Yiannis A. Levendis

Operational and Environmental Evaluation of Diesel Engines Burning Oxygen-Enriched Intake Air or Oxygen-Enriched Fuels: A Review

A method to curtail emissions of smoke and other pollutants from diesel engines is to enhance the oxygen supply to their combustion chamber. This can be accomplished by enriching either the intake air stream or the fuel stream with oxygen. Experimental studies concerning the oxygen-enrichment of intake air, have revealed large decrease of ignition delay, drastic decrease of soot emissions as well as reduction of CO and HC emissions while, brake specific fuel consumption (BSFC) remained unaffected and increasing of power output is feasible. However, this technique was accompanied by considerable increase of NO x emissions. Experimental and theoretical studies with oxygenated fuels have demonstrated large decrease of soot emissions, which correlated well with the fuel oxygen content. Reduction of CO and HC emissions with oxygenated fuels was also obtained. However, penalties in both BSFC and NO x emissions have been observed with oxygenation of diesel fuels. In both cases one has to weigh the tradeoffs in fuel economy, in power output and in the emissions of various pollutants. Moreover, fuel cost, availability and supply infrastructure, as well as equipment and operational costs, are among concerns that apply to these techniques. This manuscript presents a comparative evaluation of the two techniques regarding engine performance characteristics, environmental repercussions and economy of operation. The primary objective is to contrast the benefits and the drawbacks of the two techniques in view of economic, operational and environmental parameters. Results have shown that the overall economy of operation of the two techniques may be comparable, if the price of oxygenated fuel blends is similar to that of diesel fuel. Their impact on pollutant emissions may also be comparable, if the oxygen enrichment of either technique is limited to a low level (<23% by mass in the cylinder mixture). However, there are possibilities of increasing the power density of engines with oxygen enrichment of the intake air.

Development of multicolor pyrometers to monitor the transient response of burning carbonaceous particles

A three‐color ratio pyrometer has been developed to obtain surface temperatures and high‐temperature combustion rates of burning carbonaceous particles. The features and performance of this instrument are contrasted to those of a two‐color ratio pyrometer, constructed earlier for similar studies. The three‐color pyrometer employs a visible (0.65 μm) and two near‐infrared (0.80 and 0.975 μm) wavelengths. The instrument uses a single optical fiber to capture radiation emitted from a particle burning in a high‐temperature laminar flow furnace. Monitoring of the combustion events takes place coaxially with the particle flow, from observation windows located at the top of the furnace injectors. Thus, the temperature‐time history of burning particles can be recorded. The radiation flux is split into three beams using dichroic edge filters. Narrow (or medium) bandwidth interference filters guide monochromatic radiation to solid‐state silicon photodetectors. The associated amplification is linear and/or logarithm...

A laboratory study on the NO, NO2, SO2, CO and CO2 emissions from the combustion of pulverized coal, municipal waste plastics and tires

This is a laboratory study on the combustion emissions from pulverized solid fuels: NOx (NO and NO2), SO2, CO and CO2. Coal, waste tire crumb and waste plastics, such as poly(styrene), poly(ethylene), poly(methyl methacrylate), poly(propylene) and poly(vinyl chloride) (PVC), were burned in an electrically heated drop-tube furnace at high particle heating rates (104-105 K s−1) and elevated gas temperatures (1300–1600 K). The fuel to air bulk equivalence ratio, φ, was varied in the range of 0.4–1.8. Air or a nitrogen-free mixture of O2CO2Ar were used as oxidizing gases. Results showed that fuels which contain nitrogen generated the highest NOx emissions. Combustion of coal generated four times more NOx than combustion of tire crumb, in proportion to their nitrogen content, and ten times more NOx than that of the nitrogen-free plastics. The specific NOx emissions decreased dramatically (3–6 times) with increasing bulk equivalence ratio for all fuels. However, the NO2/NO ratio increased with the equivalence ratio in the fuel-rich region. Increasing the gas temperature, in the range 1300–1600 K, resulted in 10–25% more NOx, depending on the fuel. Atmospheric nitrogen contributed 20% of the total NOx emissions for coal, 30% for tires and 100% for the plastics. SO2 emissions of the particular coal and tire crumb tested were comparable. While absolute SO2 emissions of coal and tire increased with the equivalence ratio, specific emissions exhibited a mild downward trend. SO2 emissions were higher in the absence of atmospheric nitrogen, especially at fuel-rich conditions. Mild emissions of CO were encountered in the fuel-lean regions, but they increased exponentially in the fuel-rich region accounting for as much as 10% of the carbon at φ = 2. Overall, the CO2 emissions were proportional to the carbon content of the fuels.

Comparison of the combustion behaviour of pulverized waste tyres and coal

Abstract Comparative combustion studies were performed on particles obtained from pulverized bituminous coal and waste automobile tyres (rubber). Particle size cuts of 75–90 and 180–212 μm were burned in a thermogravimetric analyser, at low heating rates, and in an electrically heated drop-tube furnace, at high heating rates. The combustion of individual particles in the drop-tube furnace was observed with three-colour pyrometry, to obtain time-temperature histories and with high-speed cinematography to record flame particle size histories. Combustion was conducted at a gas temperature of 1450 K, in air. Upon pyrolysis, the phenomena of melting, swelling and formation of large blowholes were observed only in the case of the coal particles. The tyre particles formed chars with rough surfaces and smaller blowholes. Separate volatile and char combustion phases were detected for the coal particles studied. Tyre particles experienced an intense primary volatile combustion phase, followed by a phase of simultaneous secondary volatile combustion, of lesser intensity, and char combustion. During the initial volatile phase combustion, the peak flame temperatures were comparable for both materials, in the range 2200–2400 K. The secondary volatile/char combustion phase, observed for the type particles, was cooler, i.e. 2000–2100 K. The coal chars burned with temperatures of 1850–2000 K. Combustion was diffusionally controlled (regime III) for coal chars of both sizes and for tyre chars of the larger size cut only. Char burnout times were considerably shorter for tyre particles than coal, which can be attributed to the secondary devolatilization and the lower density of the former.

Measurements of particle flame temperatures using three-color optical pyrometry

Abstract A three-color near-infrared optical pyrometer, with wavelengths centered at 998, 810, and 640 nm, was used to monitor the combustion of polymer particles. Individual spherical poly(styrene) particles, 47–355 μm in diameter, burned in air at 1050–1400 K gas temperatures, surrounded by sooting, diffusion envelope flames. The pyrometric results were interpreted in view of two models for soot radiation: (a) A conventional model, which assumes that the flame is optically thin and isothermal and thus, the spectral emissivity is inversely proportional to the wavelength. With this method the calculated flame temperatures are averages, biased to areas with high temperature and/or soot concentration. (b) An alternative model, in which the envelope flames are assumed to be again optically thin but nonisothermal in the radial direction. The theoretical development of the latter model is included herein. For nonisothermal flames the spectral emissivity was shown to be nearly independent of the wavelength. This model in conjunction with three-color pyrometry may provide a way of estimating the highest temperature of soot in the flame, as well as the temperature gradient across the flame. Temperatures calculated this way were higher by 200–230 K than those calculated using the conventional model. Experimental results suggest that the agreement among the three individual temperatures obtained from three-color pyrometry depends on which of the above models for soot radiation is used. Based on the agreement between temperatures, the degree of isothermality of the flame may be determined and thus, indications about the controlling processes during combustion (oxygen diffusion or volatile combustion) may be obtained. However, additional work is needed, involving simpler, one-dimensional flame configurations to confirm this model.

Physical properties and oxidation rates of chars from three bituminous coals

Intrinsic oxidation rates of coal chars derived from three different bituminous coals were measured at 500 °C and the effects of char formation temperature, conversion, coal particle size and char particle size on the rates were evaluated. Characterization of the various samples including BET surface areas, mercury porosimetry, mercury and helium densities, heating values and in some cases elemental analyses were carried out to better understand the roles and interactions of the various parameters. Optical microscopy observations were also made to verify assumptions wherever possible. The results show that apparent and intrinsic rates as well as the heating values are reduced with increasing char formation temperature. N_2-BET surface areas increase an order of magnitude with conversion and exhibit maxima in the range of temperatures considered (1000 K to 1600 K). Of similar sized chars derived from different sizes of coal particles, those from the smaller coal size fractions had higher apparent reaction rates.

A study on the combustion characteristics of PVC, poly(styrene), poly(ethylene), and poly(propylene) particles under high heating rates

Abstract The combustion characteristics of four commonly encountered plastics: poly(styrene), PVC, poly(ethylene) and poly(propylene) were studied under conditions pertinent to incinerators, that is, high heating rates (in the order of 10,000 K/s) and elevated gas temperatures (1200–1500 K). Batches of spherical and monodisperse particles of these plastics were generated in the size range of 53–300 μm. Combustion of single particles, of known size and mass, was conducted in a laminar-flow drop-tube furnace, at controlled atmospheres. The radiation emitted from burning particles was monitored, along their flight path, by simultaneous three-color optical pyrometry and high-speed cinematography. With these techniques the total particle/flame combustion duration, as well as the flame temperature and diameter were measured. Results indicate that polymer particles (plastics) burned expediently with burntimes similar to those of light oil drops such as kerosene, hexadecane, etc. Both PVC and poly(styrene) burned with very luminous yellow flames, which were attributed to high soot loadings. The flame combustion of PVC was the brightest and fastest with steadily decreasing temperature and flame diameter, while that of poly(styrene) occurred mostly at constant flame diameter and mildly decreasing temperature. Combustion of both poly(ethylene) and poly(propylene) was dimmer and somewhat lengthier. Furthermore, in this temperature region, it is argued herein that the combustion of poly(styrene), poly(ethylene), and poly(propylene) occurred concurrently with, and was partially controlled by, pyrolysis reactions. PVC seemed to undergo significant pyrolysis prior to ignition; thereafter, combustion occurred in a premixed-like flame mode and, finally, dimly glowing combustion of the remaining char was observed. An energy balance during the flame combustion period enabled the calculation of the instantaneous burning rate and the average soot loading of the flame. PVC exhibited the highest soot volume fraction in its flame (3 × 10 −5 ), followed by poly(styrene) (3 × 10 −6 ), poly(propylene) (2.5 × 10 −6 ), and poly(ethylene) (1 × 10 −6 ). Moreover, it was observed that the rate of burning was the highest for PVC particles and the lowest for poly(ethylene).

Synthesis, formation and characterization of micron-sized glassy carbon spheres of controlled pore structure

Abstract A method of producing uniform-sized solid or hollow glassy carbon spheres with sizes from a few to 200 μm in times from seconds to minutes has been developed. The spheres are obtained by atomizing polymers of furfuryl alcohol containing pore forming agents in the form of either dissolved high boiling organics or dispersed carbon black. These agents were added to control the porosity, the pore size distribution and the surface areas of the chars. Heat treatment and/or partial oxidation in the range of 800 to 1600 K were then used to vary the porosity (6–50%) and surface area (2–800 m2/g), and to produce pore size distributions that include both micro and transitional pores. As the temperature rises, the size and the volume of the micropores increase at the expense of the skeletal volume and then their size distribution becomes more narrow. Partial oxidation at elevated temperatures opens and enlarges existing porosity and catalyzes multiphase graphitization of many of the mixed-polymer chars.

On the survivability and pyrosynthesis of PAH during combustion of pulverized coal and tire crumb

Abstract Results are presented on the emissions of semivolatile polycyclic aromatic hydrocarbons (PAH) from the combustion of a pulverized bituminous coal and ground waste automobile tires. Streams of fuel particles were injected at steady-state steady-flow conditions, and burned inside an isothermal drop-tube furnace, in air, at a gas temperature and gas residence time of 1150°C and 0.75 s, respectively. Combustion occurred under either very fuel-lean conditions (bulk equivalence ratio, φ 1.6 and, especially, under pyrolytic conditions in N 2 . These PAHs were mostly attributed to pyrosynthesis since none of the deuterated PAHs, adsorbed on the fuels, survived the combustion process. Small amounts of the labeled compounds, however, survived under purely pyrolytic conditions. These results were confirmed with separate experiments, where deuterium-labeled PAH standards were adsorbed on highly porous calcium/magnesium oxide or mullite particles. Again, small amounts of some PAHs survived in high-temperature pyrolytic conditions, but none in oxidative environments. These observations suggest that pyrosynthesis is the major contributing mechanism to the PAH emissions from the combustion of these fuels. Survivability of parent PAHs may be a minor mechanism at very high equivalence ratios. Finally, both fuels were mixed with powders of calcium magnesium acetate (CMA), calcium carbonate (CaCO 3 ), and calcium oxide (CaO), all of which are known sulfur reduction agents, at a molar Ca/S ratio of 1. Combustion of the fuels mixed with CMA or CaCO 3 generated enhanced amounts of PAHs, while combustion with CaO had no effect on the PAH emissions.

Oxidation kinetics of monodisperse spherical carbonaceous particles of variable properties

Synthetic chars of variable physical and chemical properties have been developed to study char oxidation mechanisms and rates. The char particles were spherical and monodisperse, with sizes ranging from a few microns to several tens of microns. The particles were made from a carbon-yielding polymer and pore-forming additives. The surface areas of the chars made from different additives varied by more than two orders of magnitude and the porosities varied by a factor of five. The pore size distributions included both micro and transitional pores. X-ray studies revealed that all chars were amorphous when heat treated to temperatures up to 1600 K in an inert atmosphere. However, upon oxidation at 1600 K, the carbon matrix underwent partial graphitization. This transformation was particularly pronounced for some of the polymer pore-former chars. Combustion experiments showed that the total surface area of the chars increased dramatically with conversion, revealing the existence of a vast network of micropores. Apparent oxidation rates were higher for the chars that contained transitional pores in a microporous matrix. When compared with the rates reported in the literature for coal derived chars, the calculated intrinsic rates were lower at intermediate particle temperatures (800-1600 K) but comparable at elevated temperatures (1800-2300 K). As the temperature was increased further, the intrinsic rates decreased consistent with the Nagle and Strickland-Constable kinetic mechanism [l].