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The amount of energy involved in the fusion of glass is an issue of great interest to the glass industry. Referring to comprehensive quantitative treatments ([7, 8] and Chapter 9.8), we will give only a brief sketch of this issue within the scope of this chapter. The approach rests on the fact that, at constant pressure, the heat (enthalpy) transferred to or drawn from a system is thermodynamically the variation of a state function: as such, the intrinsic energy demand depends only on the initial and final states of the system and it can be determined without any consideration of what is going on along the process road.

The initial enthalpy state is given by the sum of standard enthalpies H°_i at 25 °C, 1 bar, of the individual raw materials i, weighted by their respective amounts m_i in the batch:

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The final enthalpy is given by the standard enthalpies of the batch gases g, H°_GASES = ∑ m_g·H°_g, and of the glass, H°_GLASS, plus the heat content ΔH(T_ex) of the glass at the exit temperature T_ex. The standard enthalpy difference between inputs and products constitutes the chemical energy demand

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The heat content of the melt at T_ex is given by ΔH(T_ex). For convenience, all enthalpy values are inserted in absolute figures, disregarding the minus sign given in thermochemical tables. The overall intrinsic heat demand H_ex (exploited heat of the process) is given by

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 where yCULLET denotes the weight fraction of cullet per amount of glass produced.

It is true, real raw materials typically do not contain their main mineral phase only, but also contain minor amounts of side minerals. For example, a real quartz sand may contain, beside its main phase quartz, minor amounts of feldspar minerals, magnetite, spinel, etc.; a natural dolomite is typically composed of different minerals forming solid solutions in the system Ca–Mg–Fe^II–CO₃ with an overall composition not too far from the pure phase CaMg(CO₃)₂. An accurate determination of the enthalpy values H°_i of real raw materials would thus require the evaluation of multicomponent phase diagrams. However, such an approach would hardly be accepted by the technological community. Beyond this, the gain of accuracy against a simpler approach is minor only. Thus, with the reservation to a more rigorous treatment [7, 8], only the enthalpy values H°_i of pure raw materials are given here in units of MJ/kg:

Raw material i	Enthalpy H°i in MJ/kg
Pure quartz sand	15.150
Pure albite (NaAlSi3O8)	14.952
Pure dolomite CaMg(CO3)2	12.549
Pure calcite CaCO3	12.058
Soda ash	10.659
Sodium sulfate	9.782
Carbon	0.000
Calumite®	13.561

For the batch gases, the following values hold:

CO₂: 8.941; H₂O: 13.422; SO₂: 4.633; O₂: 0.000.

The energy calculation for the real glass composition of Table 2 (where the tiny amount of TiO₂ has been allotted to SiO₂) is summarized in Table 5. The position of the glass composition in the phase diagram in units of kg of equilibrium compounds per t of glass is found by the following simplified procedure:

NAS₆ = 51.440 Al₂O₃ – 55.697 K₂O,

KAS₆ = 59.102 K₂O,

hm = 6 Fe₂O₃,

FS = 7.345 Fe₂O₃,

MS = 24.907 MgO,

NC₃S₆ = 35.112 CaO,

NS₂ = 29.386 Na₂O + 19.346 K₂O – 17.867 Al₂O₃ – 10.824 CaO,

S = difference to 1000 kg.

Oxide amounts are to be inserted in wt %. For the components k, the shorthand notation hm = FeO·Fe₂O₃, F = Fe₂O₃, M = MgO, C = CaO, N = Na₂O, K = K₂O, S = SiO₂ is used. Column m(k) in Table 5 lists the resulting amounts of the constitutional components of the glass. By this procedure, one finds that the standard enthalpies of formation of the glass and melt are 14 189.7 MJ/t at room temperature and 12 665.9 MJ/t at 1300 °C, respectively. The enthalpy physically stored in the melt at 1300 °C relative to the glass at 25 °C is thus 1523.8 MJ/t. By the weighted sum of the heat capacity of compounds k, the latter value can be adjusted to any other exit temperature of the melt. For the batch given in Table 4, column “m_II(i)”, a chemical energy demand of ΔH°_chem = 461.8 MJ/t is obtained. Fusion of the selected batch with 50% cullet (y_CULLET = 0.5) thus requires an intrinsic energy demand of

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A well‐constructed and operated melting furnace (end port, air‐gas fired) reaches an efficiency of heat exploitation η_ex of 48%. Thus, the actual energy demand H_in of the melting process amounts to H_in = H_ex/η_ex = 3637 MJ/t. This result is very much in line with industrial experience. Calculations of this kind are of high importance for the evaluation of glass furnace performance [9], for furnace design, as well as for the energy optimization of batch and glass compositions.