Francesco Carbone

Challenges of measuring nascent soot in flames as evidenced by high-resolution differential mobility analysis

ABSTRACT Nascent soot particles with mobility diameters ≤10 nm were measured in an ethylene/air premixed flame to shed light on the challenges and potential artifacts affecting studies on soot inception by differential mobility analysis (DMA) techniques. The size distribution functions (SDFs) of particles with charge acquired either naturally or diffusively upon ion seeding were measured at several positions in the flame using rapid-dilution probing and a high-resolution DMA for different values of the ratio of dilution ratio to residence time (DR/Δt). The SDFs are roughly bimodal with a sub-3 nm mode and a larger one that appears either downstream in the flame or for low DR/Δts. Soot nuclei smaller than 3 nm preferentially acquire positive charge, which brings into question the assumption of steady-state charging probability of flame sampled soot nuclei in the bipolar diffusion neutralizer. The approximately polarity-symmetric lognormal SDF of larger particles is attributed to nuclei coagulation. Naturally charged particles increase in number when lowering DR/Δt, suggesting either their collisional charging by flame chemi-ions or particle nucleation by condensation of neutral molecules on ions or both. The critical conditions for suppressing particle coagulation and charge redistribution in the sampling system were not achieved under most conditions, despite the fact that values of DR/Δts were more favorable to such a suppression in the present experiment as compared to other studies in the literature. As a result, the identification of this “asymptotic” regime, which is critical to determine the parent SDFs and the charge state of nascent soot in the flame, is still elusive.

Size Distribution Functions of Ultrafine Ashes From Pulverized Coal Combustion

The major drawbacks of coal combustion are connected with the large emission of particulate matter. This paper deals with the experimental investigation of ultrafine (D < 100 nm) and nano (D < 10 nm) ashes formed during conventional high temperature pulverized coal combustion on a laboratory-scale reactor. The reactor consists of an atmospheric pressure laminar premixed flame homogeneously doped with pulverized coal particles, monodisperse in size. It allows the investigation of the early stage of ash formation. Generated aerosols, sampled using high dilution probes, are conveyed into high resolution Differential Mobility Analyzers (DMA) to perform on-line measurements of the size distribution function of ultrafine ashes. Two DMA systems are used: the first is equipped with a Faraday cup electrometer detector and identifies particles as small as 1 nm, and the second is equipped with a condensation nucleus counter and exhibits 3 nm lower detection limit. Measurements have been performed at several dilution ratios to understand the nature and behavior of detected particles. Results indicate that nanosized coal ashes are formed during pulverized coal combustion. They are the most abundant in number and also a significant fraction of ultrafine ash mass, implying they should be detected at the exhaust of pulverized coal furnaces.

The Whirl Cookstove: A Novel Development for Clean Biomass Burning

ABSTRACT An innovation in cookstove design was developed through a simple change in the natural-draft air entrainment pattern. The new concept is based on the establishment of a whirl in the combustion chamber, with air being introduced only tangentially through lateral slots in the cookstove walls. The whirl configuration creates better mixing between fuel and oxidizer, more uniform burning, and a reduction in pollutant emissions. Water boiling tests revealed significant improvements with respect to the ordinary natural draft mode of operation, the so-called rocket mode, including: a 13% average increase in the thermodynamic efficiency and a decrease in total particulate emissions by 53%. In water simmering tests, the gain in thermodynamic efficiency is estimated at 24% and the reduction in particulate mass and number are 70% and 44%, respectively. The particle abatement is more significant for sizes larger than 0.5 um. A whirl retrofit can be introduced in almost any existing cookstove design and is well suited to scalable manufacturing of inexpensive stoves by casting them from an inexpensive mold.