Can Oxygen Enrichment Boost Cupola Furnace Production?

Oxygen enrichment has the potential to enhance cupola output and lower melting costs, but other developments in cupolas through the years also can stake a claim to increased productivity.

Oxygen (O) enrichment is a versatile tool that provides important benefits for cupola operation. The benefits, reported for a wide range of foundries, are listed in Table 1 and have been recognized for more than 70 years. Not until the mid-1960s, however, was the cost of O low enough to make its use cost effective. For the most part, foundries have pursued O benefits because of economic factors. The two major factors for O use are the replacement of costly charge materials with less expensive ones and the ability of low capital means to increase cupola output. Thus far, environmental factors, such as reduced greenhouse gas emissions via reduced coke consumption, have not figured as strong incentives for foundries to further increase the use of O. This situation might change in the future.

How Oxygen Enriches

All the benefits from O enrichment derive from the fact that adding O to the blast air is the same as increasing the blast air rate, without adding extra nitrogen. Because less nitrogen is in the gas phase, the heat produced from combustion of coke increases the temperature of the gas. In turn, that increase transfers heat to the iron drops as they pass through the melt zone. Higher iron temperatures create the conditions that permit improvements in a cupola operation.

The methods used to introduce additional O into the cupola are classified into three categories: diffuser enrichment, tuyere injection and well injection. In the diffuser method, O is introduced upstream of the tuyeres through a diffuser installed in the windbox. With tuyere injection, injector tubes direct pure O through each tuyere (or every other tuyere). The third and least accepted method is well injection in which O is injected into the coke bed beneath the tuyeres using water-cooled injectors.

Modes of Enrichment

All the observed benefits of O enrichment cannot be realized simultaneously. Four basic modes of O enrichment define the nature of the benefits that are obtainable. These are:

* add O;

* add O and reduce coke;

* add O and reduce blast air;

* add O and reduce blast air and coke.

Table 2 relates the benefits found in Table 1 with each of the modes listed above. The most important benefits are highlighted in red. Salient comments concerning the benefits and needed precautions are given below.

1 Add O--This mode produces the highest iron temperatures and largest gains in carbon (C) and silicon (Si) content. It also is useful for melting at rates that exceed the capacity of the air blast system.

2 Add O and reduce coke--This mode produces maximum melt rates as it provides the highest O-to-coke ratio. The iron temperature and alloy recovery are lower than in the previous case. The amount of coke that can be removed from the charge in this mode is limited. Although O raises the temperature in the melt zone, the combustion of C supplies the energy for melting. In a perfect system, with complete combustion to carbon dioxide and no energy losses, about 5% coke is needed. Realistic values are much higher. As the amount of charged coke is reduced, the cupola atmosphere becomes more oxidizing and increasingly larger amounts of the alloy are oxidized. Eventually, the costs of the C and Si losses weigh heavily on the cost of increasing the melt rate.

Add O and reduce blast air--This mode of operation is useful when holding a constant melt rate is necessary. A benefit of reduced air volumes is a reduction on the load placed on the hot blast and emission systems. Through its studies, the British Cast Iron Research Assn. (BCIRA) established that, at a given coke level, the total O mainly determines the melt rate. Thus, replacing free air with costly O must be accompanied by an application that provides sufficient savings to make the replacement attractive. If considerable air is removed, attention needs to be given to the consequence of poorer blast penetration and reduced energy production due to the generation of greater amounts of CO.

Add O and reduce blast air and coke-Coke rates can be reduced as O is added to maintain a constant iron temperature, which is possible because of the higher combustion zone gas temperatures when the blast is enriched with O. Depending on the relative amount of blast air and coke removed, the melt rate can either increase or decrease.

Early Oxygen Delivery

Early in the application of continuous O enrichment, gas generally was introduced to the blast main. The BCIRA conducted extensive studies in the 1960s and 1970s that indicated the benefits were small compared to the introduction of the same amount of O through each tuyere (tuyere injection). This manner of introduction had the benefit of eliminating O losses due to leaks in the blast main and tuyere plumbing. More importantly, a high-velocity, injected-O stream was able to penetrate deeper into the cupola than the blast air, keeping combustion away from the cupola wall and reducing energy losses. In keeping with this philosophy, O producers developed supersonic O injectors in the 1980s as a way to further accomplish blast penetration.

In the 1960s and 1970s, when tuyere injection became popular, many cupolas used large diameter tuyeres with unheated blast air, so the velocity of the blast entering the cupola was low. In addition, the tuyeres were flush with the cupola walls, which allowed for the low-velocity gases to easily burn close to the wall, resulting in high heat losses and poor combustion efficiency. Under these conditions, high-velocity injection of O was effective in assisting to make the improvements outlined in Table 1 because a high-velocity stream can penetrate deeper into a cupola than the low-velocity blast.

Recent Improvements

Today, many foundries have taken steps to prevent such losses. Tuyere diameters have been reduced to increase the gas velocity entering the cupola, and water-cooled protruding tuyeres have been installed so the blast air is not deposited at the cupola wall. Currently, cupola manufacturers are recommending tuyere velocities about equal to those provided by subsonic tuyere injection. In these cases, the necessity for separate injection of 0 and air remains unclear.

As a case in point, identical cupola performance, except for small differences in iron temperature, was obtained in a General Motors' (GM) study in which the same 0 rate was delivered as unenriched-air blast, 0 enriched-air and lance-injected 0 to a 60-in. (152 cm) hot-blast cupola with protruding tuyeres (Table 3). These results do not conform to the benefits described in Table 1. The failure to substantially increase iron temperature, C and Si may be due to the use of much higher air blast velocities in the GM test than were used in the 1960s and 1970s cupolas (whose performance with 0 form the basis for the benefits described in Table 1).

To illustrate the differences in cupola operations (then and now) Table 4 provides velocity data from the GM study and a BCIRA study in 1977. The latter is the most authoritative study on O enrichment available. As recorded, the air blast velocity in the GM study was seven times higher than that employed in the BCIRA study. In addition, the low velocity condition was exacerbated by the use of flush tuyeres. By contrast, the velocities of the injected o in the two studies were almost the same. Logically, the low velocity operation could benefit greatly from the higher velocity 0 injection, while the higher velocity operation did not require it. The comparison suggests that the benefits attributed to improved penetration of 0 by injection for today's cupolas that operate with high air velocities and protruding tuyeres need to be re-evaluated.

The 1990s saw the introduction of still another blast air innovation, metered air to each tuyere. The devices maintain the same airflow to each tuyere, despite obstructions such as partial blocking of the tuyere by coke or slag. The concept is to create conditions for uniform combustion by providing a uniform flow of 0. No reports have been issued describing any benefits obtained.

Oxygen enrichment is a versatile tool with the potential to benefit cupola operations through its effects on melt rates and the ability to reduce nitrogen in the blast. However, some of the benefits frequently attributed to 0 enrichment may already have been achieved in modern cupolas using small-diameter, water-cooled protruding tuyeres. Further studies will be required to separate the benefits that each provides.

Table 2. Operational Benefits for the Four
Modes of Oxygen (O) Enrichmen
               Mode                  Benefits (listed in Table 1)
               Add O                       1,2,3,4,6,10,11
       Add O and reduce coke                     1,5
    Add O and reduce blast air             2,3,4,6,7,8,9,10
Add O and reduce blast air and coke            5,7,8,9
Table 3. Experimental Data from GM
Tests on Oxygen (O) Enrichment of
Cupolas
       Property         Unenriched     Enriched air     Enriched air
                        air blast   (tuyere injection)  (blast main)
      Blast air
 (cu-ft/min/cu-m/hr)    3078/5226       2557/4342        2608/4428
O (cu-ft/min/cu-m/hr)       0            117/199          113/192
   O enrichment (%)         0              3.5              3.3
      Melt rate
 (tons/hr/tonnes/hr)    9.23/8.37       9.10/8.25        9.07/8.23
Iron temperature (F/C)  2766/1519       2770/1521        2790/1532
      Carbon (%)           2.14            2.13             2.14
     Silicon (%)           3.01            2.96             3.02
Table 4. A Comparison of Gas (Air
and Oxygen) Velocities in Oxygen
(O) Enrichment Experiments
  Type of O addition             GM data
                         Velocity (ft/sec/ m/sec)
     Uneriched air               255/77.7
Blast main enriched air          216/65.8
   Tuyere-injected O             400/122
  Type of O addition            BCIRA data
                         Velocity (ft/sec/ m/sec)
     Uneriched air              38.6/11.7
Blast main enriched air         30.8/9.3
   Tuyere-injected O             394/120

Table 1. Benefits of Using Oxygen Enrichment in a Cupola Operation

* 1. Increased melt rate.

* 2. Increased iron temperature.

* 3. Increased alloy recovery.

* 4. Increased carbon pickup.

* 5. Decreased coke usage.

* 6. Increased use of difficult-to-melt scrap (steel, borings, bundles, etc.).

* 7. Reduced hot blast needs (lower use of natural gas).

* 8. Reduced blast volume.

* 9. Reduced heat losses.

* 10. More rapid cupola start-up.

* 11. Intermittent use for steady cupola control (rapid response to rectify chemistry and temperature problems).