(438e) Natural Gas to Hydrogen Via a Process Intensified Cold Atmospheric Plasma-Based Reformer

This research underscores the paradigm that challenges in reforming fossil fuels into cleaner energy sources lie not within the feedstocks themselves but in the processes utilized. By developing a "clean" chemical process via Cold atmospheric plasma-based reforming, we aim to fundamentally mitigate CO2 emissions, steering away from traditional combustion-based methods that are less efficient and environmentally harmful. Electrifying the current system has a high potential to reduce emissions and increase the efficiency of chemical processes. Technologies, including electric heating, RF, microwaves, plasma reactors, and electric furnaces, are some options that can be researched and developed for the near future.

The current research aims to develop plasma technology for chemical processes at low temperatures. Low-temperature plasma (LTP) operates outside thermal equilibrium, exhibiting higher electron and vibrational temperatures than their rotational and translational counterparts in molecules. This characteristic makes LTP especially suitable for chemical processes, initiating reactions at low temperatures and power through electron collisions that produce highly reactive species. The vibrational excitation, explained by the ladder-climbing mechanism leading to molecular dissociation and the formation of radicals via electron impacts, are foundational to the chemical processes in LTP.

In LTP, radicals form first by electron impact reactions, and once radicals are generated, propagation and recombination reactions occur to obtain final products. These reaction pathways are generally agreed upon for DC macro and micro-plasma discharges. Vibrationally excited molecules can significantly accelerate chemical reactions by effectively decreasing activation energy. The appeal of LTP extends to chemically reacting atmospheric pressure plasmas, including cold atmospheric pressure plasmas (CAP) like microplasmas, coronas, and dielectric barrier discharges utilized in ionizers. These systems operate efficiently at low temperatures and atmospheric pressure, reducing costs and system complexity. However, the kinetic effects of excited species on chemical processes in non-equilibrium plasma remain partially understood, presenting a significant research opportunity.

To some degree, all gases have some small fraction of ionized particles and free electrons for two main reasons. The first of which is their finite temperature, which leads to a small portion of particles having adequate internal energy to be ionized, as expressed by the Saha equation. The second reason is due to the cosmic sources of ionization radiation. Plasmas are created when there are a sufficient number of charged particles for collective behavior in the medium. The collective behavior is called the plasma approximation and is valid when the electron density (or charged particles) within the sphere of influence of the particle is large enough.

The fundamental inquiry of mitigating CO2 emissions led to the conceptual realization that reducing CO2 could be achieved by circumventing its production. Traditional methods of reducing emissions have predominantly centered on post-production capture and sequestration, but this approach is often costly and not entirely effective. The core concept centers around the transformative potential of plasma-based technology, offering a novel approach to reforming hydrocarbons into valuable products while circumventing the release of direct CO2 emissions. The core concept centers around the transformative potential of cold atmospheric plasma-based technology, offering a novel approach to reforming hydrocarbons into valuable products while circumventing the release of direct CO2 emissions.

Methane is chosen as the feedstock to produce hydrogen for further research on the experimentally developed CAP reformer. The backdrop against which this innovation emerges is the critical role of hydrogen as a raw material in various industrial processes, including fertilizer production, food processing, metal treatment, and petroleum refining. The dominant method for hydrogen production, steam methane reforming, currently accounts for a staggering 90% of hydrogen production in the USA. However, this process comes with significant drawbacks. Annually, it results in the emission of 800 million tonnes of CO2 in the USA alone, equivalent to the combined emissions of the UK and Indonesia. Additionally, high temperatures (around 800°C) are required to reform natural gas into hydrogen, which is both energy-intensive and environmentally harmful.

In this reforming system, the feedstock (natural gas) passes through a nonthermal plasma-based reformer and reforms to hydrogen at ambient temperature and pressure without the presence of O2. Then, the produced gases and hydrogen exit the reformer toward a membrane separation. The produced H2 separates from other produced light hydrocarbons and methane. The retentate gasses from the membrane return to the beginning of the process and mix with natural gas feedstock to reform to H2.

Basically, the system consists of three main parts: (a) the nanoseconds plasma-based reformer, (b) the membrane and separation part, and (c) the compressors for recirculating gas through the system. The nanosecond plasma-based reformer consists of two main components: (1) plasma reactor and (2) electric power supply. When a high voltage pulse is applied across electrons, neutrons and ions will be accelerated. As enough energy is implied, electrons gain enough energy to break through and flow between two electrodes. While electrons move from anode to cathode, electrons and ions, collide with other neutral molecules in the reactor, creating more electrons and ions.

Pulse parameters include two main factors: the input voltage and the pulse frequency repetition. Consequently, the power input and specific energy input can be calculated. As a result, increasing applied voltage directly increases H2 production. Observations show that the maximum H2 production reaches 36.6% of molar concentration at 30 KV. The SEI is a critical factor in evaluating the performance of plasma-based reforming. SEI is the input energy per unit of input mass of methane and is linearly related to the input energy. Changing PRF (pulse reputation frequency) increases the SEI at a higher rate in comparison to the input voltage. Hence, in order to reduce the input energy, a lower frequency is preferred, while enough pulses are required to reform natural gas.

SEI depends on the input power and flow-in rate. Generally, increasing power enhanced CH4 conversion and H2 production. The maximum hydrogen production is achieved at 30 W.

At minimum input power (10 W), 23% of methane molar flow was reformed, and at the maximum power (30 W), 60.1% of methane was converted. The H2 production at the minimum and maximum input power is 14.9% and 34 %, respectively. However, the optimum condition is defined as the highest H2 yield (kg/kJ). As experimental studies show, at higher flow rates, more pulses are required for reforming. In general, at higher flow rates, the produced hydrogen is higher. However, techno-economic analyses are required to find the most profitable operation condition.

Four scenarios are assumed to study the effect of flow rate through each reactor and the total flow of the plant on Net H2 revenue. Cases 1 and 3 studied the 1.1 l/min flow through each reactor and the total flow rate of 50000 and 100000 m3/hr. Cases 2 and 4 explain the flow rate at 2.2 l/min at each reactor and the entire plant flow of 50000 and 100000 m3/hr. As concluded, increasing the flow rate through each discharge and system flow rate increases the Net H2 revenue. The highest achieved Net H2 revenue is at 2.2 l/min flow through each reactor with a total plant flow of 100000 m3/hr. Sensitivity analyses have been done to study the effect of flow rate and find the upper and lower boundaries of operation.

Aspen Plus is utilized to simulate both input and output streams by solving the mass and energy balances. The base case scenario is a plant feedstock of 100000 m3/hr. A detailed methodology is used to perform a techno-economic analysis on the nanosecond plasma-based hydrogen plant. The results are compared with electrolysis and SMR plants with and without a carbon-capturing system. As a result, the natural gas demand for the plasma reformer plant is 53.72% lower than the SMR plant. By comparing the energy demand for plasma-based reformers with electrolysis technology, it can be concluded that plasma-based reforming requires 87.57% less energy. Based on the unit of energy, the H2 production yield for the plasma-based plant is 44.42 kg/MW, and for the electrolysis-based plant is 20 kg/MW.

Regarding the environmental impact of the hydrogen production pathways, direct emissions are exclusively higher in the SMR processes. The CAP plasma-based reformer emits zero CO2 directly in the process. A typical SMR plant without CCS releases 9.2 kg CO2 for producing one kilogram of H2. This carbon dioxide can be reduced to 1 kg CO2/ kg H2 by hiring CCS systems. The CAP plasma-based technology produces hydrogen for 2.89 $/kg H2. The Levelized cost of H2 production for electrolysis, SMRs, and SMRs with CCS are 4.51, 1.53, and 1.95 $/kg, respectively.

In summary, a novel nonthermal plasma-based reformer has been created to reform natural gas to hydrogen. A separation system is designed to separate hydrogen from other co-products. The CAP plasma-based reformer was created and went under investigation to find its optimum operating condition. The optimal operating condition for the plasma-based reformer is achieved by experimenting and studying the effect of changing reactors' flow rate, input energy, and pulse frequency repetition. The optimum operating condition is defined as the maximum accomplished H2 yield (kg/kJ). As a result, the process can compete with benchmarked technologies with zero direct CO2 emission and no water consumption.