Particulates fuels

Fuel cells have several important benefits over conventional electrical eneigy generation from sources such as coal, as they are more efficient at converting fuel sources to end-use energy. The fuel cell vehicle will have no harmful emissions such as nitrogen oxide (NO ), sulfur oxide (SOJ, or particulates. Fuel cells provide waste heat utilization in co-generation, units which serve to raise the overall energy efficiency (65-85%). 

Regulatory Emissions of pollutants (carbon monoxide [CO], oxides of nitrogen [NOJ, hydrocarbons [HC], particulates) Fuel efficiency Greenhouse gas emissions Safety... 

Bulk oxide fuels are fabricated by cold compaction followed by sintering at an elevated temperature. Loose particles or optimum mixtures of several sizes of spherical (sol-gel) particles are packed directly into the cladding, and such particulate fuel is subjected to vibratory compaction. 

A large majority of air polluting emissions comes from mobile sources. The automobile is an obvious example, but other vehicles, such as semi-trucks, trains, and aircraft also contribute. Emissions from mobile sources include CO2, volatile organic compounds (VOCs), NO, and particulates. The latter may also have heavy metals, such as lead or mercury, or hazardous organics attached. Stationary sources typically burn or produce fossil fuels - coal, gasolines, and natural gas. These produce gaseous sulfur compounds (H2S, SO2, etc.), nitrogen oxides (NOJ, CO2, and particulates. Fuel producers and distributors also typically produce VOCs. 

The polarization curve shown in Fig. 7 is of particular interest in the extension of direct carbon conversion to coal char conversion. The sample of coal was chemically cleaned by a process developed to manufacture carbon particulate fuel for gas turbines. (Ultra Clean Coal Pty. Ltd. Langley, 2004). Ash may be removed by a modified Bayer caustic digestion down to the level of 0.17%. At this level, the accumulation of ash necessary to reach 10%-volume criterion found by Weaver would require 1.5 yrs of operation at 0.1 A cm. The particulate coal (10 pm size) was charred in situ at 750°C in the presence of molten carbonate. The coal showed no tendency to agglomerate and flowed freely following pyrolysis as melt-wetted slurry. 

With respect to fuels utilized as heating fuels for industrial furnaces, or as motor fuels for large diesel engines such as those in ships or power generation sets, the characteristics of primary importance are viscosity, sulfur content and the content of extremely heavy materials (asphaltenes) whose combustion can cause high emissions of particulates which are incompatible with antipollution legislation. 

For other physical properties, the specification differences between diesel fuel and home-heating oil are minimal. Note only that there is no minimum distillation end point for heating oil, undoubtedly because tbe problem of particulate emissions is much less critical in domestic burners than in an engine. 

Gasoline engine equipment such as carburetors, injectors, intake manifolds, valve systems and combustion chambers, are subject to fouling by the fuel itself, the gases recycled from the crankcase, or even dust and particulates arriving with poorly filtered air. Three types of problems then result ... 

The main justification for diesel fuel desulfurization is related to particulate emissions which are subject to very strict rules. Part of the sulfur is transformed first into SO3, then into hydrated sulfuric acid on the filter designed to collect the particulates. Figure 5.21 gives an estimate of the variation of the particulate weights as a function of sulfur content of diesel fuel for heavy vehicles. The effect is greater when the test cycle contains more high temperature operating phases which favor the transformation of SO2 to SO3. This is particularly noticeable in the standard cycle used in Europe (ECE R49). 

In any and all cases, desulfurization of diesel fuel is a necessary condition for attaining very low particulate levels such as will be dictated by future regulations (Girard et al., 1993). 

Desulfurization will become mandatory when oxidizing catalysts are installed on the exhaust systems of diesel engines. At high temperatures this catalyst accelerates the oxidation of SO2 to SO3 and causes an increase in the weight of particulate emissions if the diesel fuel has not been desulfurized. As an illustrative example, Figure 5.22 shows that starting from a catalyst temperature of 400°C, the quantity of particulates increases very rapidly with the sulfur content. 

Influence of the sulfur content in diesel fuel on particulate emissions as a, function of the catalytic converter inlet temperature. 

Influence of hydrotreating a diesel fuel on particulate emissions. 

Diesel fuels, like gasoline, are formulated with additives that affect the process of combustion, in this case to improve the cetane number. Diesel fuels also contain detergents for irijection systems as well as compounds for improving the fuel s low tempierature rheology. Finally, decreasing particulate emissions is a problem of increasing concern, but the mechanism of action to promote this effect is not clearly understood. 

Moreover, the same surfactant structures that favor dispersion of fuel droplets in the combustion chamber most iikely play a role in reducing particulate emissions. (... 

Methanol use would also reduce pubHc exposure to toxic hydrocarbons associated with gasoline and diesel fuel, including ben2ene, 1,3-butadiene, diesel particulates, and polynuclear aromatic hydrocarbons. Although pubHc formaldehyde exposures might increase from methanol use in garages and tunnels, methanol use is expected to reduce overall pubHc exposure to toxic air contaminants. 

Fig. 2. Overall schematic of solid fuel combustion (1). Reaction sequence is A, heating and drying B, solid particle pyrolysis C, oxidation and D, post-combustion. In the oxidation sequence, left and center comprise the gas-phase region, tight is the gas—solids region. Noncondensible volatiles include CO, CO2, CH4, NH, H2O condensible volatiles are C-6—C-20 compounds oxidation products are CO2, H2O, O2, N2, NO, gaseous organic compounds are CO, hydrocarbons, and polyaromatic hydrocarbons (PAHs) and particulates are inerts, condensation products, and solid carbon products.

Formation of Airborne Emissions. Airborne emissions are formed from combustion of waste fuels as a function of certain physical and chemical reactions and mechanisms. In grate-fired systems, particulate emissions result from particles being swept through the furnace and boiler in the gaseous combustion products, and from incomplete oxidation of the soHd particles, with consequent char carryover. If pile burning is used, eg, the mass bum units employed for unprocessed MSW, typically only 20—25% of the unbumed soHds and inerts exit the combustion system as flyash. If spreader-stoker technologies are employed, between 75 and 90% of the unbumed soHds and inerts may exit the combustion system in the form of flyash. 

The use of hot gas clean-up methods to remove the sulfur and particulates from the gasified fuel increases turbine performance by a few percentage points over the cold clean-up systems. Hot gas clean-up permits use of the sensible heat and enables retention of the carbon dioxide and water vapor in the... 

Methanol, a clean burning fuel relative to conventional industrial fuels other than natural gas, can be used advantageously in stationary turbines and boilers because of its low flame luminosity and combustion temperature. Low NO emissions and virtually no sulfur or particulate emissions have been observed (83). Methanol is also considered for dual fuel (methanol plus oil or natural gas) combustion power boilers (84) as well as to fuel gas turbines in combined methanol / electric power production plants using coal gasification (85) (see Power generation). 

Natural gas is attractive as a fuel ia many appHcatioas because of its relatively clean burning characteristics and low air pollution (qv) potential compared to other fossil fuels. Combustion of natural gas iavolves mixing with air or oxygen and igniting the mixture. The overall combustion process does not iavolve particulate combustion or the vaporization of Hquid droplets. With proper burner design and operation, the combustion of natural gas is essentially complete. No unbumed hydrocarbon or carbon monoxide is present ia the products of combustioa. 

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