OCR Text |
Show IV. C. RAW MATERIAL PREHEATING TEAM PROCESS In this heat recovery scheme the sensible heat from the glass melter flue gas is utilized mainly to preheat the solid raw material constituents used in the manufacture of glass. Preheating of these raw materials, commonly referred to in the industry as batch and cul1et, is the most efficient way of utilizing the waste heat and feeding it to the me1ter. Heat absorbed by preheating batch/cullet is made available entirely for the glass melting and refining. On the other hand, only half of the heat released in natural gas firing in the furnace is used for this purpose since the rest is lost with the flue gas. PROCESS DESCRIPTION The process flow diagram for this process scheme is provided in Figure 7. As in the previous TEAM process schemes, the glass melter flue gas leaves the unit melter configured furnace at 2,200°F. At this temperature, constituents within the batch would begin to melt, particularly the soda ash. To prevent this, the hot flue gas is quenched with much colder flue gas recycled from downstream of the heat exchange operations. The flue gas is quenched with close to an equal volume of cool recycle gas in the quench tank. The resulting stream is used to preheat the natural gas and oxygen for firing into the glass melter. After this the flue gas is used for preheating the solid batch/cullet raw material feed to the glass melter. The bulk of the sensible heat in the flue gas is recovered in this direct contact heat exchange step. The flue gas drops almost 800°F in temperature as it transfers 5 MMBtu/hour of heat to the batch/cullet. The gas leaving the batch/cullet preheater passes through an electrostatic precipitator, further cooling, and a blower after which close to half of the gas is recycled for the quenching operation. The rest of the gas which is 43% carbon dioxide can be sent for carbon dioxide recovery (an attractive option for all TEAM processes) or exhausted to the stack. The important feature of the quench operation is that it conserves the enthalpy in the melter flue gas while lowering the temperature and heat quality as desired. It should be noted that this process does not require the high quality heat that is so important in the reforming and power generation heat recovery schemes. Quenching of the hot melter flue gas could also be accomplished using air or water. However both of these have disadvantages associated with their use and the following discussion outlines why recycled flue gas is best for this application. The use of water for quenching would not be as energy efficient because the latent heat of vaporization is not recovered in the process and would be lost. Using air raises the possibility of generation of nitrogen oxides, especially at high flue gas exit temperatures. Besides not being burdened by these disadvantages, the use of recycled flue gas for quenching has the advantage of not diluting the carbon dioxide content of the flue gas, as would be the case for the air or water quench options. This is a very important consideration in making the carbon dioxide recovery option economically viable. - 9 - |