Using natural gas for metal processing

Editor’s note: This article is based on a series of Energy Solution Center (ESC) seminars held from Sept. 2020 to Jan. 2021 to discuss important technical issues and marketing information on important topics related to efficient use of natural gas in industrial thermal processes used by major industries in U.S. These topics included:

• Commonly used gas furnaces/ovens
• Industrial combustion control
• Heat treating
• Aluminum melting
• Steel industry.

In addition, the content of the article and the presentations are excerpted from an upcoming booklet by the author, Arvind Thekdi, PhD, president of E3M Inc.

Dr. Arvind Thekdi has over 50 years of experience in the research and development and technical support in the areas of combustion, energy reduction and heat recovery in industrial heating systems. Dr. Thekdi started his company, E3M Inc., 20 years ago and has provided consulting services for the industries, US DOE and several utility companies including Energy Solutions Center (ESC). His focus is in improved design of process heating systems, waste energy recovery, emission control, application of combined heat and power (CHP), etc. He worked for three major furnace companies (Surface combustion, Lee-Wilson and Ipsen International) in the areas of R&D, engineering and marketing before starting E3M Inc. He has received 25 U.S. and foreign patents in subjects related to thermal systems.

 

The primary metals industry includes facilities that melt and refine metals from ores and/or scrap metal. These facilities receive primary metal sources such as iron ore for steel production, bauxite for aluminum production, metal scrap or an alternate metal source to produce molten metal, which is poured into molds to produce semi-finished shapes such as pigs or ingots, or solidified into slabs, billets or other near net shape products before it is further processed to produce plate, sheet, tubing, bar, rod, wire and other items.

Commonly used gas furnaces/ovens

The metals industry uses heating equipment known as furnaces, ovens, heaters, etc. to heat and melt a variety of materials such as steel, aluminum, copper, zinc, lead, magnesium and so on. This equipment may use fuel such as natural gas or fuel oil, or electricity as a source of heat.

The terms used for heating equipment such as a furnace and an oven are used interchangeably, particularly in a temperature range of about 800 to 1,400°F. It is based on operating temperature considerations, construction based, by a particular industry or even a plant tradition. In many cases a heating equipment operating below 1,000°F is known as an oven while the equipment operating above 1,000°F is known as a furnace. Many industries use their own terminology. For example, steel tempering equipment operating at 800°F is still called tempering furnace in heat treating shop where there are many other high temperature furnaces, while homogenizing equipment in aluminum plant may still be recognized as an oven. The chemical plant and petroleum refinery has their own terminology such as heater, reactor, etc.

In a batch furnace, the material is placed in the furnace chamber and is heated by following a certain time-temperature cycle while the load is in the furnace (see Figure 1). At the end of the desired time-temperature cycle, the load is removed from the furnace and transported to another piece of equipment such as a quench or cooling chamber. In some cases, the load is heated and cooled in the same chamber by using a cooling medium at the end of a heating cycle.

Only one set of burners are used, and the burner input is controlled by a temperature control system so the time-temperature requirement is met for the process. In a batch furnace, the heat requirement may change over a large range and the burner will respond accordingly. Here, the ratio of high firing rate to low firing rate, commonly known as the burner turndown is remarkably high.

In a continuous furnace, the material is placed or loaded directly on a material handling system such as a continuous belt or conveyor and is moved through the furnace to the discharge end (see Figure 1). While the load is moving through the furnace, the temperature of the furnace is controlled to a desired value at different locations. In furnaces, the temperature is varied in a certain length of the furnace and each length or volume associated with the length is known as a zone. For example, the furnace temperature may be 100°F at the entry location or zone and it is increased to a much higher value and held constant for one or two zones (soak zones). The load may be discharged from the soak zone or it may be cooled in subsequent zones of the furnace. Each zone temperature is controlled by firing one or more burners operated by a temperature control system.

Furnace functions and components

A typical furnace includes many functions and components for its construction and operation.

The heat generation system for gas fired furnaces includes the following components. For electrically heated furnaces, many of these components are not required and the system usually consists of an electrical system version of electricity supply and process-safety-specific components:

• Burners/heat sources
  • Gas fired burners
  • Radiant tubes
  • Infrared (IR) burners (Radiant, Catalytic, etc.).
• Combustion air supply
  • Air blower
  • Burner air supply control (valves, flowmeters, etc.)
  • Interlock equipment
  • Other components associated with process control.
• Natural gas (fuel) supply
  • Pressure regulators
  • Safety system such as shutoff valves, vent valves, etc.
  • Fuel flow control valves, etc.
  • Other components related to process control.
• Process and safety-specific components
  • Flame supervision system
  • Flue gas recirculating system
  • Oxygen injectors used for oxy-fuel or oxygen enriched air supply for combustion
  • Other process-specific components.

The burners are the most important part of the furnace and are selected based on process heat demand, type of operation (batch versus continuous), heat transfer requirement (convection versus radiation), combustion air temperature and required turndown (ratio of high fire and low fire condition heat input).

The two types of burners used in these furnaces are premix burners and nozzle mix burners (see Figure 2). In premix burners, gas and air are mixed before they enter the burner, and a flame retention device is used to stabilize the flame. In nozzle mix burners, air and gas enter the burner separately and are mixed within the burner before combustion of the mixture. These burners also use a flame stabilizer designed as part of the burner itself. Most modern industrial furnaces use nozzle mix burners.

Use of preheated combustion air

Use of preheated combustion air is perhaps the most used method of energy saving in industrial furnaces through heat recovery from exhaust gases. It is possible to save 5% to 30% energy in a furnace when heat from the furnace flue gases is used to preheat combustion air. Use of preheated air results in higher flame temperature, higher heat transfer and higher productivity. The general rule-of-thumb is for 100^°F increase in combustion air temperature, the flame temperature increases by 40^°F. It is a common belief that using preheated air results in higher amount of NO_X. The new generation of low-NO_X and ultra-low NO_X burners offer lower NO_X even with use of preheated air.

Industrial combustion control

The industry uses two definitions for defining efficiency: combustion efficiency and thermal efficiency. “Combustion efficiency,” also known as available heat, is how effectively the combustion process and the heating process is carried out in a furnace. Combustion efficiency indicates how much of the energy input leaves the furnace as flue gases. The remaining heat is distributed to meet heat demand within the furnace.

Combustion Efficiency (available heat) (%) = 100 x (1 – Heat content of flue gases/gross heat input)

“Thermal efficiency” indicates the percentage of heat input based on the gross heating value, which is approximately 1,000 Btu/standard cubic ft. (SCF) of natural gas in North America.

Thermal efficiency (%) = 100 x (Heat supplied to the load or charge/Gross heat input)

Thermal efficiency and combustion efficiency are interrelated. The following equation gives the relationship.

Thermal efficiency = combustion efficiency – (Heat loss from furnace/Gross heat input)

When thermal efficiency and available heat (combustion efficiency) are known, it is possible to calculate total furnace heat losses. In many cases, it is not possible to calculate total heat losses, and this is a simple method of doing so.

Furnace control systems

A furnace includes several different types of controls. Most used controls and their functions are as follows:

• A process control system supplies heat necessary to maintain the process temperature and other thermal conditions such as heat transfer to the material being processed.
• A burner fuel-air control system controls the amount of gas (fuel) and air to the burners to meet air-fuel ratio requirement and maintain required atmosphere in the furnace.
• A safety system that watches and controls all safety requirements such as flame supervision, overtemperature, etc., for safe furnace operation.
• A furnace pressure or draft control maintains the required pressure in the furnace.
• A furnace atmosphere control avoids explosive conditions and maintains process requirements in case of heat treating, curing of organic coatings and other similar special processes.
• Process specific, equipment specific or industry specific controls to meet special requirements.

These controls can be integrated in an intelligent computer or programmable logic controller (PLC)-based furnace control system.

The simplest form of process control for a gas fired furnace is temperature control of a zone for a single zone furnace or several zones in a multi zone furnace. The furnace temperature controller uses a temperature sensor such as a thermocouple connected to a temperature controller. The controller sends signal to the combustion control system to supply necessary heat or air-fuel supply to the furnace safely.

Safety is maintained by using a high temperature limit thermocouple together with appropriate components in the gas and air supply system sometime referred to as the gas and air train. The combustion control system is the heart of the thermal processing of the material being processed in the furnace.

Heat treating

The heat-treating industry includes thermal treatment (heating and cooling) of metal and nonmetal parts used in many industries such as automotive, construction machinery, general fabrication, etc. Heat treating is defined as controlled heating and cooling of materials to change their physical and sometimes chemical properties. Heat treating can be used to soften hard metal or to harden soft metal.

Heat treating is carried out for ferrous, non-ferrous and nonmetals including:

• Ferrous metals: steel, cast iron, alloys, stainless steel, tool steel, etc.
• Non-ferrous metals: aluminum, copper, brass, titanium, etc.
• Nonmetals: glass and ceramic materials.

However, steel accounts for about 80% of all the materials being heat treated.

Heat treatment process

The heat treatment process includes heating the material at a controlled temperature rise, holding the material temperature for a certain time, known as a soak period, followed by cooling at a controlled rate, which could be very quick as in a quenching operation (see Figure 3). The rate of heating or temperature rise within the part, soak time to equalize temperature within the part allowing the metallurgical transformations to take place and cooling rates are important in delivering the desired properties (hardness or softness) to the part.

In many processes, heating is carried out in presence of a special atmosphere or mixture of inert or reactive gases. Results of the heat-treating process for steel and other materials are affected by the following parameters:

• Amount, size, shape and form of carbon present in steel primarily determine its final properties.
• The furnace atmosphere such as inert gases (N₂ and other) or reactive (CO, H₂, CH₄ and other hydrocarbons in gaseous form) affect the surface (carbon and alloying elements) in the part.
• A controlled rate of heating and cooling (heat treatment) can change the shape, size and form of carbon in steel.
• The cooling rate after heating plays an important role in properties of the heat-treated material.
• Effects of heat treatment are often reversible, and properties of steel can be engineered through heat treatment.

Results of metal heat treating with specific compositions depends on the following parameters used during heating and cooling.

• Temperature, time and transformation (3Ts)
• Steel composition (transformation)
• Cooling temperature (temperature)
• Cooling rate (time).

Heat treatment processes of iron-carbon alloys are classified in to four categories: annealing, normalizing, hardening and tempering, and case hardening. The first two (annealing and normalizing) are used to impart softness to the parts, while the last two (hardening/tempering, and case hardening) are used to impart hardness to the parts either to the entire part or to selected section of the part.

Heat treat furnaces

Gas-fired furnaces designed for heat treating can be direct fired or indirect fired. In direct fired furnaces, the burners are fired directly in a furnace and the parts are heated while in contact with combustion products. Indirectly heated furnaces use radiant tubes or a muffle to isolate combustion products from the parts being heated. In this case, the parts are heated in a selected atmosphere.

Heat treating furnace atmospheres

Heat treating processes use different gas mixtures or “atmospheres” to protect the parts from oxidation or to add certain elements such as carbon or nitrogen that react with the base metal. The atmospheres can be classified in the following categories:

• Protective: To protect metal parts from oxidation or loss of carbon and other elements from the metal surfaces.
• Reactive: To add non-metallic (i.e., carbon, oxygen, nitrogen) or metallic (i.e., chromium, boron, vanadium) elements to the base metal.
• Purging (prevention of explosion): To remove air or flammable gases from furnaces or vessels.

Each of these atmosphere categories includes a mixture of gases such as hydrogen, carbon monoxide, methane, nitrogen or carbon dioxide. They can be generated by endothermic or exothermic reaction of natural gas and air, steam reforming process (natural gas and steam reaction) or ammonia dissociation that gives off hydrogen and nitrogen. The atmosphere also can be prepared by mixing commercially available gases mainly nitrogen, carbon monoxide, hydrogen and nitrogen.

Aluminum melting

The aluminum industry can be broadly divided into two categories: primary sector where aluminum is extracted from bauxite and, secondary sector where aluminum is produced using scrap collected from various sources. Both primary and secondary (recycled) aluminum are important manufactured products in the U.S. Primary aluminum is produced from Bauxite. It involves several steps including electrolysis of alumina to produce aluminum metal. Secondary aluminum production uses mostly recycled aluminum and some primary aluminum.

Aluminum melting furnaces use scrap or primary material as charge material. Temperature for charge material is usually close to ambient temperature in the range of 40 to 80^°F. In many cases, a scrap dryer and a preheater are used to remove moisture and organic materials before the scrap material is charged in a melting furnace. The material is heated to melting temperature, which is in the range of 1,160 to 1,210^°F depending on the alloy of aluminum. The molten liquid metal is super-heated before pouring to cast it in different shapes and sizes.

Major products shipped from aluminum plants using melt shops are ingots, sows, castings, plates, sheet coils, forgings, etc. These products are often remelted or are further processed by hot and cold rolling followed by heat treatment such as homogenizing and quenching, annealing, precipitation hardening or aging.

In North America, most of the secondary aluminum plants use gas-fired furnaces. There are many types of furnaces available and used for secondary aluminum plants. Many of these furnaces, particularly large furnaces, use heat recovery devices such as recuperators or regenerative burners to improve their thermal efficiency and energy intensity (Btu per pound of molten metal). However, use of recuperators require proper monitoring and maintenance to avoid catastrophic failure and production interruption.

During the past few years, use of regenerative burners is becoming more acceptable and many new furnaces, particularly those with near constant production level, are designed with use of regenerative burners. These systems also require scheduled maintenance. However, the possibility of catastrophic failure is much less. A few new developments have been attempted to reduce energy use, increase productivity and improve the furnace performance, but the success rate has been almost non-existent.

Scrap processing

Aluminum scrap processing uses two types of fired equipment: scrap dryer and thermal oxidizer. In most cases, they are integrated as one unit. However, for some facilities, only a scrap dyer is used and the fumes from the dryer are directed to the melter.

The secondary aluminum melting furnaces use different types of scrap as primary charge material to produce molten aluminum and products. Molten aluminum or cast products, commonly known as sows and ingots, also are used as required to achieve the required production and, in some cases, to get the required chemistry or composition of the metal.

Scrap for recycling is available in many forms. Light scrap such as used beverage containers is typically baled or briquetted to reduce transportation costs. These bales and briquettes are typically crushed, shredded or sheared and ripped to controlled flowable particle sizes for ease of charging in the furnace. A conveyor system and separation system are used to segregate particle fines for further processing.

Large volumes of aluminum scrap contain paint, enamel, lacquer or porcelain coatings, which would significantly reduce metal recovery if not removed before melting. Thermal treatment is used to remove the coatings and get clean metal for charging into the melting furnace. Some clean or noncontaminated light scrap is charged directly to the furnace hearth and is covered by additional heavier charge components. In-plant production scrap is sized to handle and conveyed to the melting furnace, usually without any pretreatment.

Steel industry

The two major routes for steel making are the use of blast furnaces to produce pig iron with the basic oxygen furnace (BOF) to produce steel in integrated mills, and the use of electric arc furnaces (EAFs) to produce steel from steel scrap and other raw materials such as direct reduced iron (DRI) or hot metal in mini mills (see Figure 4).

Figure 5: Areas of natural gas use in steel making processes. Courtesy: Arvind Thekdi, PhD, E3M Inc.[/caption]

It is likely the use of natural gas will increase in the steel industry due to the trend toward increased use of natural gas in EAFs to reduce use of electrical energy and higher productivity through pre-melting of scrap by using oxy-fuel (natural gas) burners.

– This article appeared in the Gas Technology supplement.

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