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Comprehensive Review of S olid W aste Collection and Disposal Options v2.1 158 10/22/12  Waterwall Units. This design has the furnace constructed with water tube membrane walls to recover the heat energy directly from the furnace unit. Waterwall designs are more commonly used because their thermal efficiency is higher than refractory units. Boilers convert the heat released to steam, which can be used to either generate electricity or for industrial steam applications (if a customer is nearby). Turbine-driven generators driven by the steam generate electricity. 12.3.1.3 Products of Combustion Other than the release of energy in the form of heat, the products of combustion of solid waste are fly ash and bottom ash. Each of these byproducts of combustion, air emissions, and ash, present further environmental permitting, handling, and disposal challenges for the WTE technology. Fly ash is carried in the combustion gas, which also contains a number of contaminants, including acid gases, and other products of incomplete combustion. The gases are passed through a variety of air pollution control devices for cleanup before being discharged out of the stack into the atmosphere. Bottom ash is the non-combusted material, which is discharged at the end of the grate subsystem. The bottom ash, as it is discharged from the grates, is still burning and is normally quenched by water. In the United States, the two ash streams, fly ash and bottom ash, are normally combined for management and disposal in a permitted MSW or industrial landfill. The two combined ash streams are commonly referred to as solid waste combustor ash, or just ash. In Europe, these two ash streams are not usually combined and are normally managed separately. 12.3.2 Mass Burning “Mass-burning” refers to the generic name for the type of technology used to incinerate unprocessed solid waste, and thereby releasing its heat energy. The thermal reduction of solid waste through mass-burning has been a common procedure throughout the world. There are decades of experience in constructing and operating some 500 mass burn facilities in the United States and Europe. Such facilities were in operation as early as 1896 in Hamburg, Germany, converting solid waste into electricity. 12.3.2.1 Process Description An illustration of a typical mass-fired, WTE facility is shown in Exhibit 12-1. Solid waste collection and transfer vehicles proceed into a tipping area where their waste is discharged into a large storage pit, which is usually sized to allow two to three days storage or stockpiling of refuse so that plant operations can continue over weekends and holidays when deliveries will not be accepted. There are some facilities which differ in design by utilizing a tipping floor with a front loader and belt conveyor system as their form of storage and feed system. In almost all facilities, however, the refuse is fed into the furnaces by means of overhead cranes manipulated by a crane operator. Much of the success of the operation depends upon the skill of the crane operator to remove large or unusual objects in the waste stream that would otherwise prove to be a problem if fed into the boiler. The operator is also responsible to observe the nature of the