Gas-based direct reduction iron （DRI）
Gas-based direct reduction iron (DRI), which has been commercialized, is made from natural gas or coal. In recent years, with the global attention to carbon dioxide emissions, active research has been carried out on the low carbonization of DRI processes, mainly carbon dioxide removal in existing commercial DRI processes or the use of renewable energy to produce reducing gas.
Generally, in order to meet the production requirements of DRI process and improve the reduction speed and efficiency as much as possible, the preparation of reducing gas with high temperature and high reduction degree is the key to DRI. The main components of reducing gas usually used for gas-based direct reduction are CO and H2. During the preparation and use of high reduction degree reducing gas, CO in high reduction degree reducing gas inevitably has a strong driving force of carbon deposition balance in thermodynamics.
Carbon deposition in CO:
(1) In the process of reducing iron oxide pellets, carbon deposition and reduced iron leave the system together, and there is no accumulation problem.
(2) In the preparation process of high-temperature reducing gas, carbon deposition is a solid substance, which has the risk of blocking equipment, pipelines and valves, and directly affects the production process.
(3) The carbon deposition area of CO is also directly related to pressure. In addition, the contents of H2 and CO in the reduction gas produced by different raw materials or similar raw materials can vary greatly, and the applicable direct reduction processes are also different. Therefore, carbon deposition control in the reduction gas preparation process is one of the key technologies that need to be paid attention to.
The raw material for preparing DRI reduction gas may be natural gas or coal. Considering the resource situation in our country, the available raw materials include coke oven gas, various unconventional natural gas, and tail gas from some industrial production processes, such as methanol tail gas, etc. On the other hand, DRI process can be subdivided into 8 different types according to the difference between the reduction furnace and the reduced metal condensed matter. Among them, due to the reduction characteristics of CO, DRI made from coal gasification with high CO content mostly adopts smelting reduction, while DRI made from natural gas with high H2 content mostly adopts shaft furnace reduction.
The content of reducing gas in commercial shaft furnace is kept at about 90%, the pressure is not high, and the temperature is very high, which is conducive to the production of high-quality reducing gas. At the same time, the ratio of H2/CO is 1.6-3.0. Combined with hydrogen metallurgy and reduction equilibrium curves, which have become hot spots in recent years, there is no upper limit limit on H2/CO.
According to the characteristics of China's resources, the choice of raw materials can be relatively abundant coal and various cheap methane-containing gas resources, of which the coal can be low-rank coal encouraged by the state, such as lignite. Cheap methane-containing gas resources can be natural gas, shale gas, refinery gas, coke oven gas, coalbed methane, even biogas and other gases.
