Process requirements of vertical mills for hot blast stoves and design principles of hot blast stoves

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Introduction: Slag micropowder serves as one of the important admixtures in cement production. Currently, production lines for slag micropowder have been gradually established across China and worldwide. As a byproduct of blast furnaces in steel plants, slag is no longer processed in a crude manner but undergoes further processing to increase its added value. Steel plants nationwide have implemented this practice in recent years. Imported vertical mills play a key role in this process. The challenge lies in how to configure and design the hot air furnace system so as to effectively dry the raw materials. Chinese engineers must take into account both national conditions and fuel resource availability while fully understanding the mechanical characteristics and process requirements of imported vertical mills. In terms of fuel sources, China has far more coal resources than natural gas, blast furnace gas, coke oven gas, heavy oil, or diesel. Therefore, coal should be the primary fuel for hot air furnaces in China. Nevertheless, there are still some enterprises that use blast furnace gas as fuel. Our company specializes in the research, development, design, and technical services related to industrial furnaces. To complement the slag micropowder production line, we have developed a new, highly efficient, energy-saving diffusion-type coal injection combustion furnace (hot air furnace) production line.

1. Process requirements of vertical mills regarding hot air furnaces:
1.1 Reliability: Vertical mills demand high flexibility in terms of the outlet temperature of hot air furnaces. The hot air furnace must preheat the vertical mill, with its temperature being continuously adjusted according to fluctuations in production volume until stable operation is achieved. The aforementioned diffusion-type coal injection combustion furnace is a high-temperature industrial furnace featuring excellent adjustability; its temperature can be set anywhere between 800°C and 1500°C. This new production line incorporates a coal metering system, enabling precise calculation of coal consumption by the furnace. It also includes an automated control system linking coal metering with furnace temperature, ensuring that production proceeds according to preset temperature curves. When the furnace temperature becomes too high, coal feed automatically decreases; conversely, when it drops too low, coal feed increases. The aim is to maintain optimal temperature conditions within the furnace for smooth and reliable operation. Additionally, vertical mills impose strict requirements on the negative pressure level within the furnace chamber. Since the mechanical properties of vertical mills vary across countries, their respective demands regarding negative pressure also differ. Generally speaking, foreign-made vertical mills require the hot air furnace to withstand negative pressures exceeding 300 Pa—a significant challenge for any type of hot air furnace, particularly those fueled by coal. Excessive negative pressure results in poor combustion efficiency due to rapid movement of pulverized coal particles through the furnace, leading to incomplete combustion and soot buildup within the furnace and hot air ducts. To address these issues, our diffusion-type coal injection combustion furnace features a dual-unit production line and a specially designed furnace structure: the front section serves as a combustion chamber, while the rear acts as a mixing chamber equipped with adjustable dampers. Should excessive negative pressure arise due to operational needs, these dampers can be opened to allow airflow to bypass the combustion zone without disrupting coal combustion processes. As illustrated in Figure 1, empirical evidence shows that during commissioning phases of newly installed vertical mills, if improper control procedures lead to excessive suction forces, quickly opening these dampers helps reduce negative pressure levels, thereby facilitating harmonious operation between the vertical mill and the hot air furnace. Consequently, such adjustments must occur gradually; only after operators gain thorough familiarity with both systems’ characteristics and identify equilibrium points can normal operations commence.

1.2 Response speed: Hot air furnaces must exhibit extremely rapid response capabilities, particularly in several aspects: First, as depicted in the diagram, temperature response times must be minimal. During synchronized operation between vertical mills and hot air furnaces, varying preheating durations have been reported among manufacturers—some requiring up to two hours, others merely half an hour. Optimal duration depends not only on local climate, moisture content of slag, and individual mill specifications but also heavily on the type of hot air furnace employed. Nationwide, only two furnace types are utilized in slag micropowder production lines: pulverized coal furnaces (manufactured by our company) and fluidized bed furnaces. Experience indicates that pulverized coal furnaces offer superior performance due to their high degree of automation, resulting in significantly faster temperature response times compared to fluidized bed furnaces. Commands issued by central control systems to raise or lower furnace temperatures result in mere five-second delays at the vertical mill inlet. Fluidized bed furnaces, however, exhibit considerable lag in temperature regulation. Second, considering negative pressure requirements dictated by vertical mills, hot air furnaces must adapt instantly to such changes. This aspect proves especially critical during joint commissioning phases. Our pulverized coal furnaces boast quick startup times; apart from initial heating periods during commissioning, regular operation requires only about ten minutes to reach desired temperatures. Our production line comprises two independent units capable of rapid activation depending on material type and moisture content processed by the vertical mill. Fluidized bed furnaces, conversely, demand lengthier startup periods.

1.3 Automation control systems: Vertical mills necessitate full automation across all mechanical and electrical components of hot air furnaces, including temperature regulation, atmosphere control, pressure management, and coal metering—all managed via PLC-based systems. Merely two twisted-pair cables suffice to transmit all relevant data from the furnace control panel to the central DCS system, allowing operators to monitor operational status remotely and implement unmanned operations within the furnace facility.

2. Environmental performance of hot air furnaces:
2.1 NOx control mechanism: This entire production line relies on two patented technologies. One involves a novel pulverized coal burner designed based on U.S. Department of Energy principles while accommodating domestic conditions and material constraints. It ensures optimal airflow distribution, maintaining ideal coal-air ratios for uniform mixing. The second technology utilizes non-nitrogenous combustion techniques such as staged air injection and CO₂ recirculation at elevated temperatures, generating abundant combustible gases like CO and CH₄ within the furnace’s reduction zone. These gases subsequently enter the oxidation zone where they react vigorously with fresh air, preventing substantial NOx emissions. Thus, this furnace qualifies as an environmentally friendly alternative to conventional nitrogen-emitting industrial furnaces.

2.2 SO₂ and SO₃ control: Raw coal often contains significant quantities of pyrite and organic sulfur compounds. Users must prioritize selecting low-sulfur coal varieties to minimize these contaminants