Waste heat recovery systems

More than half of the energy consumed globally is lost as heat. Most industrial processes generate large amounts of heat. The energy produced by industrial processes but not consumed, wasted, or released into the environment is considered industrial waste heat. This includes heat loss through conduction, convection, and radiation from industrial products, machinery, and thermal processes. Recovering this wasted heat can increase system efficiency, reduce fuel consumption, and lower CO₂ emissions. Estimates show that implementation of energy efficient processes in energy intensive industries can reduce global carbon emissions by 44 % in 2035. The most common carriers of residual waste heat are gaseous streams (e.g., low-quality steam, cooling air, exhaust gas, flaring gas, etc.), liquid streams (e.g., hot oil, cooling water, etc.) and solids (e.g., commodities and products, such as hot steel) with a broad temperature range of 50 °C − 1000 °C or higher.

The iron and steel industry is one of the industrial sector's largest energy consumers. The production of steel requires large amount of electricity, natural gas, coal and other energy sources. In 2018, every ton of steel produced around the world emitted on average approximately 1.85 tons of CO₂ in the atmosphere, which equates to about eight percent of the overall global CO₂ emissions. Tata Steel Netherlands alone accounts for 7% of the national CO₂ emissions. Steel production wastes considerable amount of heat which is released into the atmosphere. Waste recovery systems can thus potentially increase energy efficiency in the iron and steel industry. Waste heat from the iron and steel industry can be used to provide heating, cooling, and generate electricity. Waste heat recovery solutions has been gaining popularity for several steel making processes and numerous waste heat recovery plants were built over the last ten years. The utilisation of the energy from the off gases in the steel industry has attracted more and more attention over the last years.

The development of a waste heat recovery plant requires extensive knowledge of water-steam cycle as well as EAF process, dedusting system and downstream waste heat consumers. Large amounts of waste heat can be recovered from Electric Arc Furnace (EAF) and from Basic Oxygen Furnace (BOF). Other potential waste heat sources include Sinter Coolers and Reheating furnaces. Designing a waste heat recovery plant should also explore options for the utilization of the recovered energy, as it strongly influences the economic feasibility of the plant.

Waste heat recovery potential in Iron & Steel industry

The steel industry has the second largest heat recovery potential followed by oil & gas industry. The iron and steel industry mainly adopts two production routes: (1) Primary/Blast Furnace-Basic Oxygen Furnace (BF-BOF) route, where iron ores and scrap are used as the raw materials and accounts for 71 % of global steel production, (2) Secondary/Electric Arc Furnace (EAF) route, where steel is produced from recycled steel scrap and/or direct reduced iron which accounts for the remaining 29 % of global steel production.

Waste heat sources in an iron steel industry includes the following:

1. Hot off-gases
2. Cooling water
3. Hot intermediate products like slabs, billets, etc.
4. Hot slags

Waste heat sources in Steel industry (Source: P. Plission, et.al., 2016)

The waste heat is generally categorized into low-temperature (<200 °C), medium-temperature (200–500 °C) and high-temperature (>500 °C) waste heat. The recovery technologies for the medium and high temperature waste heat are well evolved, e.g., the heat capture of medium-temperature (350 °C) exhaust gasses, from a kiln hood clinker cooler and kiln tail preheater, using a boiler, and Coke Dry Quenching (CDQ) technology for the recovery of high-temperature (1000 °C) heat of hot coke. However, the number of efficient technologies to recover heat from low-temperature waste heat is limited. The possible high grade heat sources in the steel industry include flue gas such as Coke Oven Gas, converter gas, Electric Furnace Gas and heating furnace flue gas. High temperature liquid in steel industry include high temperature iron slag, steel slag and high temperature water, while high temperature solid waste heat includes high temperature sintering materials, high temperature coke, high temperature steel, etc. The possible medium grade heat sources include Blast Furnace gas, Sintering flue gas, and exhaust gas recovery of waste heat from the primary after flue gas. Potential low grade waste heat sources include waste steam, hot water and all kinds of low temperature flue gas and low temperature materials.

In electric arc furnaces, the waste heat is estimated to get up to one-third of the total energy supplied to the process.

Types of Waste Heat Recovery systems

WHR systems can be broadly classified into two main categories:

• Steam Control: This method does not require thermal energy buffer to manage the heat fluctuations. It uses valves to reduce mass heat stream fluctuations. The technical options to manage the fluctuations using Steam Control method include Heat source by-pass, Heat source dilution, and Working with fluid flow control.
• Thermal Energy Storage: Thermal Energy Storage has been developed and used more efficiently in handling the fluctuations of thermal energy. The technical options of TES are based on either Sensible Heat Storage (SHS), or Latent Heat Storage (LHS). SHS options include hot water tanks, and molten salts, while LHS technical options include steam accumulators and Phase Change Materials (PCMs). PCMs most commonly used are paraffin, salt hydrates, and fatty acids. LHS with PCMs applications have been developed as buffer in the thermal power fluctuations in the EAF off-gases.

Typical consumer for waste heat recovery plants are district heating, chilled water production, electric power generation or steam generation for various consumers inside and outside the plant.

Waste Heat Recovery Systems

Some of the waste hear recovery systems employed in the iron and steel industry includes but are not limited to the following:

Coke Dry Quenching (CDQ)

In this method heat is recovered by generating steam in the process of quenching and cooling red-hot cokes by using inactive gas, CO₂ , N₂, etc. Steam may be generated about 0.45 ton per hour per ton of coke to be quenched. In this system, hot coke removed from coke ovens at a temperature of approximately 1,000 °C is cooled and kept dry with inert gas and the resulting steam produced in a waste heat recovery boiler is used to generate electricity. As the sensible heat recovered by heat transfer in the cooling chamber is utilized as a heat source for steam generation, electricity generated by CDQ is clean, environmentally-friendly energy. In addition, compared to the conventional wet quenching type, CDQ brings about advantages such as the reduction of dust emissions and improvement of coke quality. In 1976, Nippon Steel constructed the first CDQ plant at its Yawata Works with a capacity of 56 t/h.

Top pressure Recovery Turbine (TRT)

A Top pressure Recovery Turbine generates electric power by employing the heat and pressure of blast furnace top gas to drive a turbine generator. Blast furnace gas (BFG) with a pressure of 2 to 2.5 kg/cm²g, can drive a gas turbine generator after being cleaned from dust. Blast-furnace gas of 4.2 X 10⁵ Nm³/h can generate about 9000 kW electricity and the exhaust gas from the gas turbine is available as heat energy source. After the blast furnace gas is used in power generation, it is used as a fuel in iron and steel manufacturing processes. TRT comprises of dust collecting equipment, a gas turbine, and a generator. Generating methods may be wet or dry depending on the BFG purification method. Dust is removed by Venturi scrubbers in the wet method and by a dry-type dust collector in the dry method. When dust is treated by the dry method, the gas temperature drop is small in comparison with the wet method, and as a result, generated output is at maximum 1.6 times greater than with the wet method.

Slab Cooling Boiler

The heat energy radiated from half-finished red hot goods, e.g., steel slabs after blooming, is available for steam generation during cooling by water tube walls of a boiler which the red-hot goods pass through. CO₂ and H₂ gas as well as steam are available for recovering heat.

Skid Boiler

Heat can be recovered from steam generated by cooling water that pass through skid pipes installed for supporting steel slabs inside the heating furnace. Heat to be absorbed by cooling water is about 8% of heat necessary for heating steel, i.e., 400 X 103 kcal/t of steel.

Converter gas recovery

Converter gas or Basic Oxygen Furnace Gas (BOFG) is a by-product of the steel manufacturing process in the primary steel production process. It is generated during the oxidation of pig iron to steel in the oxygen converter. Crude BOFG contains approximately 56–70% CO, 13–20% N₂, 15–21% CO₂, and small amounts of 1–4% H₂. It is a low calorific gas with a calorific value between 8.0 and 9.0 MJ/Nm³. As a low calorific gas, it features a relatively slow combustion rate. The gas finds application as a fuel for a tunnel furnace that heats up steel sheets either for a hot rolling mill, steel hardening, or both, in Jenbacher gas engines, etc.

Exhaust Gas from Heating Furnace, Soaking Pit and Sintering Machine

Heat of exhaust gas at 600 °C discharged from heating furnaces and soaking pits can be recovered by using exhaust gas boilers. Taking a heating furnace for example, 27,800 Nm³/h of exhaust gas can generate 52.3 t of saturated steam, 14 kg/cm²g pressure, per hour. If steam of pressure 41 kg/cm²g can be produced, the steam can generate 2,320 kW of electricity and 60.1 t/h of 14 kg/cm²g saturated steam.

Organic Rankine Cycle (ORC)

The working principle of an ORC is similar to operational principle of a conventional steam Rankine cycle, with an organic fluid with lower boiling temperature being used as working fluid in an ORC instead of steam. ORC shows great flexibility in using of moderate temperature heat source. An ORC is composed of a fluid pump, heat recovery system (HRS), turbine and the condenser. The HRS transfers heat from the waste heat source into the working fluid.

Barriers for Waste heat recovery systems implementation

The major barriers of the widespread implementation of heat recovery systems to achieve optimum economic performance, efficient waste heat recovery and practical feasibility are as follows:

• Lack of information and knowledge
• Lack of optimization approach for the network of heat conversation
• Space limitation for installation of waste heat recovery equipment
• Primary funding and operational expenses
• Inconsistency between the customer demand and source of waste heat on the energy level, time and the space
• Risks of the technological change to the industrial processes
• Inaccessibility
• Temperature and chemical constraints
• Long payback