Combustion systems waste

Given the mechanisms and temperatures, waste combustion systems typically employ higher percentages of excess air, and typically also have lower cross-sectional and volumetric heat release rates than those associated with fossil fuels. Representative combustion conditions are shown in Table 11 for wet wood waste with 50—60% moisture total basis, municipal soHd waste, and RDF. 

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. 

Multi-stage preheating, pre-calciners, kiln combustion system improvements, enhancement of internal heat transfer in kiln, kiln shell loss reduction, optimize heat transfer in clinker cooler, use of waste fuels Blended cements, cogeneration... 

Boilers may be direct fired or indirect fired. Energy supply designs account for various combustion methods using fossil fuels, municipal waste, process residues, waste heat, and by-products. Special boiler combustion systems to reduce pollution or improve efficiency include fluidized-bed and combined cycle. 

Future applications are likely to involve real-time measurements and control of various combustion systems including gas turbines, waste incinerators, and aeropropulsion systems. 

Fluidized-bed combustion systems use a heated bed of sandlike material suspended (fluidized) within a rising column of turbulent air to burn many types and classes of waste fuels. The vendor claims that this technique results in improved combustion efficiency of high moisture content fuels and is adaptable to a variety of waste -type fuels. The scrubbing action of the bed material on the fuel particle is said to enhance the combustion process by stripping away the carbon dioxide and char layers that normally form around the fuel particle. This allows oxygen to reach the combustible material much more readily and increases the rate and efficiency of the combustion process. 

The zero-emission energy recycling system (ZEROS) is a closed-loop thermal oxidation process that incinerates waste and recycles flue gas emissions for electrical co-generation. The technology uses a two-stage plasma torch combustion system, energy recovery system, and combustion gas cleanup systems. 

Pope, K. M. 1991. Tires to energy in a fluidized bed combustion system. Proceedings 1991 Conference on Waste Tires as a Utility Fuel. Electric Power Research Institute, Palo Alto, CA, EPRI GS-7538, 11/1-11/9. 

The combustion system requires cleaning of the boiler tubes after every 30 days of operation. A typical incinerator run is 7 to 17 days. This type of duty cycle is suitable for solid waste disposal at the tire plants, although it would be unacceptable for steady production of electric power. 

Particulate emissions may increase if combustion is not complete. As seen in Tables 1-2 and 1-3, sulfur emissions may decrease if the tires or TDF replace higher sulfur coal, but may increase if tires or TDF replace wood waste containing little sulfur. NOx emissions, likewise, may increase or decrease based on the relative nitrogen content of the fuel. Also, NOx emissions may increase if additional excess air enters the combustion system to facilitate the feed of the tires or TDF. 

Pope, Kent M. Tires to Energy in a Fluidized Bed Combustion System. Presented at EPRI Conference Waste Fuels in Utility Boilers. San Jose, CA. January 28, 1991. 

The largest scrap tire combustion system is the Oxford Energy plant in Modesto, California. It consumes about 4.9 million tires per year and generates 14 MW of power. A second Oxford Energy power plant, designed to bum about 9 to 10 million tires per year has been build in Connecticut. Two others may be built in Nevada and New York. Also, two Firestone plants have installed pulsating floor furnaces to dispose of scrap tires and other solid wastes here, process steam is generated as a by-product. 

Application of the sodium-based dual alkali process to high chloride gas streams is of interest not only to the Army but also to other operators of combustion systems using conventional fuels such as coal where toxic chloride-containing wastes are co-fired for destruction purposes. 

Theoretical studies into the mechanisms of formation of PCDD/Fs in combustion systems, with the specific aim of elucidating emission control strategies, commenced in the early 1980s. Seminal contributions7-9 examined free radical, homogeneous gas-phase reactions in the hot, combustion zone of municipal solid waste (MSW) incinerators, and concluded that this formation mechanism could not account for measured concentrations of PCDD/Fs in incinerator stack emissions. Measurements taken at various stages of the combustion and gas cleaning train of a MSW incinerator at Tsushima, Japan,... 

Adsorptive Processes. The use of activated carbon, sprayed into a dry/semi dry scrubbing unit along with lime or less frequently packed in an adsorption unit positioned after the particulate removal device and prior to the stack, has become a standard component in gas cleaning trains as a means of PCDD/F control on all sizes of plant fed with MSW or clinical waste. Other adsorptive media such as zeolites are also being tested. The inclusion of an adsorptive device in combustion systems fired with wood and agricultural wastes is not normally contemplated, and as noted above, an interesting issue to be resolved is whether different waste types generate flyash of different activities relative to PCDD/F formation. 

The impetus for displacing fossil fuels with biomass in heat and power combustion systems is typically to reduce emissions of C02, and of NOx, SOx, and other air toxics, or to improve utilization of biomass residues and wastes. The threat of increased global warming has subjected the use of fossil fuels to increasing scrutiny in terms of greenhouse gas and pollutant emissions. Renewable and... 

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