We first evaluate a base case scenario with fixed values for these two parameters, which renders a detailed cost breakdown and yields insight into the impact of process flexibility on key design choices. Subsequently, the analysis scope extends to explore the

Discussion

The results of this study show clear potential of process flexibility in facilitating the integration of variable renewable energy (VRE) into a chemical process, by accommodating the variability of renewable sources with reduced reliance on expensive VRE over-provision and its subsequent storage. However, for a viable flexibility implementation there are two prerequisites derived from the interplay triangle shown in Fig. 4: (1) the cost of the core flexible chemical process units, i.e., those

Conclusions

Using methanol production as an example for electrification and the use of renewable energy in chemical industry, this work reveals clear potential of incorporating load flexibility in the chemical processes to improve the economics of integrating variable renewable energy. The demonstrated benefits of process flexibility arise from reduced expensive energy (H2) storage (when the renewable generation is expensive) and, in certain cases, improved energy utilisation due to the reduction of

Nomenclature

Abbreviations
CapExCapital expenditure
CFCapacity factor
ESSEnergy storage system
LCOMeOHLevelised methanol cost
LECLevelised energy cost
MEAMonoethanolamine
MVRMechanical vapour recompression
OpExOperational expenditure
PVPhotovoltaic
SOFCSolid oxide fuel cell
TFCCTotal fixed capital cost
VREVariable renewable energy
Parameters
akConstant utility coefficient for subsystem k
bConstant material coefficient
cjUnit price of utility j ($/kg or $/kWh)
CjAnnualised cost of utility j ($/year)
CkAnnualised cost of

CRediT authorship contribution statement

Chao Chen: Conceptualisation, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Writing – original draft. Aidong Yang: Conceptualisation, Formal analysis, Methodology, Software, Supervision, Validation, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

CC thanks the China Scholarship Council and Jesus College, Oxford for funding the studentship. The authors wish to thank Dr. Guoping Hu for a critical review of the manuscript.

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