Hot exhaust gases discharged from Zone 3 are ducted into a heat exchanger to recover thermal energy. A number of options are available for recovering the heat. In one arrangement, the heat can be transferred directly to a sludge drying circuit. In most cases, this eliminates the need for an additional energy source for sludge drying. The amount of heat available is approximately equal to the amount of heat needed to dry sludge with solids contents from 25% up to 90% to 95% solids. Another configuration would recover the heat as steam. In applications where more heat is released from the Minergy process than is required to dry the sludge, standard energy recovery technologies (steam, thermal oil, gas/gas heat exchanger) can be integrated into the system for heating, power generation or other process needs.
Although most of the inorganic material is melted in the GlassPack® melter, a fraction of dust can be present in the gas stream. Normally, a collector can be installed on the exhaust of the heat recovery unit to collect particulate material. An exhaust fan is used to maintain draft and induce flow through the process. The exhaust is further cooled and water vapor produced from combustion is condensed in a direct packed tower style condenser. The exhaust gas is cooled to 90 to 120 degrees Fahrenheit into the gas recycle header. A portion of the recycle gas is exhausted out of the process. Depending on air regulations and air permit requirements, air quality control equipment can be installed at this location in the process. The equipment would be compact and economical due to the low gas volumes exhausted by the process.
The remainder of the recycled gas is boosted in pressure through a small fan and enriched with oxygen. The end result is synthetic air, which is injected into Zone 1 of the GlassPack® melter. Unlike normal air which is always 21% oxygen, synthetic air can be mixed to any ratio of oxygen necessary. This allows simultaneous optimization of both melting temperatures and combustion conditions, which cannot be done with conventional air-fired combustion.
Interlocks Assure Process Safety
The GlassPack® Closed-Loop Combustion System is thoroughly instrumented with high quality and reliable equipment, which is calibrated and functionally checked at startup. The control component provides numerous interlocks to prevent the system from operating outside its normal design parameters. Both the oxygen supply and the startup natural gas supply system use double block and bleed valves to guarantee isolation of the energy supply to the combustor in the event that any critical process parameter exceeds the limitations.