Homogeneous high-temperature gas-phase processes for energy storage and carbon dioxide utilization

One of the key goals of this decade is halving greenhouse-gas emissions by 2030 to minimize the devastating effects of climate change. Therefore, incorporated technologies must be sustainable and immediately applicable while remaining cheap and safe to be socially and politically acceptable. A promising solution is the application of established and technically matured technologies with alternative operating strategies. Combustion engines are established techniques that make it possible to provide various forms of energy in all sectors according to demand and to guarantee energy supply based on renewable energies. However, instead of the usually fuel-lean engine operation and the associated provision of mechanical or electrical energy, the use of fuel-rich mixtures in the combustion engine leads to the simultaneous provision of heat, mechanical or electrical energy, and valuable base chemicals. The engine can also complement the future energy system in the context of "power-to-gas", as the electrical or mechanical energy supplied can lead to the formation of valuable, high-energy chemicals. Then, the engine would be a reciprocating compressor whose cylinder is essentially a chemical reactor, not for producing work, but for chemical production. Due to the work supplied during the compression stroke and the corresponding temperature increase, the pyrolysis or the dry reforming of methane or natural gas can occur without the use of a catalyst. These processes produce chemicals (e.g., hydrogen, olefins, or aromatics) with significantly higher chemical exergy than their reactants. Thermodynamically, this con be called exergy storage since the work supplied is converted into chemical exergy, which can be stored. The CO2 neutral equivalent in the context of carbon capture and utilization occurs when CO2 and methane or natural gas is converted to synthesis gas.

Based on the first considerations, this work aims to investigate the feasibility and efficiency of such engine-based exergy storage and dry reforming processes and to identify possible operating conditions and principles. For this purpose, systematic investigations were conducted by performing engine simulations and validation experiments. The engine simulations allow the kinetic and exergetic investigation and assessment of engine-based pyrolysis and dry reforming without having to perform expensive or hazardous experiments. However, the outcome of these engine simulations is affected by the thermodynamic-kinetic models describing the state-dependent fluid properties and reaction rates of chemical reactions. Consequently, the simulations are complemented by validation experiments in a shock tube to identify what chemical reactions occur under the conditions and to validate thermodynamic-kinetic models. The shock tube experiments were performed at 1800–2700 K and 1 atm, investigating the CO formation during the dry reforming of alkanes at a reaction time of 3 ms. The experimental data will also be analyzed extensively using reaction pathway and sensitivity analyses.

The results of the simulations show that maximum temperatures in the engine of 1400–1800 K are required to convert ~70-80% of the reactants, methane, natural gas, or CO2. However, due to the high heat capacity of the reactants, an argon dilution of up to 97% is required to achieve the needed maximum temperatures. The main products are hydrogen, acetylene, ethylene, and benzene, as well as CO if CO2 is present in the initial mixture. The variation of the initial conditions and the mixture composition allows for the production of certain chemicals on demand: Ethylene and benzene are favored when the maximum temperatures are small or intermediate (1300–1600 K). In contrast, acetylene and hydrogen are favored when the maximum temperatures are high enough (>2000 K). An undesirable by-product is soot, but, by adding hydrogen to the initial mixture, the product distribution is shifted toward C2-species, such as acetylene and ethylene, inhibiting soot formation. The addition of CO2 has a similar effect since the O-atoms in CO2 cause oxidation reactions, shifting the product distribution slightly toward C2-species and the equilibrium product CO. The exergetic analysis showed that exergy losses due to entropy production in chemical reactions are minimal; accordingly, exergetic efficiencies achieve values of up to 75%.

The results obtained by the engine simulations are supported by the results of the experimental investigation, as they show that the temperatures and time frames of the engine are sufficient for the chemical processes. The analysis of the experimental data showed that the driving force for CO formation during the dry reforming of hydrocarbons is the formation and consumption of hydrogen radicals. Specifically, this occurs from abstraction or addition by or from C2 hydrocarbons. The experimental results generally agree with those of the kinetic shock tube simulations. Sensitivity analyses performed as part of this work revealed potential improvements for more accurate predictions.


Citation style:
Could not load citation form.


Use and reproduction:
All rights reserved