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2. ENERGY PERFORMANCE ASSESSMENT OF FURNACES

2.1       Industrial Heating Furnaces

Furnace is by definition a device for heating materials and therefore a user of energy. Heating furnaces can be divided into batch-type (Job at stationary position) and continuous type (large volume of work output at regular intervals). The types of batch furnace include box, bogie, cover, etc. For mass production, continuous furnaces are used in general. The types of continu- ous furnaces include pusher-type furnace (Figure 2.1), walking hearth-type furnace, rotary hearth and walking beam-type furnace.(Figure 2.2)

The primary energy required for reheating / heat treatment (say annealing) furnaces are in the form of Furnace oil, LSHS, LDO or electricity

Figure 2.1: Pusher-Type 3-Zone Reheating Furnace Figure 2.2: Walking Beam-Type Reheating Furnace

2.2       Purpose of the Performance Test

• To find out the efficiency of the furnace
• To find out the Specific energy consumption

The purpose of the performance test is to determine efficiency of the furnace and specific energy consumption for comparing with design values or best practice norms. There are many factors affecting furnace performance such as capacity utilization of furnaces, excess air ratio, final heating temperature etc. It is the key for assessing current level of performances and find- ing the scope for improvements and productivity.

Heat Balance of a Furnace

Heat balance helps us to numerically understand the present heat loss and efficiency and improve the furnace operation using these data. Thus, preparation of heat balance is a pre-requirement for assessing energy conservation potential.

2.3       Performance Terms and Definitions

2.4       Reference Standards

In addition to conventional methods, Japanese Industrial Standard (JIS) GO702 "Method of heat balance for continuous furnaces for steel" is used for the purpose of establishing the heat losses and efficiency of reheating furnaces.

2.5       Furnace Efficiency Testing Method

The energy required to increase the temperature of a material is the product of the mass, the change in temperature and the specific heat. i.e. Energy = Mass x Specific Heat x rise in temperature. The specific heat of the material can be obtained from a reference manual and describes the amount of energy required by different materials to raise a unit of weight through one degree of temperature.

If the process requires a change in state, from solid to liquid, or liquid to gas, then an additional quantity of energy is required called the latent heat of fusion or latent heat of evaporation and this quantity of energy needs to be added to the total energy requirement. However in this section melting furnaces are not considered.

The total heat input is provided in the form of fuel or power. The desired output is the heat supplied for heating the material or process. Other heat outputs in the furnaces are undesirable heat losses.

The various losses that occur in the fuel fired furnace (Figure 2.3) are listed below.

1. Heat lost through exhaust gases either as sensible heat, latent heat or as incomplete combustion
2. Heat loss through furnace walls and hearth
3. Heat loss to the surroundings by radiation and convection from the outer surface of the walls
4. Heat loss through gases leaking through cracks, openings and doors.

Furnace Efficiency

The efficiency of a furnace is the ratio of useful output to heat input. The furnace efficiency can be determined by both direct and indirect method.

2.5.1                 Direct Method Testing

The efficiency of the furnace can be computed by measuring the amount of fuel consumed per unit weight of material produced from the furnace.

Thermal efficiency of the furnace =

Heat in the stock

Heat in the fuel consumed

The quantity of heat to be imparted (Q) to the stock can be found from the formula

2.5.2                 Indirect Method Testing

Similar to the method of evaluating boiler efficiency by indirect method, furnace efficiency can also be calculated by indirect method. Furnace efficiency is calculated after subtracting sensi- ble heat loss in flue gas, loss due to moisture in flue gas, heat loss due to openings in furnace, heat loss through furnace skin and other unaccountedlosses from the input to the furnace.

In order to find out furnace efficiency using indirect method, various parameters that are required are hourly furnace oil consumption, material output, excess air quantity, temperature of flue gas, temperature of furnace at various zones, skin temperature and hot combustion air temperature. Efficiency is determined by subtracting all the heat losses from 100.

Measurement Parameters

The following measurements are to be made for doing the energy balance in oil fired reheating furnaces (e.g. Heating Furnace)

i)            Weight of stock / Number of billets heated

ii)            Temperature of furnace walls, roof etc

iii)            Flue gas temperature

iv)            Flue gas analysis

v)            Fuel Oil consumption

Instruments like infrared thermometer, fuel consumption monitor, surface thermocouple and other measuring devices are required to measure the above parameters. Reference manual should be referred for data like specific heat, humidity etc.

Example: Energy Efficiency by Indirect Method

An oil-fired reheating furnace has an operating temperature of around 1340°C. Average fuel con- sumption is 400 litres/hour. The flue gas exit temperature after air preheater is 750°C. Air is pre- heated from ambient temperature of 40°C to 190°C through an air pre-heater. The furnace has 460 mm thick wall (x) on the billet extraction outlet side, which is 1 m high (D) and 1 m wide. The other data are as given below. Find out the efficiency of the furnace by both indirect and direct method.

Surface temperature other than heating and soaking zone = 85 °C

Solution

1. Sensible Heat Loss in Flue Gas:

Excess air

(Where O₂ is the % of oxygen in flue gas = 12% )

O₂%

= ———— 21–O₂%

´ 100

=   12 x 100 / (21 - 12)

=   133% excess air

Theoretical air required to burn 1 kg of oil                 =   14 kg (Typical value for all fuel oil) Total air supplied     =      Theoretical air x (1 + excess air/100)

Total air supplied                                                         =   14 x 2.33 kg / kg of oil

=   32.62 kg / kg of oil

Sensible heat loss                                                         =   m x C_p x DT

m                                                                   =   Weight of flue gas

=   Actual mass of air supplied / kg of fuel + mass of fuel(1kg)

=   32.62 + 1.0 = 33.62 kg / kg of oil.

C_p                                                                   =   Specific heat of flue gas

=   0.24 kCal/kg/°C

DT                                                                  =   Temperature difference

Heat loss = m x C_p x DT                                               =   33.62 x 0.24 x (750- 40)

=   5729 kCal / kg of oil

5729 x 100

%    Heat loss in flue gas                                             =

——————  =    57.29%

10000

2.            Loss Due to Evaporation of Moisture Present in Fuel

M {584 + 0.45 (T_fg–T_amb)}

% Loss      =    ———————————   ´ 100

GCV of Fuel

Where,

M               -     kg of Moisture in 1 kg of fuel oil (0.15 kg/kg of fuel oil) T_fg    -       Flue GasTemperature

T_amb           -     Ambient temperature

GCV          -     Gross Calorific Value of Fuel

0.15 {584 +0.45 (750-40)}

% Loss      =    --------------------------------    x 100

10000

=    1.36 %

3.            Loss Due to Evaporation of Water Formed due to Hydrogen in Fuel

% Loss            9 x H₂ {584 + 0.45 (T_fg-T_amb)}

=                                                       

GCV of Fuel

x 100

Where, H₂ – kg of H₂ in 1 kg of fuel oil (0.1123 kg/kg of fuel oil)

=    9 x 0.1123 {584 + 0.45 (750-40)}

                                                            x 100

10000

=    9.13 %

4.            Heat Loss due to Openings:

If a furnace body has an opening on it, the heat in the furnace escapes to the outside as radiant heat. Heat loss due to openings can be calculated by computing black body radiation at furnace temperature, and multiplying thesevalues with emissivity (usually 0.8 for furnace brick work), and the factor of radiation through openings. Factor for radiation through openings can be determined with the help of graph as shown in figure 2.4. The black body radiation losses can be directly computed from the curves as given in the figure 2.5 below.

Figure 2.4 Factor for Determining the Equivalent of Heat Release from Openings to the Quality of Heat Release from Perfect Black Body

Figure 2.5 Graph for Determining Black Body Radiation at a Particular Temperature

The reheating furnace in example has 460mm thick wall (X) on the billet extraction outlet side, which is 1mhigh (D) and 1m wide. With furnace temperature of 1340°C, the quantity (Q) of radiation heat loss from the opening is calculated as follows:

The shape of the opening is square and D/X          =    1/0.46 = 2.17 The factor of radiation (Refer Figure 2.4)      =                                                                                 0.71

Black body radiation corresponding to 1340°C     =    36.00 kCal/cm²/hr (Refer Figure 2.5 On black body radiation)

Total heat loss = Black body radiation x area of opening x factor of radiation x emissivity

Method 1: Radiation Heat Loss from Surface of Furnace

The quantity of heat loss from surface of furnace body is the sum of natural convection and thermal radiation. This quantity can be calculated from surface temperatures of furnace. The temperatures on furnace surface should be measured at as many points as possible, and their average should be used. If the number of measuring points is too small, the error becomes large.

The quantity (Q) of heat release from a reheating furnace is calculated with the following formula:

where

Q    : Quantity of heat release in kCal / W / m²

a     : factor regarding direction of the surface of natural convection ceiling = 2.8, side walls = 2.2, hearth = 1.5

t_l     : temperature of external wall surface of the furnace (°C) t₂     : temperature of air around the furnace (°C)

E    : emissivity of external wall surface of the furnace

The first term of the formula above represents the quantity of heat release by natural con- vection, and the second term represents the quantity of heat release by radiation.

Method 2 : Radiation Heat Loss from Surface of Furnace

The following Figure 2.6 shows the relation between the temperature of external wall surface and the quantity of heat release calculated with this formula.

Figure 2.6 Quantity of Heat Release at Various Temperatures

From the Figure 2.6, the quantities of heat release from ceiling, sidewalls and hearth per unit area can be found.

6.      Unaccounted Loss

These losses comprise of heat storage loss, loss of furnace gases around charging door and opening, heat loss by incomplete combustion, loss of heat by conduction through hearth, loss due to formation of scales.

=    66.6 Litre of fuel /tonne of Material (stock)

2.5.4    Factors Affecting Furnace Performance

The important factors, which affect the efficiency, are listed below for critical analysis.

• Under loading due to poor hearth loading and improper production scheduling
• Improper Design
• Use of inefficient burner
• Insufficient draft/chimney
• Absence of Waste heat recovery
• Absence of Instruments/Controls
• Improper operation/Maintenance
• High stack loss
• Improper insulation /Refractories

2.6       Data Collection Format for Furnace Performance Assessment

The field-testing format for data collection and parameter measurements are shown below

Stock

Fuel Analysis

Flue gas Analysis

Cooling water

Temperature of combustion air          = Ambient airtemperature       =

The Table 2.1 can be used to construct a heat balance for a typical heat treatment furnace

2.7  Useful Data

Radiation Heat Transfer

Heat transfer by radiation is proportional to the absolute temperature to the power 4. Consequently the radiation losses increase considerably as temperature increases.

In practical terms this means the radiation losses from an open furnace door at 1500°C are 11 times greater thanthe same furnace at 700°C. A good incentive for the iron and steel melters is to keep the furnace lid closed at all times and maintaining a continuous feed of cold charge onto the molten bath.

Furnace Utilization Factor

Utilization has a critical effect on furnace efficiency and is a factor that is often ignored or under-estimated. If thefurnace is at temperature then standby losses of a furnace occur whether or not a product is in the furnace.

Standby Losses

Energy is lost from the charge or its enclosure in the way of heat: (a) conduction, (b) convec- tion; or/and (c)radiation

Furnace Draft Control

Furnace pressure control has a major effect on fuel fired furnace efficiency. Running a furnace at a slight positive pressure reduces air ingress and can increase the efficiency.

Theoretical Heat

Example of melting one tonne of steel from an ambient temperature of 20°C . Specific heat of steel = 0.186 Wh/kg/°C, latent heat for melting of steel = 40 Wh/kg/°C. Melting point of steel

= 1600°C.

Theoretical Total heat = Sensible heat + Latent heat

Sensible Heat      =    1000 kg x 0.186 Wh /kg °C x (1600-20)°C = 294 kWh/T

So the theoretical energy needed to melt one tonne of steel from 20°C = 334 kWh. Actual Energy used to melt to 1600°C is 700 kWh

Efficiency = 334 kWh x 100 = 48%

700 kwh