1.5.Waste heat recovery

The EU industrial, residential, and commercial/institutional sectors account for 65.2% from all energy used 2010 (Fig. 1). Efforts to improve the energy efficiency are focused on reduction of energy consumed by equipments, changing the processes or technologies used in the industrial, residential, and commercial/institutional facilities and on the capture and reuse of waste heat.

Figure 1. Final energy consumption of EU-27 in 2010.

The first step in improving energy efficiency is the energy audit of the systems used in industrial, commercial and institutional facilities. This analysis shows the path of energy input, use and output and identifies the potential energy cost savings. The energy audit may also indicate the waste heat sources. The waste heat is the energy associated with the waste streams of air, exhaust gases, and/or liquids that leave the boundaries of a plant or building and enter the environment, and is related to equipment inefficiencies and thermodynamic limitations of equipments and processes. The waste heat should be first reduced or eliminated and then recovered and used. Recovery and use of waste heat can be performed when there is a demand for recovered heat and also a match between available heat and required heat.

The waste heat recovery requires three main components: an accessible source of waste heat; a recovery technology and a use for recovered energy (Fig. 2). Once the waste heat recovery system is selected, its economical evaluation is required.

As direct benefit the recovery of waste heat has the reduction of utility consumption and costs. The indirect benefits are: pollution reduction, equipment size reduction and reduction of auxiliary energy consumption.

Figure 2. Components of a waste heat recovery system.

1.5.1.Characteristics of waste heat

In a waste heat recovery projects the following characteristics of waste heat source are important: quantity, quality, availability and compatibility with heat demand.

Heat quantity

When a substance absorbs or releases heat, either the temperature of substance changes or the state of substance changes. The heat released or absorbed by a body or a substance during a constant-temperature process (change of state of matter) is called latent heat. Latent heat is mainly dependent on material properties, temperature and pressure. The heat exchanged by a body or substance that has as its sole effect a change of temperature is called sensible heat.

The quantity of heat in a substance depends on its heat capacity, its temperature variation and whether it suffers or not change of state (phase). It can be expressed by using the enthalpy. In the waste heat recovery systems, both sensible and latent heat can be recoverable.

The amount of sensible heat contained in a substance can be calculated using the following equation:

[kJ] (1)

where: m – mass of substance, kg;

c_p – specific heat of substance, kJ/(kg·K) (amount of heat absorbed by one kilogram of a substance to rise its temperature with one Kelvin);

h – enthalpy of substance at temperature T and pressure p, kJ/kg.

The amount of heat required for evaporation of a liquid or condensation of a gas can be calculated with the equation:

[kJ] (2)

where h_l is latent heat of vaporisation or condensation, kJ/kg.

The heat which can be recovered from exhaust gases can be calculated as a difference between the enthalpy at the beginning of process and enthalpy at the end of recovery process.

The enthalpy of gas, considered as a mixture of dry gas and water vapour, at the beginning of process, can be calculated with equation:

[kJ/kg] (3)

where: c_p,dg - specific heat of dry gas, kJ/(kg·K);

c_p,w - specific heat of water, kJ/(kg·K):

(4)

T₁ – gas temperature at the beginning of recovery process;

h_l,w - latent heat of water vaporisation, kJ/kg;

x – specific gas humidity, kg water vapour/kg dry gas: ;

m_w – mass of water vapour content in exhaust gases, kg;

m_dg – mass of dry gas content in exhaust gases, kg.

The gas enthalpy at the end of recovery process can be expressed as follow:

• when the final gas temperature is above the dew point (without vapour condensation):

[kJ/kg] (5)

• when the final gas temperature is below the dew point (with vapour condensation):

[kJ/kg] (6)

where: T₂ - gas temperature at the end of the process, K;

x_s – specific gas humidity at saturation:

[kg water vapour/kg dry gas] (7)

p_ws – partial pressure of water vapour at saturation, Pa:

[Pa] (8)

p_t – total pressure of gases, Pa.

Many waste heat sources from industrial sector are within the low temperature range. For example, the boilers with recovery system usually exhaust the combustion gases at temperature about (150-180)°C. From the combustion exhaust gases can be recovered a large part of contained heat if the gases are cooled below the water dew point and the latent heat of vaporization is released. The water dew point is the temperature at which a given volume of gas should be cooled, at constant pressure and water content, to reach a state at which the partial vapour pressure is equal to saturation vapour pressure. Figure 3 shows the significant increase in combustion plant efficiency when the temperature of exhaust gases at the recovery unit outlet decreases below the dew point. In order to increase the recovery efficiency by cooling the waste heat below the temperature of acid vapour condensation a technology that minimizes the chemical attack of acid condensate is required.

Figure 3. Efficiency of combustion plants with waste heat recovery unit as a function of stack inlet temperature of flue gases.

When the exhaust gases contain gaseous species like SO₃, SO₂, HCl or NO₂ and the temperature drops to dew point, the sulphuric acid, chloride acid, nitric acid and water begin to condense leading to a new dew point with values higher than the water dew point and called acid dew point. The acid dew point temperature is that temperature at which the actual acid vapours pressure equals the acid vapour saturation pressure. below the acid dew point, a very corrosive process occurs on surfaces of equipments (steel, plastics, and cement) leading to operational failure.

To avoid the corrosive issues requires knowing the dew acid point. There are certain empirical formulas for acid dew point prediction [1]:

• flue gas sulphuric acid dew point :

(9.a)

or

(9.b)

• flue gas nitric acid dew point:

(10)

• flue gas hydrochloric acid dew point:

(11)

where is partial pressure of water vapour, sulphur trioxide, sulphur dioxide, nitric acid and chloride acid respectively in mm Hg.

The mixed acid dew point can be approximated by the sulphuric acid dew point. The sulphuric acid dew point temperature depends on the SO₃ and water content of the flue gas resulted from the sulphur, hydrogen, and free water content of the fuel; the relative humidity of the air; and the amount of excess air used.

Heat quality

The quality of heat is given by its temperature level. The higher the temperature, the more available the waste heat for using in processes and more cost effective is the heat recovery. For waste heat with high temperature, a cascade system for heat recovery may be used to obtain the maximum amount of heat.

The quality of waste heat can be also described by the work potential. The work potential represents the maximum possible work that could be extracted from a heat engine operating between the waste heat temperature and ambient temperatures [5]. the work potential of the heat is calculated by multiplying the heat amount by the Carnot efficiency:

(12)

where: T_H –temperature of waste heat source, K;

T₀ – ambient temperature, K.

The waste heat at temperature close to ambient temperature has a little use and the use of heat pump can improve the quality of waste heat economically over a limited range of temperatures around environmental temperature.

According to the temperature level, the waste heat sources are classified into three ranges [2]:

• high range (t ≥ 650°C);

• medium range (230°C ≤ t ≤ 650°C);

• low range (t ≤ 230°C).

This classification matches the classification of commercial waste heat recovery systems. The waste heat in the high-temperature range (Table XXV) results from the direct-fired processes and is suitable to generate mechanical/electrical power by using steam or gas turbines.