Effect of methane reforming before combustion on emission and calorimetric characteristics of its combustion process

Highlights

• Combustion characteristics of methane/water mixture and products of steam methane reforming were investigated.

• Products of steam methane reforming give higher NOx emission.

• Increasing in oxidant temperature leads to increasing in NOx emission.

• Water addition to fuel gives decreasing in NOx emissions for methane and syngas by 57% and 28%, respectively.

• Maximum flame temperature is higher for syngas at equal combustor heat release.

Abstract

In this study, combustion and emission characteristics of methane mixed with steam (CH₄/H₂O) and the products of methane reforming with steam (CO/H₂/H₂O) were compared. Four fuel compositions were analysed: CH₄+H₂O, CH₄+2H₂O, and products of complete methane reforming in these mixtures, respectively. A comparison was carried out through the numerical model created via Ansys Fluent 2019 R2. A combustion process was simulated using a non-premixed combustion model, standard k-ϵ turbulence model and P-1 radiation model. The combustor heat capacity for interrelated fuel compositions was kept constant due to air preheating before combustion. The inlet air temperature was varied to gain a better insight into the combustion behaviour at elevated temperatures. The effect of steam addition on the emission characteristics and flame temperatures was also evaluated. NO_x formation was assessed on the outlet of the combustion zone. The obtained results indicate that syngas has a higher combustion temperature than methane (in the same combustor heat capacity) and therefore emitted 27% more NO_x comparing to methane combustion. With the air inlet temperature increment, the pollutant concentration difference between the two cases decreased. Steam addition to fuel inlet resulted in lesser emissions both for methane and syngas by 57% and 28%, respectively. In summary, syngas combustion occurred at higher temperature and produced more NO_x emissions in all cases considered.

Introduction

Hydrocarbon fuels will remain the main source of primary energy in the coming years, despite advances in renewable energy sources. Natural gas is one of the main hydrocarbon fuels. Natural gas is the cleanest burning and fastest-growing fossil fuel, contributing to almost 30% of total energy demand growth through the last decade, more than any other fuel. Being the cleanest burning fossil fuel, natural gas provides a number of environmental benefits compared to other fossil fuels, particularly in terms of air quality and greenhouse gas emissions. However, the combustion of natural gas causes an increase in NO_x emissions.

NO_x emission is one of the main issues for human health and the environment [1]. NO_x forms primarily by the two formation mechanisms: fuel NO and thermal NO. Fuel NO formation is driven by the oxidation of nitrogen-bearing species (HCN, NH₃) that are present in fuel. That mechanism has a small influence if fuel was refined prior to combustion. Authors of this article successfully implemented that technology for the syngas combustion in medium-Btu fueled combustor [2].

Recently, the requirements for NO_x emissions have become more and more stringent. Therefore, scientists and engineers from many countries are working to reduce NO_x emissions [[3], [4], [5]]. Thermal NO is the dominant process, therefore NO_x formation is strongly dependent on the flame temperature [6]. In the scientific articles, a lot of effective ways to reduce NO_x emission were suggested:

• water injection [[7], [8], [9]];

• flue gas recirculation [10,11];

• staged combustion [12];

• low NO_x burners with use of fuel staging, air staging or fiber-matrix, to reduce peak temperature [13,14];

• oxy-fuel combustion [15,16];

• reburning to convert NO to N₂ [17];

• selective catalytic and non-catalytic reduction [18,19].

Among these ways of NO_x emission reduction, water addition to flame is one of the most widely used methods. Water vaporises, thus taking heat from the combustion process, and dilutes combustion products resulting in decreased maximum temperature level and thus NO_x emissions. That technique has some limitations and can be used in systems, where heat loss is tolerable and increased combustion product volume could be beneficial for heat transfer properties.

In recent years, the study of methane dilution with water to reduce NO_x emissions has been of great scientific interest. Solokolou [20] developed a numerical model to analyse the influence of H₂O fuel dilution on the NO_x pollutant formation. 2D premixed combustion model included k-e turbulence model and eddy dissipation model for the turbulence-chemistry interaction. Results showed that with the increment in H₂O diluent concentration, sharp NO_x and slight temperature decrement is observed. The aforementioned effect occurs mostly because of high-temperature region reduction as well as combustion energy absorption by the water diluent. Also, a higher H₂O diluent concentration influenced the maximum temperature distribution, which was found to shift closer to the inlet.

Dai et al. [21]. carried out simulation studies on methane combustion with N₂ and H₂O diluents in hot oxygen coflow. Oxygen concentration in coflow varied from 3% to 85%. Combustion characteristics were influenced by N₂ and H₂O diluents sharper for the lower O₂ concentration. It was observed that combustion occupies a smaller volume and occurs at lower temperatures with H₂O dilution rather than with N₂ dilution. One of the findings is that higher H₂O concentration intensifies CO to CO₂ reaction conversion rate. That reaction is responsible for the major fraction of released heat in the course of methane-air oxidation reaction and therefore combustion volume shrinks. The other reason is that fuel jet entrains more O₂ from the H₂O/O₂ hot coflow. The more oxidant is present in the reaction space the higher is combustion reaction rate, that further decreases the reaction volume. Reduced flame temperature is caused by the water vapour higher thermal capacity, which effectively consumes heat of reaction. It also occurred that H₂O diluent has a chemical effect on the combustion reaction, which is expressed by the ignition delay. Ignition delay was observed during MILD combustion. As a result, the flame moved downstream from the fuel nozzle jet. Authors also found out that H₂O diluent has several advantages over CO₂ dilution such as energy efficiency and compact configuration. These advantages are caused mainly by air separation and CO₂ sequestration.

Water addition to the combustion chamber can be organized in different ways. One of those ways is water injection to fuel-air mixture. If we consider a mixing of fuel and water before the combustion chamber, we can expect a fuel reforming reaction. Initial research in the field of fuel reforming before combustion was performed in the United States in 1968. This investigation was conducted by Wimmer and Lee [22]. The main goal of this investigation was to reduce pollutant emissions from spark-ignition engines. Wimmer and Lee were the first who demonstrated that products of the fuel reforming showed better lean burning behaviour and produced lower emissions than propane and methane.

Moreover, in recent years, scientists and engineers are paying more attention to reforming hydrocarbon fuels before combustion by using waste heat [23]. This way of fuel reforming is called thermochemical recuperation (TCR). The main idea of TCR is the endothermic reforming of hydrocarbon fuels to generate hydrogen-rich fuel (syngas or synfuel). Waste heat is energy source for the fuel reforming processes. As a result of this endothermic reforming, a low heating value (LHV) of new synfuel is (in terms of a unit mass of the original fuel) higher than LHV of the original fuel. This new synfuel is used either partially or completely as fuel in such fuel-consuming plants with TCR. In the TCR systems, there is a transformation of flue gases enthalpy into the chemical energy of new synfuel.

One of the most discussed techniques of TCR in the scientific articles is TCR by steam methane reforming [[24], [25], [26], [27]]. The steam methane reforming products mainly consist of hydrogen and carbon monoxide. As mentioned above, one of the most important ways to suppress nitrogen oxides is water injection into the combustion chamber. Fig. 1a shows a schematic diagram of this method. The main concept is a direct injection of water into the combustion chamber. In fuel-consuming plants with pre-combustion methane reforming organized with the help of the thermochemical waste-heat recuperation system, initially water is used for the methane reforming reaction (Fig. 1b). In other words, the process of fuel combustion in fuel-consuming equipment with TCR is divided into two stages. The first stage is the reforming of methane with steam, the second stage is the combustion of reforming products.

Among the technical problems for an industrial application of the thermochemical waste-heat recuperation systems is investigation of synthesis gas combustion. A distinctive feature of synthesis gas combustion is the presence of hydrogen and carbon monoxide. The development of numerical modelling tools, such as Ansys, Comsol, Star CCM++, OpenFOAM, greatly simplified the study of the emission and calorimetric characteristics of synthesis gas combustion [28]. Norbert Peters showed that up to 70% of combustion problems can be solved using CFD-modeling [29].

‘Lopez-Ruiz et al. [30] examined NO_x formation during the pure hydrogen combustion process. They investigated the impact of different fuel consumptions on thermal NO_x formation. Case geometry was a 2D axisymmetric model of a domestic multi flame diffusion boiler. The finite-rate chemistry model was implemented in Ansys Fluent 19.0. The authors concluded that NO_x formation could be lowered by 2.2 times at 0.8 kW if the flame-splitting method is used.

Habib et al. [31] carried out simulation studies on emission characteristics of methane and syngas in a package boiler. Combustion of methane and three various syngas compositions were compared. Scientists concluded that boiler exit temperature elevated with the increment in H₂ concentration in syngas due to its slow reaction kinetics. Heat transfer from the flame to the convection part of the burner was higher for the 33%CO/67%H₂ fuel which was accompanied by decreased average temperature. Contours of NO_x emissions strongly correlated with maximum temperature. Also, pollutant concentration was decreased by 30% when the amount of excess air was increased from 5% to 25%.

Scribano [32] numerically investigated the influence of different H₂:CO ratios, equivalence ratios and Reynolds numbers on CO and NO_x emission characteristics in the process of syn-gas combustion in the radial burner. It was found that the lowest CO and NO_x emissions occurred at the highest H₂:CO fuel ratio. Optimum fuel combustion conditions were observed at a low equivalence ratio and high Reynolds number.

Asgari et al. [33] modeled premixed syngas combustion at different H₂/CO ratio (from 0.25 to 1) and equivalence ratio (from 0.5 to 1) in a McKenna burner. The numerical model was investigated in Open Foam Framework under 2D axisymmetric conditions. It was found that both near flame temperature and NO concentrations decreased with the increment in H₂/CO ratio. Also, authors established that above the flame NO₂ concentration steadily decreases due to a NO₂–NO conversion reaction.

Lee [34] experimentally investigated the impact of flame temperature and flame structure on NO_x and CO emissions under various CO/H₂/CH₄ fuel composition in a partially-premixed gas turbine model combustor. Results indicated that NO_x formation is mostly influenced by flame temperature and residence time in a high-temperature region. Residence time is affected by flame structure, which depends on the fuel composition.

Alavandi [35] performed an experimental study to gain better insight into lean-premixed combustion of varying CH₄/CO/H₂ mixture in two-section porous burner. Results indicated that NO_x and CO emissions decreased for the higher percentage of CO and H₂ in fuel. However, experiments were conducted at an adiabatic flame temperature below 1600 °C, which corresponds to insignificant thermal NO_x production.

Chaouki [36] compared gas temperature, CO₂ and NO_x emissions for different syngas compositions and methane in a gas turbine. For this purpose, the 3D numerical model was developed consisting of k-e turbulence model, P-1 radiation model and probability density function (PDF) combustion model. Exit average CO₂ emissions per unit of energy generation (kg/kJ) were found lower by 12% only for syngas with high hydrogen content, the other syngas compositions showed higher concentrations than that of methane. Assessing NO_x emissions per unit of energy generation (kg/kJ) the reduction by 33% in comparison to methane combustion was only observed for one syngas composition with high water concentration (22,6%).

Giles et al. [37] analysed the impact of syngas composition and diluents effect on NO_x emission characteristics of syngas non-premixed counterflow combustion. They developed a numerical model using the OPPDIF code with the Chemkin package. Authors found that H₂O is the most efficient diluent comparing to CO₂ and N₂ due to its high specific heat which efficiently lowers the flame temperature. Also, it was observed that the thermal mechanism is dominant in NO_x formation for 50%H₂:50%CO mixture.

Such a variety of articles on CFD-modeling of synthesis gas combustion is due to the fact that the composition of synthesis gas can vary over a wide range of component concentrations. In addition, the development of the thermochemical waste-heat recuperation technology and the on-board hydrogen production technology makes it extremely relevant to answer the question: “How does the reforming of methane affect the emission and calorimetric characteristics of its combustion process?” In this article, using CFD-modeling, we tried to show the effect of methane reforming on the emission and calorimetric characteristics of its combustion process.