Techno-economics optimization of H2 and CO2 compression for renewable energy storage and power-to-gas applications

The decarbonization of the industrial sector is imperative to achieve a sustainable future. Carbon capture and storage technologies are the leading options, but lately the use of CO2 is also being considered as a very attractive alternative that approaches a circular economy. In this regard, power to gas is a promising option to take advantage of renewable H2 by converting it, together with the captured CO2, into renewable gases, in particular renewable methane. As renewable energy production, or the mismatch between renewable production and consumption, is not constant, it is essential to store renewable H2 or CO2 to properly run a methanation installation and produce renewable gas. This work analyses and optimizes the system layout and storage pressure and presents an annual cost (including CAPEX and OPEX) minimization. Results show the proper compression stages need to achieve the storage pressure that minimizes the system cost. This pressure is just below the supercritical pressure for CO2 and at lower pressures for H2, around 67 bar. This last quantity is in agreement with the usual pressures to store and distribute natural gas. Moreover, the H2 storage costs are higher than that of CO2, even with lower mass quantities; this is due to the lower H2 density compared with CO2 . Finally, it is concluded that the compressor costs are the most relevant costs for CO2 compression, but the storage tank costs are the most relevant in the case of H

Thermal Energy Storage in Concentrating Solar Power Plants: A Review of European and North American R&D Projects

Thermal energy storage (TES) is the most suitable solution found to improve the concentrating solar power (CSP) plant’s dispatchability. Molten salts used as sensible heat storage (SHS) are the most widespread TES medium. However, novel and promising TES materials can be implemented into CSP plants within different configurations, minimizing the TES costs and increasing the working temperature to improve the thermal performance of the associated power block. The first objective of this review is to provide an overview of the most widespread CSP technologies, TES technologies and TES-CSP configurations within the currently operational facilities. Once this information has been compiled, the second aim is to collect and present the existing European and North American TES-CSP Research and Development (R&D) projects within the last decade (2011–2021). Data related to these projects such as TES-CSP configuration path, TES and CSP technologies applied, storage capacity, power block associated and the levelized cost of electricity (LCOE) of the commercial up-scaling project are presented. In addition, project information such as location, research period, project leader and budget granted are also extracted. A timeline of the R&D projects launched from 2011 is built, showing the technology readiness level (TRL) achieved by the end of the project

Calcium looping as chemical energy storage in concentrated solar power plants: Carbonator modelling and configuration assessment

This paper addresses the analysis of different configurations of carbonator for thermochemical energy storage for concentrated solar applications. The design of this equipment is different from the previous experience of calcium looping cycle for carbon capture. The use of fluidized beds and large particles are not feasible for this novel application of calcium looping. New reactors and different arrangements for the carbonation process are necessary. The design of a carbonator reactor for a specific Calcium Looping-Concentrated Solar Power application has not been addressed yet in detail in literature. In this work, a comparison of single stage reactor, two parallel reactors and two reactors in series with intercooling are simulated to calculate conversion rates, gas temperatures and flow rates, and heat transfer rates to the external cooling fluid. The modelling encompasses fluid dynamics, lime conversion kinetics and heat transfer, which are solved using a 1-D discrete mesh. The third arrangement results in the most reasonable sizes, and larger conversion rates, avoiding the occurrence of internal reactor zones in which the reaction is inhibited. Energy balance components are also quantified for each configuration

Comparative study of optimized purge flow in a CO2 capture system using different sorbents

AbstractOne of the most promising options for CO2 capture in large power generation facilities is the system based on the CO2 sorption loop. This method has gained rapid importance due to promising carbonator CO2 capture efficiency, the existence of low cost sorbents and the fact that no gas pre-treatment unit is needed before entering the system. The sum of these features results in a competitively low cost CO2 capture system when using low cost natural sorbents. Different regenerable sorbents are being investigated for large-scale CO2 capture purposes and high temperature Mg-based, Li-based and Ca-based sorbents are considered as suitable candidates. This study considers the applicability of lithium orthosilicatum, hydrated limestone and raw natural limestone. A basic configuration that makes use of two interconnected circulating fluidized beds (carbonator and calciner) has been studied. Among the key variables that influence the performance of these systems, the carbonation conversion of the sorbent and the heat requirement at calciner are the most relevant. Both variables are mainly influenced by sorbent/CO2 ratio and make-up flow (purge) of solids. Purge is necessary to mitigate the sorbent deactivation. Large sorbent/CO2 ratios improve the carbonation conversion but also increase the cost of the system due to a more intensive solid circulation. Large make-up flow also improves the extent of sorption phenomena and hence the CO2 capture, but increases the heat demand at calciner and the fresh sorbent cost. The aim of this paper is to calculate the optimum make-up flow of fresh sorbent and sorbent/CO2 ratio for a set of these regenerable sorbents in order to minimize the capture cost of the system integrated into a power plant. Resulting optimal values are compared to assess the energetic performance and CO2 capture cost of the cycle for each sorbent material

Improved Flexibility and Economics of Combined Cycles by Power to Gas

Massive penetration of renewable energy in the energy systems is required to comply with existing CO2 regulations. Considering current power pools, large shares of renewable energy sources imply strong efficiency and economic penalties in fossil fuel power plants as they are mainly operated to regulate the system and constant shutdowns are expected. Under this framework, the integration of a combined cycle power plant (CCPP) with an energy storage technology such as power to gas (PtG) is proposed to virtually reduce its minimum complaint load through the diversion of instantaneous excess electricity. Power to gas produces hydrogen through water electrolysis, which is later combined with CO2 to produce methane. The main novelty of this study relies in the improved flexibility and economics of combined cycles by means of using power to gas as a tool to reduce the minimum complaint load. The principal objective of the study is the quantification of cost reduction under different scenarios of shutdowns and conventional start-ups. The case study analyses a combined cycle of 400 MWe gross power with a minimum complaint load of 30% that can be virtually reduced to 20% by means of a 40- MWe power-to-gas plant. Eight scenarios are defined to compare the reference case of conventional operation under hot, warm, and cold start-ups with power-to-gas-assisted operation. Additionally, PtG-assisted operation scenarios are analyzed with different loads (30–50–70%). These scenarios also include the consideration of a temporary peak of demand occurring in a period in which dispatch is below the minimum complaint load. Under this situation, the response time of conventional plants is very limited, while PtG-assisted CCPP can rapidly satisfy the peak. The techno-economic model quantifies the required fuel, gross and net power, and emissions as well as total costs and incomes under each scenario and net differential profit in an hourly basis. The analysis of the obtained results does not recommend the operation of the PtG-assisted CCPP at minimum complaint load for hot, warm, or cold start-ups. However, important marginal profits are achieved with the proposed system for part-loads operation over 50% for every sort of start-up, avoiding shutdowns and extending the capacity factor

Decision-making methodology for managing photovoltaic surplus electricity through Power to Gas: Combined heat and power in urban buildings

Power to Gas technology, which converts surplus electricity into synthetic methane, is a promising alternative to overcome the ¿uctuating behavior of renewable energies. Hybridization with oxy-fuel combustion provides the CO2 ¿ow required in the methanation process and allows supplying both heat and electricity, keeping the CO 2 in a closed loop. The complexity of these facilities makes their management a key factor to be economically viable. This work presents a decision-making methodology to size and manage a cogeneration system that combines solar photovoltaic, chemical storage through Power to Gas, and an oxy-fuel boiler. Up to 35 potential situations have been identi¿ed, depending on the surplus electricity, occupancy of the intermediate storages of hydrogen and synthetic methane, and thermal demand. For illustration purposes, the methodology has been applied to a case study in the building sector. Speci¿cally, a building with 270 kW of solar photovoltaic installed power is analyzed under nine energy scenarios. The calculated capacities of electrolysis vary from 65 kW to 96 kW with operating hours between 2184 and 2475 h. The percentage of methane stored in the gas grid varies from 0.0% (no injection) to 30.9%. The more favorable scenarios are those with the lowest demands, showing temporary displacements beyond the month between injection and utilization

A review on CO2 mitigation in the Iron and Steel industry through Power to X processes

In this paper we present the first systematic review of Power to X processes applied to the iron and steel industry. These processes convert renewable electricity into valuable chemicals through an electrolysis stage that produces the final product or a necessary intermediate. We have classified them in five categories (Power to Iron, Power to Hydrogen, Power to Syngas, Power to Methane and Power to Methanol) to compare the results of the different studies published so far, gathering specific energy consumption, electrolysis power capacity, CO2 emissions, and technology readiness level. We also present, for the first time, novel concepts that integrate oxy-fuel ironmaking and Power to Gas. Lastly, we round the review off with a summary of the most important research projects on the topic, including relevant data on the largest pilot facilities (2–6 MW)

Techno-economic feasibility of power to gas–oxy-fuel boiler hybrid system under uncertainty

One of the main challenges associated with utilisation of the renewable energy is the need for energy storage to handle its intermittent nature. Power-to-Gas (PtG) represents a promising option to foster the conversion of renewable electricity into energy carriers that may attend electrical, thermal, or mechanical needs on-demand. This work aimed to incorporate a stochastic approach (Artificial Neural Network combined with Monte Carlo simulations) into the thermodynamic and economic analysis of the PtG process hybridized with an oxy-fuel boiler (modelled in Aspen Plus®). Such approach generated probability density curves for the key techno-economic performance indicators of the PtG process. Results showed that the mean utilisation of electricity from RES, accounting for the chemical energy in SNG and heat from methanators, reached 62.6%. Besides, the probability that the discounted cash flow is positive was estimated to be only 13.4%, under the set of conditions considered in the work. This work also showed that in order to make the mean net present value positive, subsidies of 68 €/MWelh are required (with respect to the electricity consumed by PtG process from RES). This figure is similar to the financial aids received by other technologies in the current economic environment

Lab-scale experimental tests of Power to Gas-Oxycombustion hybridization: system design and preliminary results

Power-to-Gas (PtG) represents one of the most promising energy storage technologies. PtG converts electricity surplus into synthetic natural gas by combining water electrolysis and CO2 methanation. This technology valorises captured CO2 to produce a ‘carbon neutral’ natural gas, while allowing temporal displacement of renewable energy. PtG-Oxycombustion hybridization is proposed to integrate mass and energy flows of the global system. Oxygen, comburent under oxy-fuel combustion, is commonly produced in an air separation unit. This unit can be replaced by an electrolyser which by-produces O2 reducing the electrical consumption and the energy penalty of the carbon separation process. The aim of this work is to present the design, construction and testing of a methanation reactor at laboratory scale to increase the knowledge of the key component of this system. Experimental data are used to validate the theoretical kinetic model at different operating temperatures implemented in Aspen Plus. CO2 conversions about 60-80% are found for catalyst temperature between 350 and 550 ºC. These values agree well with expected theoretical conversions from the kinetic model

CO2 recycling in the iron and steel industry via power-to-gas and oxy-fuel combustion

The iron and steel industry is the largest energy-consuming sector in the world. It is responsible for emitting 4-5% of the total anthropogenic CO2. As an energy-intensive industry, it is essential that the iron and steel sector accomplishes important carbon emission reduction. Carbon capture is one of the most promising alternatives to achieve this aim. Moreover, if carbon utilization via power-to-gas is integrated with carbon capture, there could be a significant increase in the interest of this alternative in the iron and steel sector. This paper presents several simulations to integrate oxy-fuel processes and power-to-gas in a steel plant, and compares gas productions (coke oven gas, blast furnace gas, and blast oxygen furnace gas), energy requirements, and carbon reduction with a base case in order to obtain the technical feasibility of the proposals. Two different power-to-gas technology implementations were selected, together with the oxy blast furnace and the top gas recycling technologies. These integrations are based on three strategies: (i) converting the blast furnace (BF) process into an oxy-fuel process, (ii) recirculating blast furnace gas (BFG) back to the BF itself, and (iii) using a methanation process to generate CH4 and also introduce it to the BF. Applying these improvements to the steel industry, we achieved reductions in CO2 emissions of up to 8%, and reductions in coal fuel consumption of 12.8%. On the basis of the results, we are able to conclude that the energy required to achieve the above emission savings could be as low as 4.9 MJ/kg CO2 for the second implementation. These values highlight the importance of carrying out future research in the implementation of carbon capture and power-to-gas in the industrial sector. © 2021 by the authors