Common sense science: An interview with Harry Tabor, solar energy pioneer

Nicknamed the ‘father of solar energy exploitation in Israel’, Dr Harry Tabor, has made many important contributions to the technical development of solar energy. Although he puts many of his discoveries down to ‘luck’, others would call them good science. Now retired, he explained to Nina Morgan why he still believes using renewable energy sources makes very good sense.     

Importance of chlorine speciation on de novo formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans

The role of chlorine speciation on de novo formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDDs/Fs) has been studied thoroughly in an entrained flow reactor during simulated waste combustion. The effects of gas-phase chlorine species such as chlorine (Cl2), hydrogen chloride (HCl), and chlorine radicals (Cl*), as well as ash-bound chlorine, on PCDD/F de novo formation were isolated for investigation. The ash-bound chlorine alone was observed to be a sufficient chlorine source for PCDD/F formation. The addition of HCl to the system did not influence the yields of the PCDDs/Fs nor the degree of chlorination due to its poor chlorinating ability. Addition of 200 ppm of Cl2 to the ash-feed system resulted in increased PCDD/F yields, especially for the octa- and hepta-chlorinated congeners. Altering the reaction temperature to enable the presence of only Cl2 to the system did not change the yields of PCDD/F compared to those when both Cl2/Cl* were present. However, comparison between ash-bound and gas-phase chlorine, the latter at a concentration typical of a realistic combustion process, revealed ash-bound chlorine to be the more important chlorine source for de novo formation of PCDD/F in a full-scale incinerator.     

Effect of moisture, charge size, and chlorine concentration on PCDD/F emissions from simulated open burning of forest biomass

Loblolly pine (Pinus taeda) was combusted at different charge sizes, fuel moisture, and chlorine content to determine the effect on emissions of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDDs/Fs) as well as copollutants CO, PM, and total hydrocarbons. The experiments were performed in an enclosed chamber under conditions simulating open, prescribed burns of forest biomass. Burn progress was monitored through on line measurement of combustion gases and temperature while PCDD/F concentrations were determined by ambient sampling methods. PCDD/F toxic equivalency (TEQ) and total (tetra- to octa-CDD/F) emission factors were independent of charge size (1-10 kg) and moisture content (7-50%). However, the lower chlorinated, mono- to tri-CDD/F compounds were increased by poor combustion conditions: combustion efficiency lower than 0.919 was generally found when the moisture content was higher than 30%. The increase of fuel matrix chlorine from 0.04% to 0.8% using a brine bath resulted in about a 100-fold increase of PCDD/F to about 90 ng TEQ/kg of carbon burned, C(b). These emission factors were linearly dependent on Cl concentration in the biomass. PCDD 2,3,7,8-Cl-substituted congeners and homologue patterns were also influenced by the addition of chlorine resulting in emissions with a higher abundance of the most toxic congeners (TeCDD and PeCDD). When both chlorine and moisture content were increased in the fuel, a simultaneous effect of the two parameters was observed. The increased TEQ values expected from higher Cl concentrations were mitigated by the presence of water, giving MCE = 0.868, promoting formation of mono- to tri-PCDD/F, and lowering the TEQ value. Open burn simulations were used to study PCDD/F formation in different combustion conditions providing a mathematical correlation between PCDD/F emissions and chlorine and moisture content in the fuel.     

Origin of carbon in polychlorinated dioxins and furans formed during sooting combustion

The importance of solid- and gas-phase carbon precursors for the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/Fs) during sooting combustion was investigated in an entrained flow reactor (EFR). Experiments were performed at various methane (CH4) flame equivalence ratios with or without gas-phase chlorine (Cl2) and fly ash, to provide a realistic environment for carbon reactions and PCDD/DF formation. Selected experiments were conducted with labeled 13CH4 and 37Cl2 to investigate the relative roles of different carbon and chlorine species for the formation of PCDD/DF. The presence of soot and ash were the two major factors controlling the PCDD/DF yields. The 16 PCDD/DF homologues as well as other analyzed chlorinated aromatics were formed by reaction pathways that varied with degree of chlorination. The mono- and dichlorinated homologues were formed by gas-phase, catalytic, or noncatalytic flame product reactions, occurring during soot formation in the near flame zone and/or at lower reaction temperatures (<650 degrees C) in the postcombustion zone. Meanwhile, the higher (tri- to octa-) chlorinated homologues were mainly formed in the postcombustion zone (<650 degrees C) by fly ash-catalyzed de novo synthesis of the soot. Of these, the PCDD/DFs were formed from high carbon number (>C12) fragments in the solid soot structure, while the PCDDs, at least in part, were also formed by reaction of two C6 fragments. The tri- to hexachlorinated DD/DF homologues were formed via a relatively fast de novo synthesis occurring during the first minutes of reactions on the continuously formed soot particles, whereas de novo synthesis on an aged soot matrix was the major pathway for the hepta- and octachlorinated congeners.     

Friction and Shear Strength of Polymers

The authors have measured the shear strength and friction of a number of polymers. The shear strength measurements were made on thin films of the polymers deposited on hard, smooth substrates. The values of the coefficients of friction obtained from these shear strength measurements have been compared with the values obtained when bulk polymer slides either on itself or on a hard, smooth surface. The results support the view that in the sliding of bulk polymer energy is dissipated within a thin region close to the sliding interface and it is the pressure in the contact which largely governs the magnitude of the interfacial shear strength.     

Evidence for Self-Organized Sentence Processing: Digging-In Effects.

Dynamical, self-organizing models of sentence processing predict "digging-in" effects: The more committed the parser becomes to a wrong syntactic choice, the harder it is to reanalyze. Experiment 1 replicates previous grammaticality judgment studies (F. Ferreira & J. M. Henderson, 1991b, 1993), revealing a deleterious effect of lengthening the ambiguous region of a garden-path sentence. The authors interpret this result as a digging-in effect. Experiment 2 finds a corresponding effect on reading times. Experiment 3 finds that making 2 wrong attachments is worse than making 1. Non-self-organizing models require multiple stipulations to predict both kinds of effects. The authors show that, under an appropriately formulated self-organizing account, both results stem from self-reinforcement of node and link activations, a feature that is needed independently. An implemented model is given. (PsycINFO Database Record (c) 2010 APA, all rights reserved)