Economic Evaluation of Novel C-Zero Processes for the Efficient Production of Energy, Chemicals, and Fuels ^†

Abstract

The aim of this study is to provide a comprehensive analysis of the outcome of two separate techno-economic studies that were conducted for the scaled-up and industrially relevant processes of a) synthetic natural gas (SNG) production from captured (cement-based) CO₂ and green-H₂ (via renewable-assisted electrolysis) and b) combined electricity and crude biofuel production through the integration of biomass pyrolysis, gasification, and solid oxide fuel cells. As was found, the SNG production process seems more feasible from an economic perspective as it can be comparable to current market values.

1. Introduction

The aim of this study is to provide a comprehensive analysis of the outcome of two separate techno-economic studies that were conducted for the scaled-up and industrially relevant processes of (a) synthetic natural gas (SNG) production from captured (cement-based) CO₂ and green-H₂ (via renewable-assisted electrolysis) and (b) combined electricity and crude biofuel production through the integration of biomass pyrolysis, gasification, and solid oxide fuel cells. The overall results have been previously published in [1,2] and a more generic description of the methodology and the respective results is provided hereafter along with a future vision. To this end, this short article will briefly present the methodology followed, the process description, and, finally, the economic evaluation and comparison of the two processes, adhering to the initiatives of recent Energy and Climate Change Policies [3,4].

2. Steps Followed Towards the Economic Evaluation of C-Zero Processes

While the two processes might not be fully aligned, due to the different feedstock, the end-user products, and the integrated processes taken place, the overall methodology that was followed was similar in both cases. Below, the reader can find the applied steps as discussed in a simplified way [1,2,5,6]:

Step 1: Development of a realistic process flow diagram (PFD) by including all necessary equipment (e.g., reactors, heat exchangers, fluid transportation, storage, recycling streams). Simulation of the proposed integrated process in Aspen Plus (or other relevant software) and analysis of the mass and energy balances in a way that will be meaningful in the scaled-up implementation.

Step 2: Estimation of the Major Equipment Cost (prior installation) by using common empirical equations that take into account key sizing parameters and are widely available in engineering handbooks. In order for the estimation to be realistic, the user can incorporate available market prices for standard equipment and also apply cost escalation (through Marshall and Swift indices), as well as a direct comparison with the literature. This part is always useful as it takes into account monetary variations and cost inflations.

Step 3: Estimation of the yearly utility needs (e.g., steam, cooling water, energy, catalysts, solvents, cleaning consumables) and assignment of operating and financial factors such as plant operation factor, plant useful life, income tax rate, risk factor, depreciation, and annual interest rate.

Step 4a: Estimation of the total fixed capital investment (or most commonly known as Capital Expenditure/CAPEX) based on direct, indirect, and other costs that are evaluated on the basis of the Major Equipment Cost (MEC) in Step 1.

Step 4b: Estimation of the annual production expenditures (or most commonly known as Operating Expenditure/OPEX) based on direct, annual fixed, and general production costs that are evaluated on the basis of the yearly utility needs in Step 2.

Step 5: Estimation of critical economic indices such as gross and net profit, payback period, return on investment, and, most importantly, the break-even product prices (or levelized costs) that will render the venture feasible (minimization of the Net Present Value or a similar economic objective function).

3. Process Description and Scaled-Up Simulation

3.1. SNG Production Through Captured CO₂ and RES-H₂

Figure 1, as adapted from [1], presents the integrated concept of SNG production and reuse in cement industries (other heavy industries could also be included and refer to refineries, petrochemicals, steel, etc.) through the exploitation of flue gas emissions (ca., CO₂) and RES-powered electrolytic H₂. As a brief description, a mono-ethanol amine (MEA) absorber captures the CO₂ emissions from a cement-based flue gas stream and provides a ready-to-use CO₂ stream with a high (>90%) purity at a mass flow of 150–200 t/h. Moreover, a 6–7 GWp-sized hybrid RES system with intermediate storage supports the production of 4 times the captured CO₂ (in molar basis) and green-H₂ via water electrolysis at a mass flow rate of more than 30 t/h. In this way, the two streams are fed to a catalytic methanation reactor for the production of more than 70 t/h SNG (end product). As a marketable co-product, the electrolytic oxygen is also exploited and considered a marketable product ready to be used.

3.2. Electricity and Crude Biofuel Production Through Olive Kernel Valorization

Figure 2, as adapted from [2], presents the concept of olive kernel slow-pyrolysis, bio-char gasification, and power generation in a combined SOFC–steam turbine cycle. As a brief description, biomass (olive kernel) is fed to a pyrolyzer at a feed rate of 0.85 t/h. The biochar and the aqueous phase of pyrolysis enter an autothermal gasifier for the production of syngas along with recycling streams. The overall syngas stream is rich in CO/H₂ and enters a solid oxide fuel cell (SOFC) for the net electricity production of 1.4 MWp (including afterburners and steam turbines). As a marketable co-product, bio-oil is exploited.

4. Economic Analysis of the Novel C-Zero Processes

Table 1. Total fixed capital investment and annual production expenditures for the coupled process systems.

Cost Type	MEA-CO2	RES-H2	SNG Production (Including Utilities)	Biomass to Power and Biofuels
CAPEX, EUR	159,326,500	9,060,667,150	487,710,300	35,179,298
OPEX, EUR/y	50,352,200	39,811,900	55,315,000	8,283,850

presents a summary of the economic evaluation results and more information can be found in [1,2]. Figure 3 shows the respective break-even prices of the main products in both processes.

4.1. SNG Production Through Captured CO₂ and RES-H₂

As can be seen, SNG production through the captured CO₂ and RES-H₂ integrated process is highly dependent on the CAPEX of the RES-assisted water electrolysis that facilitates the continuous production of immense amounts of green-H₂. Meanwhile, the overall CAPEX of the MEA-CO₂ absorption and the catalytic SNG production units (including auxiliaries such as cooling tower, steam provision, etc.) is almost 14 times lower than the CAPEX for the RES-H₂ system. Regarding the overall OPEX, it is seen that it is ~1.5% of the overall CAPEX and insignificantly affects the levelized cost of SNG and electrolytic oxygen (Figure 3a). The break-even price of SNG can be gradually improved by increasing CO₂ penalty costs at 50–100 EUR/tn and 13%, subsidizing the RES-H₂ system (500 EUR/tn SNG with values reported in 2021 in [1]).

4.2. Electricity and Crude Biofuel Production Through Olive Kernel Valorization

As can be seen in Table 1, electricity and crude biofuel production through olive kernel valorization requires quite a low CAPEX, but in this concept, the OPEX is the driving economic force for the levelized cost of energy and crude biofuels (pyrolytic bio-oil) as it is almost 25% of the overall CAPEX. Employing the current technology costs and assuming progressive reduction up to 70% of the Major Equipment Cost, the CAPEX and the OPEX can be reduced. In this sense, the marketable electricity prices could be reduced to 0.274 EUR/kWh for a 70% subsidization of the fixed capital investment and to a further 0.249 EUR/kWh, for increased bio-oil prices (Figure 3b).

4.3. Critical Analysis and a Future Outlook

Integrated processes such as the above aim towards a carbon-neutral future, as posed by the recently announced EU Green Deal strategy, while the proposed processes clearly follow the needs for the mitigation of carbon emissions via the implementation of renewables under the circular economy demands. The introduction of RESs in energy production systems not only ensures compliance with the direction towards greener energy systems, but also provides access to the so-far unexploited and presently abundant energy resources. However, while the need for constant development in the form of process design, intensification, and detailed technical and economic analyses is evident in order to render such systems viable, the intermittency and variability in the availability of the RES-based sources dictate the need for developing appropriate storage options for the efficient management and utilization of the energy flows [7,8].