Thermochemical Characterization of Bio- and Petro-diesel Fuels Using a Novel Laser-Heating Technique

Abstract

A state-of-the-art, rapid laser-heating technique, referred to as the laser-driven thermal reactor, was used to characterize National Institute of Standards and Technology Standard Reference Material (SRM) diesel and biodiesel fuels, as well as a prototype biodiesel fuel. Also described are the various issues associated with carrying out these measurements under different operating conditions (i.e., temperature, pressure, heating rate, and sample mass). The technique provides measurement of various relevant thermochemical characteristics; for this investigation the focus was on the sample endothermic/exothermic behavior, specific heat release rate and total specific heat release. The experimental apparatus consists of a copper sphere-shaped reactor mounted within a vacuum chamber, along with integrated optical, gas-supply, and computer-controlled data-acquisition subsystems. At the center of the reactor, the sample rests on a thermocouple. The reactor is heated from opposing sides by a near-infrared laser to achieve nearly uniform sample temperature. The change in sample temperature with time (i.e., thermogram) is recorded and compared to a baseline (no sample) thermogram obtained prior to the experiment. Then processed (using an equation for thermal energy conservation) for the thermochemical information of interest. Results indicated that the modification of the baseline is attributed to residue remaining after completion of reactions and a change in the oxide layer of the reactor sphere outer surface. Thus, the sphere must be pre-oxidized in air using the laser prior to any sample or baseline measurement. This investigation provides preliminary evaluation of SRM biodiesel fuels, with the results being consistent with distillation curve work reported in the literature.

Laser-Driven Calorimetry of Single-Component Liquid Hydrocarbons

“…The change of the liquid mass with time while reacting is also accounted for, although its effect on the total mass is small. The time corresponding to complete consumption of reactants and termination of chemical reactions occurs typically near the peak in the sample-baseline (sample temperature) temperature difference profile; this point also matches the return in the sample derivative profile to that of the respective baseline at higher temperatures 42 - see Fig. 6.…”

“…It was determined from an earlier investigation 42 that operating under reduced pressure inhibits liquid reactivity with air. However, operating at atmospheric pressure reduces the lifetime of the reactor sphere to about 10 runs before replacement.…”

“…However, this is true only if the Cu 2 O layer (with a reddish appearance) that forms on the sphere surface at higher temperatures (above 973 K) during an experiment is re-oxidized back to CuO; namely, as existed before the run. 42 This is accomplished before the next run by reheating the sphere surface with the laser beam to 773 K, as described earlier. Following this procedure reverts the apparatus back to its original condition, and thus prepares the reactor for the next run, eliminates repeatability issues, and reduces measurement uncertainty.…”

“…As discussed in Presser et al, 42 a change in the reactor sphere oxide layer during a run with sample can reduce the maximum temperatures achieved for subsequent baseline measurements, requiring re-oxidation of the sphere back to the initial CuO state before running the next experiment. Also, other systemic changes in the operating conditions (e.g., replacement of the reactor sphere) may have a similar effect.…”

“…This work builds upon an earlier work 42 with biofuel standard reference materials from the National Institute of Standards and Technology. In the earlier investigation, the fuels were mixtures composed of known fatty-acid-methyl-ester constituents with the results showing well-defined exothermic behavior at specific temperature regimes near the boiling point of each liquid.…”