Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
2001-10-03
2003-07-15
Andrews, Melvyn (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S958000, C266S156000
Reexamination Certificate
active
06592646
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of producing direct reduced iron with a gas obtained by coal gasification, and more particularly to a direct reduced iron producing method provided with steps of gasifying coal to a coal-derived gas containing a reducing gas, and reducing iron ore by the reducing gas to produce reduced iron.
2. Description of the Related Art
Production of reduced iron has been widespread because a plant for direct reduction of iron ore can be built at relatively low cost and easily operated. In addition, such a production can be economically practical even with a small-scaled plant. It is a common practice of using natural gas as a fuel (also as a reducing agent) for the production. Specifically, it has been a customary practice to reform natural gas into synthesis gas by H
2
O or CO
2
so as to reduce iron ore by the synthesis gas.
Synthesis gas having substantially the same ingredients as the above synthesis gas can be produced by gasifying coal in a furnace designed for coal gasification. Synthesis gas produced by gasifying coal (hereinafter, referred to as “coal-derived gas”) contains reducing gas consisting of CO and H
2
as main ingredients, and CO
2
, H
2
O, H
2
S, etc. as sub ingredients.
Since the reduced iron production consumes a large volume of fuel, it is often the case that plants for reduction of iron ore are built close to fields of fuel (around gas fields). However, potential demands for producing reduced iron by using coal as a fuel (also as a reducing agent) instead of natural gas cannot be neglected especially in a region where inexpensive natural gas is hard to obtain but abundant coal is available. Particularly, producing direct reduced iron with use of coal-derived gas is regarded as a most practical technology because each process thereof is performed in a satisfactorily sophisticated manner. As a matter of fact, constructing parties and builders of plants for producing reduced iron researched processes of producing direct reduced iron with use of coal-derived gas. The results of their research were disclosed in T. A. Lepinski, M. R. Jones, Iron and Steel Engineer, Oct. 1982, pp. 23-28, P. E. Duarte, E. O. Gerstbrein, H. Smegal, Proceedings, AIC Conferences 3rd Annual Asian Steel Summit, 1997. This fact reveals that interest is increasing in this technical field.
However, a commercial plant aiming at producing direct reduced iron with use of coal-derived gas has not yet been put into practice. This is because building such a plant involves economical problems since invariable cost (fixed cost) such as construction cost for a coal gasification furnace and peripheral facilities is high. The fixed cost has not been successfully suppressed because each of the production processes are sophisticated and cannot be further simplified. Therefore, in order to lead this technology to a commercial success, required is an idea of suppressing variable cost by (a) reducing fuel cost due to improvement of heat efficiency or (b) utilizing inexpensive coal resources which has not been available in the conventional technology.
Considering (a) improvement of heat efficiency in producing direct reduced iron with use of coal-derived gas, the most important matter is how to utilize waste heat resulting from (A) coal-derived gas and (B) exhaust gas emitted from top part of a furnace for reducing iron ore (hereinafter, simply referred to as “top gas”). Waste heat obtained from coal-derived gas (A) and top gas (B) each amounts to 200 to 400 kcal per kg of reduced iron (namely, in terms of calorie per 1 kg of reduced iron product, 200 to 400kcal=200×4.18605 to 400×4.18605 kJ=836 to 1672 kJ). The sum of waste heating value of coal-derived gas (A) and top gas (B) occupies about 20% with respect to the sum of the theoretical heating value requirement for producing the reduced iron and the waste heating value throughout the production processes.
It is desirable to perform hot feeding of coal-derived gas (feeding coal-derived gas to a predetermined facility such as a furnace for reducing iron ore at a sufficiently high temperature without being cooled to an atmospheric temperature) in order to most efficiently utilize waste heat obtained from coal-derived gas (A). On the other hand, taking into account an adverse affect that sulfur compounds such as H
2
S contained in coal-derived gas may impart to quality of resultant reduced iron product, it is desirable to desulfurize coal-derived gas. In view of these, it is desirable to perform hot desulfurization with respect to coal-derived gas (namely, desulfurizing coal-derived gas at a sufficiently high temperature suitable for desulfurization without cooling the gas to an atmospheric temperature).
As to the idea of how to utilize waste heat derived from top gas (B), it is required to fabricate a novel heat recovery system for the top gas (B) having relatively low pressure and temperature.
The following technology has been provided with respect to hot feeding and hot desulfurization of coal-derived gas (A). Specifically, U.S. Pat. No. 4,260,412 proposes an idea of obtaining coal-derived gas in a fluidized bed gasification furnace internally equipped with a desulfurizer and performing hot feeding of gas to an iron ore reducing furnace by way of a reheating furnace. U.S. Pat. No. 4,173,465 does not disclose a specific type of gasification furnace but suggests a process of hot desulfurizing coal-derived gas (desulfurization out of a furnace) on a movable bed of limestone. In any case, mixing coal-derived gas with top gas (B) which has been cleaned and cooled (hereinafter, referred to as “recirculating gas”) enables to lower the temperature of the coal-derived gas to a suitable level for hot desulfurization. This is conceived one of the effective and economical techniques of utilizing waste heat derived from coal-derived gas from the viewpoint of heat balance because sensible heat resulting from coal-derived gas is directly utilized in the process.
However, neither U.S. Pat. Nos. 4,260,412 nor 4,173,465 discloses the idea of utilizing top gas (B).
Top gas (B) has such a large fluid rate as 1.5 to 3Nm
3
per kg of reduced iron product, but has a relatively low temperature and pressure (about 400° C. and 2 bar), respectively. Specifically, since the temperature of the gas (B) is relatively low despite its large calorific capacity as a heat source, it is not easy to recover heat from the gas. Although the heat recovery can be attained by a heat exchange between the top gas and recirculating gas in order to meet heat balance in the process, the efficiency of such a heat exchange is considerably low due to low gas-to-gas heat transfer coefficient. Therefore, an expensive heat exchanger having a satisfactorily large heat transmission area is required. Even in the technical field of producing direct reduced iron with use of natural gas which has been primarily conducted nowadays, the heat recovery from top gas has been given up in most of the cases. However, there is a need of finding an effective heat recovery in the field of producing direct reduced iron with use of gas obtained by coal gasification. In this field, suppressing variable cost is a more significant task.
Regarding effective use of waste heat derived from coal-derived gas (A), both of U.S. Pat. Nos. 4,260,412 and 4,173,465 disclose direct cooling by mixing with recirculating gas to set the temperature of the coal-derived gas to a suitable temperature for hot desulfurization (400 to 900° C.). This is one of the inexpensive and effective waste heat utilizing techniques as mentioned above. However, neither U.S. Pat. No. 4,260,412 nor 4,173,465 discloses effective measures for a case that a pressure in the gasification furnace is greater than that in the iron ore reducing furnace.
Reduced iron production plants currently under operation produce about 500,000 ton/year as a minimum unit on a commercial scale. In view of this, it is required to build a gasification furnace capable of producing coa
Makino Eiichiro
Shigehisa Takuo
Tomuro Jinichi
Andrews Melvyn
Kabushiki Kaisha Kobe Seiko Sho
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