CFD modelling of an ammonia cracker for the on-board generation of NH3/H2 mixture as a COx-free fuel
Session chaired by Dr. Agustin Valera-Madina
The climate change caused by global warming has become the main crisis for human race in the last century, and the major player is carbon dioxide having a large portion of greenhouse gas emissions. The CO2 emissions caused by human activities mostly come from electricity, transportation, industry, residential and agriculture. Among these sections, transportation has a noticeable portion of CO2 emission as well as NOx, and unburnt hydrocarbons. Using electric vehicles and fuel cell technology are two well-known solutions. Both of these technologies are comparably immature and expensive with considerable limitations. In search of zero-carbon fuels, apart from nuclear energy, hydrogen has been attracted much attention in the last few decades. Hydrogen can be used either in internal combustion engines or in fuel cells, and in both cases the exhaust product is water. However, hydrogen technology has not been successful in the industry mainly due to the limitations on fuel storage and transportation. Alternative solutions to H2 delivery and storage obstacles are using chemical or thermal processing of hydrogen carriers for on-demand generation of hydrogen. Ammonia (NH3) has been repeatedly reported in the literature as a potential H2 carrier for the on-site production of hydrogen. Ammonia has superior H2 content (17.8 wt%) and energy density compared to alternative carriers with well-established production and transportation infrastructures. It is worth noting that H2 can be produced directly from NH3 by cracking with the aid of a heterogeneous catalyst. Hence, in case of transportation applications hydrogen could be generated on-site and directly injected into the internal combustion engine together with ammonia. Therefore, expensive and large consuming H2 tanks are not required, and this makes the on-board NH3 cracking feasible for vehicular applications. Ammonia decomposition is an endothermic reaction that yields high conversions at high temperature and low-pressure conditions in the presence of heterogeneous catalysts. Ru-based catalyst series were reported as the most active for the ammonia cracking reaction; however, numerous studies have focused on enhancing the reaction conversions at low temperature using novel catalysts. Hence, the overall operation efficiency can be improved by decreasing the reaction temperature, and the process costs can be reduced using abundant, inexpensive catalysts. While the main investigation area is related to the application of ammonia as a COx-free hydrogen carrier, it can also be used as an alternative fuel for combustion systems, e.g., internal combustion engines (ICE). Nevertheless, in conventional ICEs, NH3 should be co-fed together with a combustion promoter. Due to the low laminar burning velocity, high ignition energy and narrow flammability limits of pure NH3, it is feasible to use ammonia as a primary fuel together with other fuels used as combustion promoters such as H2 for combustion. Hydrogen, on the other hand, is characterised by low ignition energy and wide flammability range, and the complementary properties of NH3/H2 mixture could be operated in combustion engines. Some studies proposed H2 as a potential promoter, reporting that a small ratio of H2 in ammonia/air mixture (> 1 wt%) could speed up the combustion significantly and allow satisfactory engine running. The required H2 ratio can be directly generated from ammonia through decomposition reaction at the upstream of the engine system. For example, 14% conversion of NH3 is sufficient to produce approximately 2.5 wt% hydrogen mixture. This work presents a numerical methodology to model a macro-scale heater-reactor system for hydrogen generation from the ammonia decomposition reaction. A kinetic rate expression was developed and for the reaction and implemented in the CFD model based on the experimental data obtained. The kinetic model parameters for the macro-scale cracker were calculated and used based on a commercial catalyst. A CFD solver was used to solve the governing equations, and measuring the reacting flow properties from temperature, velocity and density distributions to the species mole fraction profile and outlet NH3 conversions. The CFD models were solved at various reaction temperatures and NH3 flow rates and compared with the empirical data obtained from the literature. The solved CFD model showed good accuracy with respect to the experimental data in the model. Therefore, the developed rate expression could effectively predict the experimental kinetic conversions along the ammonia decomposition reactors. Furthermore, a heat transfer analysis was performed on the heater-reactor model to assess the temperature gradients all over the heater section and the reactor part and to optimise the internal heating system. The heat transfer analysis for the macro-scale heater-reactor showed that increasing the heater lengths increases the surface contact and reduces the overall heat flux in the heater and reactor parts leading to lower temperature gradients. Therefore, there is less possibility of hot spots inside the system, and the overall process efficiency could be enhanced. Finally, a scale-up calculation was conducted to analyse the feasibility of the NH3/H2 mixture generation process in larger scales for internal combustion engines.
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