Prediction and Measurement of the Product Gas Composition of the Ultra Rich Premixed Combustion of Natural Gas: Effects of Equivalence Ratio, Residence Time, Pressure, and Oxygen Concentration

Abstract

The ultra rich combustion (partial oxidation) of natural gas to hydrogen and carbon monoxide is theoretically and experimentally investigated. The effect of the process parameters equivalence ratio, residence time, pressure, and composition of the oxidizer is explored. Computations are performed with the use of the chemical kinetics simulation package CHEMKIN. First, the ultra rich combustion process is modeled as a freely propagating flame in order to establish the rich flame propagation properties. An Arrhenius correlation of the laminar flame speed with the adiabatic flame temperature is found with activation temperature 20,000 K. Subsequently, perfectly stirred reactor (PSR) computations were performed. From these, it is concluded that optimal natural gas conversion to hydrogen and carbon monoxide requires a residence time of at least 50 ms and, depending on residence time, an equivalence ratio between 2 and 4. To reach chemical equilibrium in ultra rich mixtures, the residence time is very long (>1000 ms). The model predictions are validated by experiments on ultra rich combustion of natural gas by means of air enriched to 40% oxygen concentration at up to 3 bar and 300 kW. The effect of equivalence ratio at residence time 50 ms was investigated. Good comparison was found between measurements and model predictions on carbon monoxide, hydrogen, and the soot precursor acetylene. It can be concluded that the model provides reliable information on product gas concentrations as a result of ultra rich combustion of natural gas.

Experimental study of hydrogen production and soot particulate matter emissions from methane rich-combustion in inert porous media

“…This difference of residence time in the post-flame zone is quite important for the conversion of reactants to H 2 and CO. As it is known, the time evolution of reactions within a premixed rich-flame can be divided into two distinct zones; the flame front region where fast oxidation reactions take place and the post-flame region, which is dominated by slow endothermic reforming reactions [11,12,18,28]. Such reactions increase the concentrations of H 2 and CO and decrease the respective ones of CH 4 and H 2 O, towards equilibrium conditions.…”

Analysis of an idealized counter-current microchannel-based reactor to produce hydrogen-rich syngas from methanol

“…[40] (rich limit measured at 60 C). and fuel leaks downstream, where it is consumed by slow endothermic reforming reactions [5] (e.g., CH 3 OH þ H 2 O / CO 2 þ 3H 2 ) followed by water-gas-shift reactions (e.g., CO þ H 2 O / CO 2 þ H 2 ) that bring the species concentrations to equilibrium values. Products from freely-propagating flames at high equivalence ratios require extremely large residence times (tens of seconds) to reach the equilibrium concentrations.…”

“…The obtention of large hydrogen conversion rates from hydrocarbons in small-volume reformers requires completion of the chemical process within a short residence time. As shown for freely propagating, adiabatic rich premixed flames [5], this chemical process is limited by the kinetics, in particular by slow reforming reactions. For this reason, catalyst layers [6e11] or heat recirculation [12e16] are frequently employed in order to accelerate these reactions.…”

Analysis of the Autoignition Process under the Industrial Partial Oxidation Conditions Using Detailed Kinetic Modeling

“…Industry and academia have been pursuing the conversion of natural gas to higher valued products. The noncatalytic partial oxidation (NCPOX) technique converts natural gas into acetylene and synthesis gas via combustion of fuel-rich natural gas/oxygen mixtures. , Compared with the conventional production of synthesis gas or hydrogen alone from natural gas by the NCPOX method without flame quenching technique, − the NCPOX method with rapid flame quenching can achieve the coproduction of acetylene and thus is more economically attractive. The process does not require a catalyst, thus it is not limited by the catalyst lifetime and the cost of the catalytic materials.…”