(124b) Fossil Fuel Assets and Wastewater Resources Integrated SOLID OXIDE Electrolytic Cell for a NOVEL Hydrogen Production

Water and energy are two inseparable resources needed for the existence, wellbeing and economic sustainability by human. The production of energy requires substantial amount of freshwater. There is an increase in energy demand due to several factors like the population growth. During the continuous expansion of an energy sector, the mix of the deployed technologies to produce fuels and generation of electricity controls the associated impact on freshwater resources in that area. This work explores the nexus model by focusing on the design, performance and economic estimation of a novel, highly efficient and environmentally benign hydrogen production process which can be integrated with several power production units, including coal, natural gas-fired power plants and renewable energy sources. The system employs flue gas from coal power plants and various wastewater sources to generate steam as the process's feedstock for the SOEC unit. The SOEC and the balance of plant components are designed, simulated, and integrated using Aspen HYSYS Software. The SOEC model is experimentally and numerically validated, and a piece-wise performance evaluation of the system is carried out. The result of this innovative system shows overall thermal-to-hydrogen efficiency of 56.3% and 61.97% if the heat of the high-temperature exiting species are put to use and the SOEC shows a voltage efficiency of 92.9% which can be improved depending on the energy source and by full energy Integration. The techno-economic analysis of the system gives a Levelized hydrogen cost of $2.99/kg, which can be upgraded to meet the U.S. Department of Energy (DOE) targets and the life cycle analysis shows using solar PV to power the system is of greater economic prospect . The study offers insights into water-energy nexus for the researchers and policymakers for decision making and technology development.

Keywords: Solid oxide electrolytic cell (SOEC), Hydrogen production, Flue gas, Water-energy nexus, techno-economic analysis (TEA)

INTRODUCTION

The production of hydrogen from renewable and sustainable sources is a key to the green hydrogen economy. Water electrolysis has gained attention and looks promising as the process is greenhouse gas-free and yield 100% pure hydrogen. Even though water is abundant, its accessibility is a great challenge, and its use for this process will undermine water security for domestic and agricultural use. One of the major crises faced in this generation due to its increasing population is the availability of clean drinking water, particularly in developing countries. Even the developed ones can only access 2% of the available water[1]. China, most especially its coal-rich region, is faced with water scarcity[2] and projected to be severe in 2050 and the arid and semi-regions like the Middle East and the western U.S. [3].

There are three major commercialized and conceptualized water electrolytic cells for the generation of hydrogen in recent times [4] which include alkaline electrolytic cell (AEC), the most matured technology [5], polymer electrolyte membrane (PEM) electrolytic cell (PEMEC) and solid oxide electrolytic cell (SOEC). The AEC challenges range from low current density to low operational pressure, which aimed to be solved with the advent of PEMEC[6]. Likewise, the PEMEC suffers from high capital cost, pure water requirement, and increased system complexity, stemming from its operational pressure [6, 7]. However, the SOEC is an evolving and promising technology that is anticipated to overcome the setbacks plaguing these two electrolyzers [8, 9]. Solid ion-conducting ceramics are used as electrolytes in the SOECs, which operate at high temperatures ranging from 650-1000 ℃ [9]. Several works have been done on this technology both in the laboratories [8] and industries to commercialize it [9]. Kazempoor and Braun[10] modeled and evaluated a SOEC system to serve as a primary system-level performance prediction tool. Several contemporary studies on SOEC focused on hydrogen production directly from water and steam [11-13]. For better system optimization high round-trip efficiency, SOEC has been designed and studied alongside its reverse counterpart, solid oxide fuel cell (SOFC), to form regenerative solid oxide cell (rSOC) [14-16]. To eliminate the uncertainty concerning future cost and high capital cost of this technology, recent literature evaluates its techno-economic viability to encourage investments[17-19].

This work is distinctive in its water electrolysis in that it explores the flue gas from power plant and wastewater as a feedstock into the hydrogen production unit integrated with a different energy source. The net energy and techno-economic analyses are carried out on the entire system for performance evaluation and sensitivity analysis on the system's critical parameters.

SYSTEM AND MODEL DESCRIPTION

The proposed system is shown in fig. 1. It comprises two sections, namely the upper steam generating section and the high-temperature SOEC unit- hydrogen production unit.

In the upper section, high pressurized steam is generated from the wastewater reservoir after it is pumped to the water treatment and recovery unit (WTRU) where it is evaporated at atmospheric pressure and 101 ℃. This is mixed with the steam, which is concurrently generated from the power plant flue gas. The flue gas is cooled to 25℃ in the condenser for moisture recovery, and the heat regained is used in the WTRU. The recovered water is immediately evaporated and combined with the steam from the WTRU. The mixed stream is sent to the pre-heater to elevate its temperature to 750-850℃ before being fed into the SOEC to lower the required entropic heat for decomposition. Air from the ambient environment is compressed, preheated, and supplied to the system to flush out the produced oxygen from the electrolysis reactions in the SOEC. The exiting hydrogen product is stored in a compressed hydrogen tank.

The water decomposition reaction taking place in the SOEC is represented by the anodic oxidation and cathodic reduction reaction below

Planar model configuration was used in the SOEC design with the system comprising of the balance of plant (BoP) such as condenser, heater, compressor, mixer, evaporator, separator, pump, heat exchanger to finally actualize a working model which are thermodynamically model using hysys. All other model assumptions are similar to that of Kazempoor and Braun[10].

RESULTS AND DISCUSSION

This section summarizes some of the important results of this work. The SOEC is in the developing stage, and this novel work is evaluated and validated. The system generated 242400 kg/day of hydrogen from the flue gas from a power plant with a capacity 333.7 MW. The voltage efficiency of the SOEC system is 92.9% at 850 ℃, and the overall thermal efficiency of the designed plant is 61.97%, which seems to be very prosperous compared to other plants of the same size. Even though it is shown from hysys that is potential to save over 25% of the utilized energy in the system which reduces cost of production and optimize productivity. To live up to the progressive development in different sources of energy, the model is designed to be integrated with several energy sources. The Levelized cost of hydrogen (LCOH) production with some of these energy sources is evaluated and has shown in fig. 2. The LCOH ranges from $2.99-5.96/ kg hydrogen with the solar photovoltaic (P.V.) given the cheapest cost assuming the U.S. Department of Energy (DOE) target of 3 cent/kWh by 2020 and beyond. As such, the life cycle analysis (LCA) from cradle to gate for the plant is performed in SimaPro using the solar P.V. and the coal power plant to measure the severity of the environmental impact. Obviously, the COx emission with the coal power plant is higher. However, SimaPro revealed the higher effects of some other critical parameter factors like the human health, ecosystem and particulate matter formation associated with it as shown in fig. 3.

CONCLUSIONS

The novel model designed in this work shows the possibilities of different energy integrations for hydrogen production and redefined a new approach to water-energy nexus. The LCOH of hydrogen can be as low as $2.99 per kg hydrogen, and this is a close route to meeting the DOE target for the cost of hydrogen for distribution and as potential energy storage. Powering the system with solar PV is of high economic prospect and further work needs to be embarked on to establish the exact environmental impact of large-scale hydrogen production to meet up the global requirement for sustainability and a green economy.