The impact of oxygen exothermicity on energy quality of biofuels and catalytic upgrading

• Main Author: Merckel, Ryan David
• Other Authors: Labuschagne, F.J.W.J. (Frederick Johannes Willem Jacobus)
• Language: en
• Published: University of Pretoria 2020
• Subjects: UCTD
• Online Access: http://hdl.handle.net/2263/77849; Merckel, RD 2019, The impact of oxygen exothermicity on energy quality of biofuels and catalytic upgrading, PhD Thesis, University of Pretoria, Pretoria, viewed yymmdd <http://hdl.handle.net/2263/77849>

Energy systems and products have always focused heavily on the fuel rather than the oxidant. That oxygen is the definitive factor in the exothermicity of combustion reactions rather than the fuel is demonstrated through the application of the principles governing classical thermodynamics. Through a rigorous analysis of the changes in bond (dissociation) energies undergone by each element during combustion for 359 fuels, it is observed that oxygen contributes more than 100 % towards the (net) higher heating value (HHV), with only a few exceptions. Hydrogen present in the fuel contributes only weakly to the HHV (with a maximum contribution of ~16 %). The oxidation of carbon typically consumes energy evolved during combustion, thereby contributing negatively to the HHV, and in a rare case has a maximum contribution of just ~18 %. These results suggest that the reduction half- reaction, rather than the oxidation half-reaction (concerning the fuel elements), is responsible for the exothermicity of the combustion reaction. An accurate one-parameter correlation for estimating the HHV as a function of the mass of oxygen consumed during combustion is derived from an approximation of the general thermodynamic relation, as ∆7ℎ°1 = −13.87 𝑚1 (MJ kg71). The constant is obtained from the simplification of a modifier function 𝜇n = −13.87 𝑒71.17171i (MJ kg71) in the equation ∆7ℎ°1 = 𝜇n 𝑚1i − ∆ℎ (MJ kg71), which is a function of the mass fraction of oxygen consumed, 𝜔1 . The resulting correlation of ∆7ℎ°1 = −13.87 𝑚1 performs well statistically when evaluated against 1 087 fuel combustion data of wide chemical composition with a chemical formula of C71 H77 O71 N77 S7i P77 . For this correlation, a coefficient of determination of 𝑅1 = 0.981, a root-mean-squared error of RMSE = 1.5 MJ kg71 and a mean bias error of MBE = 1.8 % are achieved. This pivotal contribution of oxygen to combustion exothermicity and the possibility that reduction half-reactions are, in general, exothermic may have serious repercussions for the biofuel arena, but also for electrochemical cell technologies and energy systems overall. As energy products, biofuels may be evaluated in terms of energy quality, in both their synthesis and upgradation. An expression for the change in energy quality, ∆𝐸7, which occurs as a result of the synthesis of biofuel from biomass or by the upgradation of biofuel, is derived mathematically as a function of the ratio of the mass of oxygen consumed for the product over the feed, as ∆𝐸n = 1𝑚1n 7171n7171n1𝑚1n 7711n1 − 1. This equation implies that any improvement to energy quality must accompany an increase in the consumption of oxygen. This improvement is, however, limited as it has been found that the ∆𝐸n of any system is bounded according to the expression ∆𝐸n = 𝑚7⁄𝑚 − 1, where 𝑚7 and 𝑚 are the initial and final masses of combustible material. As a consequence, if 𝑚 = 𝑚7 (i.e. no change in 𝑚7 occurs), then no change in energy quality occurs, while ∆𝐸n → ∞ when 𝑚 → 0 in cases where the oxidation potential of 𝑚7 is increased. That is to say, reductions to mass yields that accompany improved oxidation potential result in increases in energy quality. Furthermore, an analysis of more than 16 million generated fuel data shows that maximising carbon mass yields limits the change in energy quality that is possible to 116 %—a much higher value of ∆𝐸n may be achieved if this metric is not adopted. These results contradict the prima facie objectives of increasing total and carbon-specific mass yields, which are applied to biofuel synthesis and upgradation, and especially with regard to the manufacture of pyrolysis oil. Together with these aforementioned results, a novel technique of analysis comprising three methods is developed for use with the analytical technique of pyrolysis-GC/MS (py-GC/MS) which is well suited to the study of pyrolysis and catalytic upgradation of fast pyrolysis oils. These three methods use a graphically based approach whereby cumulative atomic ratios, calorific values and elemental masses are plotted against the cumulative masses for compounds characterized via py-GC/MS. This technique allows the accurate estimation of the atomic ratios, elemental compositions and calorific values of pyrolysis oils via linear extrapolation. Following the demonstration and validation of these three methods with the use of 19 sets of data from the literature, this technique is used in the evaluation of four catalysts, namely bentonite, zsm-5 zeolite, the oxide C2013 and the oxide M1213. The comparisons between uncatalyzed (neat) pyrolysis oil and catalysed pyrolysis oils are based on their ability to increase the calorific value and decrease the oxygen content of pyrolysis oil. These objectives were achieved (for Eucalyptus grandis as the feedstock) in the following order: neat pyrolysis oil (−27.6 MJ kg17 and 30.4 % oxygen) < zsm-5-catalyzed pyrolysis oil at 500 °C (−29.1 MJ kg17 and 28.2 % oxygen) < bentonite-catalysed pyrolysis oil (−30.1 MJ kg17 and 28.1 % oxygen) < C2013-catalysed pyrolysis oil (−31.3 MJ kg17 and 24.9 % oxygen) < zsm-5-catalysed pyrolysis oil at 300 °C (−32.5 MJ kg17 and 23.5 % oxygen) < M1213-catalysed pyrolysis oil (−39.1 MJ kg17 and 14.2 % oxygen). The results presented in this thesis rely strongly on the revelation that oxygen contributes significantly to the chemistry of combustion, but this does not appear to be applicable solely to oxygen. Fluorine, too, seems to demonstrate a similar exothermicity when undergoing reduction. When combusted with methane, ethane, propane and butane, it is found to contribute 110.6 %, 108.5 %, 107.6 % and 107.2 % towards to the HHV, respectively. This finding warrants further investigations of other energy systems that follow similar chemical changes relating to redox chemistry. === Thesis (PhD)--University of Pretoria, 2020. === Chemical Engineering === PhD === Unrestricted