(535e) Experimental and Modeling Analysis of N-Butane, Isobutane, Isobutylene and 1-Butene: Implications for Solid-Oxide Fuel Cell Operation

Fossil fuels have been the world's dominant source of energy. This dependence on oil and other fossil fuel resources presents many difficulties, including resource depletion, accelerating global warming, escalating cost of oil, and national security issues. One way to address this problem is to employ more efficient energy conversion devices such as fuel cells. Solid-Oxide Fuel Cells (SOFCs) in particular are promising because they can operate directly on hydrocarbons. Typical operating temperatures for SOFCs range from 600-800 C. Because of these high operating temperatures, thermal decomposition of the hydrocarbon fuel within the anode channel is significant and is likely to influence SOFC operation. Gas phase chemistry directly affects SOFC performance. For example, pyrolysis produces smaller alkanes, alkenes, and H2. Any remaining parent alkanes, as well as the smaller alkanes and alkenes can then react within the catalytic porous anode. Typically, these will be steam reforming reactions to form a mixture of CO and H2 (the steam is produced by electrochemical oxidation of the H2 at the three phase boundary). CO further reacts with the steam via the water-gas-shift reaction to form additional H2. Thus, the ultimate rate of H2 production, which governs the fuel cell performance, is contingent upon both the gas-phase and catalytic reactions. Another way in which gas-phase kinetics can affect SOFC performance is that molecular-weight-growth reactions can lead to deposit formation, decreasing the efficiency and the life of the SOFC. Therefore, it is necessary to characterize the thermal decomposition of the fuel and determine its impact on SOFC operation. We will discuss our experiments and detailed kinetic modeling predictions for pyrolysis of n-butane, isobutane, 1-butene, and isobutylene. There were several reasons for including the olefins in this study. Not only are they products of alkane pyrolysis, but our earlier analysis strongly suggests that they play a key role in forming deposits. In particular, reactions of the resonantly-stabilized radicals produced by hydrogen abstraction from the olefins are thought to be a major pathway to molecular weight growth. Experiments were performed over a range of temperatures and residence times such that data were collected over the full range of conversion of the parent fuel. The reaction mechanism used in this study is an improved and updated version of a previous C4 mechanism. In particular, the model employs the results of CBS-QB3 calculations to provide improved estimates of the rate coefficients for hydrogen abstraction and isomerization reactions. Butane pyrolysis experiments (50% n-butane/50% N2 mixture) were performed using two different reactors. In one system, reactors and products were monitored with a mass-spectrometer at relatively high temperatures (700 - 850 C) and short residence times (t ~ 1 s). The other system used GC detection and operated at lower temperatures (600 - 725 C) and longer residence times (t ~ 5 s). The main products were hydrogen, methane, ethane, ethylene, and propylene. Noticeable molecular weight growth started at 725 C and t ~5 sec, where n-butane conversion was greater than 95%. The model was able to quantitatively describe the effect of both temperature and residence time on conversion and product selectivity, as illustrated in Fig. 1. Similar experiments have been performed with isobutane, isobutylene, and 1-butene. It was observed that isobutene was more reactive than n-butane at similar experimental conditions. The main products of isobutane were hydrogen, methane, propylene, and other C4 isomers. The pyrolysis of isobutane formed considerably less C2 species than n-butane pyrolysis. That is expected because the branched nature of isobutane does not allow C-C b-scission from the parent isomer to form C2 species. Noticeable molecular weight growth did not start until 750 C. At similar temperatures and residence times, there were fewer C4 species from 1-butene than from n-butane, isobutane and isobutylene pyrolysis. The main products of 1-butene were hydrogen, methane, ethane, ethylene, propylene, and C4 isomers. There was significant molecular weight growth- C5 species started appearing at 625 C, while C6 species started appearing at 650 C. At temperatures 675 - 700 C, the amount of C5 gradually decreased while the amount of C6 gradually increased. Isobutylene did not show significant reaction until 675 C, significantly higher than the other C4 species studied. The main products of isobutylene pyrolysis were hydrogen, methane, ethylene, propylene, and isomers of C4. Surprisingly C5 and C6 species started appearing at 700 C, where the conversion was still very low. At 725 C, the amount of C5 and C6 observed experimentally increased. At 750 C, the amount of C5 decreased while the amount of C6 increased. This wide array of data will be compared to the detailed kinetic model predictions.