TPi October 2012

The need for material upgrade in recuperators in steel mills by Johan Wallin and Tommy Sand, Sandvik Materials Technology, Sandviken, Sweden

Introduction

The saving in fuel also means a decrease in combustion air and therefore stack losses are decreased not only by lowering the stack gas temperatures but also by discharging smaller quantities of exhaust gas. For the recuperator used in conjunction with the hot rolling mills of steel mills, the tube steel grades that historically have been used range from low alloyed carbon steel to high alloyed high temperature stainless steel grades. Depending on the steel grade and the service parameters, the lifetime and efficiency of the recuperator tubes varies to a large extent. The environment in which the recuperator operates are relatively extreme, with temperatures of the flue gases coming from the furnace ranging typically between 925°C and 1,050°C [1] . For a typical recuperator in connection to an annealing furnace at a rolling mill, you will have corrosion problems originating from the flue gases at these high temperatures. , the hot flue gases can contain several corrosive compounds and impurities such as SO x , Cl – and H 2 S, depending on what fuel is used. Common fuel types used in furnaces in a steel mill differ from mill to mill and the type of furnace. Typically, different kinds of fossil fuels like oils, coke, coal, lignite and natural gas are used. The tubes will be exposed to, for example, oxidation, sulphidation, nitridation and carburisation environments, in which different corrosion mechanism will lead to the destruction of the tubes (see Figure 2). One of the energy/fuel saving goals with the furnace operation is to recover as much heat/energy from the flue gases as possible, by preheating the incoming air as much as possible. This will then in the end reduce the fuel cost. At the same time, another task has been to extend the life/service time of the recuperator tubes. The recuperator efficiency has been one of the top priorities from an energy saving point of view in steel mills, and has been under constant development and improvement for the past few decades. Apart from CO 2 , H 2 O, N 2 and O 2

Summary A steel mill in Turkey experienced low waste energy recovery and short lifetime from its recuperator in one of its rolling mills. By upgrading to Sandvik 253 MA, the mill lowered its fuel consumption and extended the tube lifetime more than three times. History and background of the application A recuperator is a heat exchanging gas-to-gas device used in high temperature environment usually over 500°C. A recuperator can be used in many different industries where energy recovery is possible. The recuperators used in steel mills are usually tube/shell heat exchangers, where the convective heat exchange takes place between the hot flue gases and the cold incoming combustion air through metallic tube walls. Tubes carry the cold air for combustion to be pre-heated; the outside contains the waste heat stream. The hot gases are cooled by the incoming combustion air, which now carries additional energy into the combustion chamber. This is energy that does not have to be supplied by the fuel; consequently, less fuel is burned for a given furnace loading. This energy saving will in turn reduce fuel consumption and consequently, fuel costs. An example of fuel savings can be seen in Figure 1 [3] .

Figure 1 : Savings in fuel consumption at different waste gas and air preheat temperatures

Table 1 : Service parameters for the recuperator at the rolling mill furnace

Recuperator service conditions

Service parameters

Shell side

Tube side

Media

, N

, H

O

Fresh combustion air

CO

2

2

2

Impurities

Cl – , H

S

N/A

2

Temp in (C°) Temp out (C°)

800-850 450-500

0-25

480

Dimension OD x WT x L (mm)

45 x 3 x 3,250 45 x 3 x 3,301

N/A

77

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October 2012 Tube Products International