A cold-blast cupola typically consumes about 10–15% of the metal charge as coke, or approximately 100–150 kg of coke per tonne of iron melted [S8].
The figure is an operating benchmark, not a universal design ratio: furnace efficiency, blast temperature, coke quality, burden composition and required tapping temperature can move actual consumption away from that range [S2][S3].
What coke rate means at the furnace floor
Coke consumption is normally expressed against metal charged, metal melted or hot metal tapped, so the denominator must be fixed before comparing a foundry with another operation. A 10–15% coke rate means 100–150 kg for every 1,000 kg of metal charge; against 1,000 kg of liquid iron tapped, the mass relationship remains approximately 100–150 kg of coke, subject to inventory and yield differences [S8].
The basic cupola furnace generates heat by burning coke around descending metal and slag-forming additives. Carbon combustion releases 33,260 kJ per kilogram of carbon for the reaction C plus O₂ forming CO₂, while the reduction of CO₂ with carbon to form CO consumes 14,009 kJ per kilogram of carbon [S2].
Not all coke carbon reaches the intended combustion reaction. Typical cupola carbon pickup is reported at 5–18%, with only 82–95% of coke used in combustion, although the exact balance depends on the furnace and burden [S3]. Carbon dissolved into iron is desirable to a point, but excessive carburization or carbon carried into the charge reduces fuel efficiency and can affect casting chemistry.
Cold blast versus hot blast
A cold-blast cupola blows ambient air into the combustion zone and generally handles melting rates of about 2–10 t/h [S2]. This arrangement offers low capital cost and straightforward operation, but carbon monoxide leaves with the exhaust gas, increasing specific coke use and limiting the ability to reach high tapping temperatures without sacrificing melting rate [S2].
A hot-blast cupola recovers heat from CO-rich exhaust gas and uses it to preheat the blast to approximately 400–600°C [S2]. The preheated blast supports more scrap in the burden, improves control of spout temperature and narrows variation in tapped-iron composition, while reducing the fuel penalty associated with cold-blast operation [S2].
The comparison is straightforward: cold blast favors simple construction and low capital cost, while hot blast favors thermal efficiency, temperature control and fuel reduction. A gas-assisted arrangement can also displace part of the coke because the burner supplies heat directly, but the result depends on gas price, burner efficiency and the operating point of the coke bed [S1].
Rate, temperature and burden effects
Coke consumption decreased by 22%, with no reduction in metal tapping temperature, while the melting rate increased from 26 to 36 t/h, approximately 38% [S5]. That result demonstrates why coke consumption should be normalized to tonnes of hot metal rather than compared as a raw daily fuel total.
Charge composition changes the correct coke rate because pig iron, steel scrap, returns and foundry iron differ in carbon content, density and oxidation state. A dirty, mixed, low-density scrap burden can generally be processed more readily by a cupola than by an electric furnace, although zinc-coated material and excessive contamination can still increase slag, oxidation losses and refractory wear [S3].
Flux additions also affect the operating result. Limestone is charged with the burden to form slag, while the slag must be controlled because its quantity, viscosity and chemistry affect heat transfer, metal loss and the working condition of the furnace lining. Material and charge records should therefore accompany every coke-rate calculation.
Useful performance benchmarks beyond coke mass
TPK, defined as tonnes of hot metal per hour for each 1,000 standard cubic feet of equivalent blast rate, evaluates more than coke consumption alone [S3]. It incorporates fuel use, thermal efficiency, sensible heat loss, Boudouard reactions and slag generation, making it a better operational comparison than kilograms of coke per tonne [S3].
Reported U.S. TPK ranges are 2.3–2.8 for low-performance operation, 2.8–3.3 for medium performance and above 3.3 for high-performance operation [S3]. The low range is associated with factors such as an unlined shell, cold blast, a short stack or poor charging practice, while the higher range is associated with a refractory-lined furnace, a tall stack, hot blast and tightly controlled charging [S3].
Furnace efficiency of about 60% is typical for the cold-blast design described in the foundry reference, but that figure should not be transferred directly to every furnace [S2]. The same reference notes that only about 40% of the energy produced by coke combustion is devoted to metal melting and overheating beyond the melting point, with the remainder represented by other thermal and chemical losses [S4].
Options for lowering coke intensity
Hot-blast recuperation is the strongest conventional improvement where exhaust gas can be recovered below the throat and used to preheat the blast [S2]. A cold-blast installation cannot apply that arrangement effectively because extraction above the throat admits false air during charging, which reduces the recoverable exhaust-gas energy [S2].
Secondary air above the melting zone and oxygen injection can reduce energy losses, while the holding furnace can buffer temperature and chemistry after melting without adding coke to the cupola charge. These measures address different stages: recuperation raises useful combustion heat, oxygen improves local combustion, and the holding furnace provides process control downstream of the melt.
For a foundry replacing or supplementing a cupola, an induction furnace offers electrical melting and tighter temperature control, but it changes the raw-material and electricity-cost case rather than merely reducing the coke rate of the existing furnace. A comparison of cold-blast and hot-blast cupola operation is therefore more relevant than transferring an induction-furnace energy value to cupola accounting.
What to record for an auditable coke balance
A defensible benchmark requires at least four measurements: coke mass charged, metal mass charged, hot-metal mass tapped and the analysis of tapped iron [S1][S2]. The calculation should also distinguish purchased coke from other carbon-bearing additions, since carbon in anodes, carburizers or fuel injectants changes the effective carbon rate [S1].
For each campaign, retain the average coke-bed height, blast volume, blast temperature, equivalent blast rate, melting rate and tapping temperature. The Bureau of Mines evaluation used fuel and air combinations to measure coke rate, coke-bed height, energy input and melting rate, showing why a single worksheet cannot explain differences between furnaces [S1].
The operating target should be stated as kilograms of coke per tonne of hot metal, with separate limits for iron chemistry and spout temperature. If coke use rises above about 150 kg/t without a required increase in tapping temperature, examine blast temperature, combustion air distribution, burden oxidation, coke quality, charge sizing and slag control. If coke use falls below about 100 kg/t, verify that the charge has enough carbon and that measured hot-metal output has not been overstated.
The trackable signal is a three-month rolling coke rate paired with TPK, tapping temperature and carbon pickup. A sustained reduction in kilograms of coke per tonne, combined with stable chemistry and higher TPK, demonstrates fuel improvement rather than a transfer of heat demand downstream.