Monolit kanallı katalizörlerde doğalgazdan hidrojen eldesinin ve doğalgaz - hidrojen karışımlarının yakılmasının deneysel ve teorik analizi

Abstract

Günümüzde, kömür, doğalgaz gibi fosil yakıtlar ile biyokütle ve benzeri enerji kaynaklarının hem hâlihazırdaki teknolojiler, hem de sessiz, çevreci ve yenilikçi enerji teknolojileriyle verimli ve yaygın kullanılması büyük önem arz etmektedir. Bu konuda, özellikle hem elektriğin hem de ısıtmanın aynı anda sağlandığı evsel birleşik ısı-güç üretim sistemleri üzerine yapılan araştırma çalışmaları yoğun olarak devam etmektedir. Bu sistemler, yaşam alanlarındaki gaz hatlarından sağlanan hidrokarbon gaz yakıtın kullanılmasıyla elektrik enerjisinin ulusal şebekeden bağımsız olarak ihtiyaç duyulduğu yerde üretilmesini mümkün kılmaktadır. Evsel birleşik ısı-güç üretim sistemlerinde enerji üreteci olarak güneş kolektörleri, rüzgâr türbinleri, endüstriyel/mikro türbinler, doğal gaz motorları, içten yanmalı motorlar, stirling motorları, mikrotürbinler ve yakıt pilleri gibi farklı teknolojiler kullanılabilmektedir. Bu teknolojilerin arasında enerji kullanımında verimin artmasını, emisyonların azaltılmasını ve sessiz çalışma imkânını sağlaması ile yakıt pilli enerji dönüşüm sistemleri yenilikçi sistemler arasında yerini almıştır. Yakıt pili için gerekli yakıt hidrojendir. Hidrojen patlayıcı ve uçucu bir gaz olması nedeniyle üretildiği yerde tüketilmesi güvenlik açısından bir gerekliliktir. Yakıt pili kullanan sistemlerinin içerisinde doğalgazı/LPG/fueloili vb. hidrokarbon yakıtları hidrojene çeviren yakıt hazırlama sistemleri bulunmaktadır. Bu yakıt hazırlama sistemleriyle üretilen hidrojenin kalitesi yakıt pili performansını belirleyen ve etkileyen önemli bir unsur olduğundan bu sistemlerin optimum üretim ve işletme koşullarının tespit edilmesi gereklidir. Bu tezin amacı evsel yakıt pilli birleşik ısı-güç üretim sistemlerinde kullanılmak üzere bir yakıt hazırlama sisteminin teorik analizlerinin yapılması, tasarlanması, kurulması ve optimum üretim ve çalışma koşullarının belirlenmesidir. Çalışma iki ana bölümden oluşmaktadır: Çalışmanın ilk bölümünde monolit kanallı katalizörde doğalgazdan hidrojenin eldesi ele alınmıştır. İkinci bölümde ise monolit kanallı katalizörde hidrojenin ve doğalgaz - hidrojen karışımlarının yakılması incelenmiştir. Bu incelemeler yapılırken çalışma deneysel ve teorik olmak üzere iki temel araştırma yöntemi ile yürütülmüştür. Tez çalışmasının teorik çalışma kısmında, yakıt hazırlama sistemini temsil eden katalizör kinetiğine bağlı bir teorik model başarıyla oluşturulmuştur. Teorik çalışmada hem katalizörden bağımsız kimyasal denge hali hem de katalizör kinetiğine bağlı çözümlemeler yapılmıştır. Sistemin kinetik modeli oluşturularak detaylı parametrik analizler yapılmıştır. Bu model, yakıt hazırlama sisteminde gerçekleştirilen deneysel çalışmalar ile de doğrulanmıştır. Deneysel çalışmayla ise gaz hattından başlayarak yakıt piline hidrojen besleme noktasına kadar olan tüm sistem tasarlanmış, kurulmuş, teorik çalışma ile elde edilen parametreler ışığında deneyler yapılmış ve de teorik ve deneysel sonuçlar birbirleriyle kıyaslanarak sistem performansı tartışılmıştır. Bu tez çalışmasında, 2kWt (termal) kapasitede hidrojen üretebilen bir yakıt hazırlama sistemi tasarlanmış, kurulmuş, modellenmiş ve deneyleri gerçekleştirilerek başarılı bir şekilde çalıştırılmıştır. Gerçekleştirilen detaylı deneysel ve teorik çalışmalar neticesinde en iyi işletme koşulu belirlenmiştir. Belirlenen en iyi koşulda gerçekleştirilen deneyde, 0,307 m3/h debideki doğalgazdan %78,53'lük verim (alt ısıl değere göre) ile 0,725 m3/h hidrojen elde edilmiştir. Üretilen yakıt, %44,75 mol oranında hidrojen ve %1,3 oranında karbonmonoksit içermektedir. Bu içerik 1kWe (elektriksel) kapasiteye sahip yüksek sıcaklık PEM (Polimer Elektrolit Membranlı) yakıt pilini besleyecek kalitededir. Yakıt hazırlama sistemi mikro-ölçekli birleşik ısı-güç enerji dönüşüm sistemlerine uygun olarak kurulmuştur. Bu enerji dönüşüm sistemlerin kapasiteleri 5kWe (10kWt) kadar çıkabilmektedir. Tez çalışması ile ortaya konan yakıt hazırlama sisteminin teknolojisi bu kapasitelere de uygulanabilmektedir. Tezin kapsamında oluşturulan teorik kinetik modelle, istenen elektrik üretim kapasitesine bağlı olarak yakıt hazırlama sisteminin tasarım değerleri kolaylıkla tespit edilebilmektedir. Bu kinetik model ile kapasite artırımına yönelik olarak bir çalışma yapıldığında, örneğin 3kWe için gerekli doğalgazın 0,925m3/h olacağı ve hidrojenin 2,25m3/h olarak %80,5 verimle üretilebileceği öngörülebilmektedir. 5kWe kapasiteye çıkıldığında ise yakıt hazırlama sisteminin 1,545m3/h doğalgaz harcamasıyla 3,8m3/h hidrojen üretimini yapabileceği ve sistemin %82 verim değerine ulaşılabileceği öngörülmüştür.

The idea of higher efficiency in energy systems provides an innovative consideration of "distributed generation of energy". Distributed energy resources (DER) are parallel and stand-alone electric generation units located within the electric infrastructure or near the end user such as generators, back-up generators, or on-site power systems. Today, several new technological alternatives are being developed or improved toward commercialization. Systems with different capacities are possible but one of today's focus for DER is the micro-scale energy systems especially small scale combined heat and power units (<5kWe) for residential applications. The most promising of DER system are proton exchange membrane fuel cell (PEMFC) based energy production systems because of low greenhouse gas emissions, simple maintenance and particularly its high efficiency. High temperature proton exchange membrane fuel cells (HT-PEMFC) are quite promising for the last decades since phosphoric acid doped polybenzimidazole (PBI) membranes provide to operate at higher temperature and carbon monoxide level. An HT-PEMFC system consists of a fuel processor, which converts any hydrocarbon-based fuel to hydrogen, a fuel cell and auxiliary units. The most preferred fuel is natural gas for stationary applications since its infrastructure pipeline for households are already well established. Natural gas fuel processors include a hydrogen production reactor (steam reformer- SR or an autothermal reformer-ATR), gas clean-up reactors (high/middle/low-temperature shifts reactors (HT/MT/LT-WGS), preferential oxidation reactors-PrOx, membrane reactors or pressure swing absorption systems) and various auxiliary subunits. In other words, the fuel processor requires chain reactor process, whose efficiencies are highly dependent on catalysts performance. Hence, the critical problems are addressed as the responses of the fuel processor under various system configurations, operational conditions and strategies. This work focused on the fuel processor unit and approaches the selected topic from an operational condition point of view and it deals with the experimental and theoretical investigation of the parametric effects on the fuel processor implementable to a 1.0kWe fuel cell. The ultimate aim is to investigate the operational conditions on the fuel processor performance. The operational conditions are determined as steam-to-carbon, oxygen-to-carbon and inlet temperature of the reactors. A parametric study is performed to determine their effects on the hydrogen yield, fuel conversion, thermal efficiency, hydrogen and carbon monoxide mole fractions. In this work, both experimental and theoretical studies are performed. The experimental findings are discussed comparatively with theoretical findings. First, an experimental apparatus of the fuel processor is designed and built. The fuel processor has a hydrogen production capacity of 2.0Wth and applicable in a residential-scale high temperature proton exchange membrane fuel cell (HT-PEM). The system consists of an auto-thermal reformer (ATR); gas clean-up units, namely high and low-temperature shift reactors (HTS, LTS) and utilities including pump, evaporator and heat exchangers. The system also consists of a catalytic burner to combust the anode-off of the fuel cell. The burner provides the heating requirement of the system and can combust the mixture of hydrogen and natural gas. The advantage of the catalytic off-gas burner is to ignite the combustion reaction at room temperature without need an ignitor source. The simulation of the fuel processor has been carried out by using commercial process design software, to present both kinetic and thermodynamic equilibrium solutions. First time in this study, a detailed reaction mechanism including reforming, combustion, water gas shift reactions, is modelled for monolith-type catalyst and compare with experimental findings. Besides, as in first in this study, both steam reforming and combustion kinetics for ethane is included in the model. To do best of our knowledge, no study has yielded kinetic simulations of fuel processor consisting of monolith type ATR, HT-WGS and LT-WGS catalysts to investigate operational conditions effects. The comparison has also involved the results of equilibrium calculations in order to identify the limitations of the commercial monolith performance. The main practical problem is the kinetically simulation of the monolith catalyst. To the best of our knowledge, powder form catalyst studies are presented in the literature without given effectiveness factors for the pellet form or monolith form catalysts. Even selection of the reactions should be a well-thought-of step for a micro/small scale catalyst. We examined the most well-known Xu and Froment steam reforming reaction mechanism combined with methane combustion, ethane combustion and ethane steam reforming kinetics. The final model has included six reactions along with the detailed kinetic expressions. The selection of the mechanism for the autothermal reforming of natural gas on monolith catalyst was done according to the three sets of experiments. The experimental conditions were full and partial capacities, the O2/C ratios of 0.4 and 0.5, H2O/C ratio of 3.0, and the inlet temperature of 450°C. The experiments, in terms of flow rates and temperature, were inputted in the simulation and the kinetic model was run. The pre-exponential values are adjusted while keeping the activation energies and adsorption energies constant and the reaction mechanism was verified according to the commercial monolith catalyst within error limitations of 5% and lower. We determined that the kinetic results are in good agreement with the experimental findings. The combined mechanism with updated pre-exponential factors provided a description of an experimental fuel processor. After the validation of the kinetic model, a parametric experimental study is conducted and the results were discussed by comparing with this model and the thermodynamics. The kinetic model is also used for further investigation of inlet temperature parametric effects. In the study, we compared the experimental results of the same autothermal reformer, operated at O2/C ratios of 0.4 - 0.5 and H2O/C ratios of 1.5 to 3.0, to our validated kinetic model and the thermodynamic equilibrium calculations. The first expectation from the fuel processor is to provide high-hydrogen-low-carbon monoxide reformate gas. The main impact among the parameters that affect the monolith performance is determined as oxygen-to-carbon ratio. Monolith catalysts performance is in agreement with thermodynamics, especially for lower oxygen feeding. High O2/C ratio resulted in high fuel conversion, high outlet reactor temperature, low hydrogen mole fraction, and high carbon monoxide mole fraction. Whereas, high steam-to-carbon ratio led to low fuel conversion, stable outlet temperature, low hydrogen mole fraction, and low carbon monoxide mole fraction. Both efficiency and hydrogen yield are peaked at O2/C of 0.4 and then noticeably decreased. In terms of fuel conversion, both O2/C ratio and inlet temperature has a dominant effect whereas H2O/C had an observable positive effect if the operational O2/C ratio is preferred as 0.4. It was determined that hydrogen mole fraction has been influenced strongly by O2/C, comparing to H2O/C. CO mole fraction is affected strongly by H2O/C, comparing to O2/C. Increase in the air (oxygen) flow accelerated the combustion but simultaneously suppressed steam-reforming despite higher steam feeding. Increasing oxygen flow rate may cause the catalyst to diverge from the equilibrium but still, the catalyst performance is comparable with the equilibrium results. The favourable operating conditions for the fuel processor loaded with commercial monolith catalyst are determined from the parametric study. The steam-to-carbon ratio of 3,0, and the oxygen-to-carbon ratio of 0.5 provide the optimum operation. The preferred inlet temperatures are 450°C, 400°C and 310°C for autothermal reformer, high temperature water gas shift and low-temperature water gas shift, respectively. The hydrogen yield is obtained as 2.53. The fuel conversion of 93,5% and the thermal efficiency of 82% are obtained, which are comparable with the literature. For the favourable conditions, the predicted outlet temperatures of 756°C to 840°C were in limits of the catalyst temperature resistance. Furthermore, the obtained hydrogen amount is 31 mol/h meaning 0.725 (std) m3/h or 2.0kW in chemical power. Consequently, the size of the monolith was appropriate and the reactor has proven to provide the required power. The fuel processor is designed and built for 1kWe PEM fuel cell micro-combined heat and power system for one family household or a small scale business building. The design can be implementable up to 5kWe class fuel cell or 10kWth fuel processor. Hence, just increasing the system capacity supplies higher energy demands. To understand the efficiency changes during the scale-up, the validated kinetic model is applied. It is found that, system efficiency is increasing from 82% to 85.7% for the production of 1kWe to 5kWe, respectively. The natural gas consumption is increased linearly up to 1.43 (std)m3/h. The fuel processor system can produce 3.8 (std) m3/h hydrogen for a 5kWe residential fuel cell applications. It is concluded that the kinetic model for the present fuel processor can be used for designing larger scale system. In this thesis, a fuel processor, containing monolith reactors, capable of producing 2kW hydrogen is designed, manufactured, modelled and operated successfully.