Furnace charge preheating, Is it a “necessary” или “useless” tool?

In the electric arc furnace (ЭДП) steelmaking and foundry industries, “предварительный нагрев лома” has always been a hot topic. Some hail it as the ultimate tool for energy saving and cost reduction, while others criticize it as an overrated space-hogger with high oxidation losses—essentially turning an electricity hog into a gas hog.

Is a scrap preheating system (whether utilizing natural gas or induction furnace off-gas) truly essential or just a gimmick? Let’s skip the fluff and break down the numbers and underlying logic.

1. The Core Ledger: How Much Power Does Preheating Actually Save?

We can calculate theoretical energy savings using thermodynamic formulas.

Suppose we preheat 1 тонна (1000 кг) of scrap steel from a room temperature of 20°C to 500°C:

Theoretical Power Savings Calculation

The average specific heat capacity ( C_p ) of steel in the range of 20°C to 500°C примерно 0.56 kJ/(kg · ℃). The theoretical heat energy ( вопрос ) required for preheating is:

Q = m · C_p · ΔT

Q = 1000 kg x 0.56 kJ/(kg · ℃) х (500℃ – 20℃) = 268,800 кДж

Converting heat energy into electrical energy (где 1 kWh = 3600 кДж):

E_theory = 268,800 / 3600 ≈ 74.7 кВтч

Considering that the comprehensive electrical-to-thermal efficiency (или) of an induction furnace is generally between 75% и 80%, the actual grid electricity saved during the melting period is:

E_actual = 74.7 / 0.75 ≈ 99.6 kWh/t

Real-World Production Adjustment: Accounting for heat loss during the transfer of scrap from the preheating section into the furnace, effectively preheating the scrap to 500° C. reduces actual melting power consumption by 70 к 90 кВтч per ton. For a plant with an annual output of 50,000 тонны, the electricity savings alone can easily amount to hundreds of thousands of dollars.

2. Pros and Cons of Mainstream Preheating Systems

В настоящее время, the market is dominated by two main preheating methods: Natural Gas/Coal Gas Preheating и Furnace Off-Gas Waste Heat Recovery.

2.1 Natural Gas Preheating: Trading Gas for Electricity

• Плюс: High energy density, fast heating speed, and excellent temperature control (easily reaching 300°C to 600°C). It completely burns off moisture and oil on the scrap surface, eliminating the risk of splashing or explosions caused by wet, cold material entering the furnace.
• Минусы: It is a form of “energy conversion,” not a free lunch. If local gas prices are high and electricity prices are low (например, during off-peak night hours), the economics of “trading gas for electricity” drop significantly.

2.2 Off-Gas Waste Heat Recovery: The Theoretical “Free Lunch”

• Плюс: Utilizes the high-temperature flue gas emitted by the индукционная печь (typically 200°C to 400°C), making the operating cost extremely low.
• Минусы: Induction furnaces operate in batches, meaning off-gas temperature and flow rate fluctuate wildly and are highly unstable. The off-gas contains dust, which easily clogs preheating pipes and heat exchangers, leading to high maintenance costs. The preheating temperature usually only reaches 150°C to 250°C, yielding limited power savings (о 20 к 40 kWh/t).

3. Why Do Some Call It a Gimmick? (The Unignorable Hidden Costs)

The reason preheating systems have not achieved 100% adoption is that they hit several pain points for manufacturers:

• Oxidation and Burn Loss (The Ultimate Pain Point): Steel exposed to air at temperatures above 400°C oxidizes rapidly. If the atmosphere inside the preheater is poorly controlled (high oxygen levels) or the preheating time is too long, a thick oxide scale forms on the scrap surface. Каждый 0.5% increase in metal burn loss can completely wipe out the financial gains from electricity savings.
• ВторостепенныйОтносящийся к окружающей среде Загрязнение: A lot of cheap scrap contains cutting fluids, краска, or plastics. В “half-baked” temperatures between 200°C and 400°C, these oils and impurities turn into thick smoke containing heavy VOCs or even dioxins. Without investing in expensive secondary combustion chambers or deodorizing dust collectors, passing environmental inspections is virtually impossible.
• Footprint and Capital Investment (Капвложения): Preheating bins, conveyor tracks, flue gas ductwork, and matching charging cars require significant crane bay space. For brownfield retrofits, plants often face the awkward reality of having zero space to spare.

4. Комплексное сравнение: Which One to Choose?

Заключение: “Must-Have” или “Overrated Gimmick”?

Scrap preheating is definitely not a gimmick, but it is a double-edged sword that tests a plant’s management prowess.

It is a 【Must-Have Super Weapon】 if you meet the following conditions:

1. Емкость Bottlenecks: Your transformer capacity has maxed out, and you need to shorten melting cycles to boost throughput (preheating acts as a proxy for expanding transformer power).
2. Бедный & Varying Scrap Quality: Scrap is stored outdoors long-term or gets heavily soaked during rainy seasons. To guarantee absolute operational safety, preheating is required to drive off moisture.
3. High Electricity Rates or Steep Peak-to-Valley Spreads: In regions with high power costs, the ROI of swapping gas or waste heat for electricity is incredibly high.

Однако, it can easily turn into a 【White Elephant】 if:

1. Floor Space is at a Premium: Forcing the system into a tight layout disrupts logistics and chokes crane efficiency, costing more than it saves.
2. Focusing on High-Grade Specialty Steels/Точность Кастинг: Your process is highly sensitive to raw material oxidation and cannot tolerate even a 0.5% surface burn loss or chemical composition fluctuations.
3. Operating in an Environmentally Hyper-Sensitive Zone: The plant cannot absorb the additional emission-treatment costs triggered by the burning of oily scrap residues during preheating.