The cross-flow vertical furnace is also called a KHD furnace. It is characterized by an ejector for circulating gas inside. The limestone is decarbonated by the sensible heat of a large amount of circulating gas. The calcining gas passes horizontally through the lime filling layer to give the limestone decomposition heat. The gas that has descended after calcination is circulated to the combustion chamber by the ejector, and after being heated, it enters the furnace again as calcining gas.

The advantages of this furnace are as follows:

(1) Because calcination is carried out with high-temperature gas generated by combustion, the limestone does not come into contact with the flame, so

there is no overburning;

(2) The furnace has a small cross-section and there is no quality fluctuation caused by gas deviation;

(3) The furnace is easy to operate and convenient for quality management;

(4) It can calcine smaller limestone (20-40mm);

(5) The furnace has a small volume, the product is less pulverized in the furnace, and the yield is high.

1. Equipment Overview

The furnace is shown in Figure 3-1 and consists of a raw stone supply device, a furnace body, a combustion device, a heat exchange device, and an exhaust gas dust removal device. First, the raw stone is transported to the furnace top by a belt conveyor after thorough washing and screening, and then evenly put into the preheating zone by the belt conveyor (B·D) on the furnace top. The limestone is heated by the exhaust gas in the furnace in the preheating zone, and after being preheated to a temperature close to the decomposition temperature, it enters the upper calcining zone (6). The exhaust gas passes through the saddle (Saddee) and is evenly drawn through the dust removal device (5) for dust removal before being discharged from the common chimney.

In the upper calcining zone, the high-temperature gas flowing perpendicularly to the limestone provides heat for decomposition of the limestone,

causing most of the limestone to decompose. The uncalcined limestone at the upper gas outlet enters the high-temperature zone of the lower calcining zone (10) and is completely decomposed. The calcined lime exchanges heat with the cooling air sucked from the furnace by the cooling fan (20) in the cooling zone (14), is discharged by the discharger (15) after cooling, and is then transported away by the belt conveyor under the furnace.

The high-temperature cooling waste gas during the lime cooling process exchanges heat with the secondary air blown in the heat exchanger (17), passes through the dust collector (18) for dust removal, and is discharged by the cooling fan. The secondary air heated by the heat exchange also serves as the red driving medium of the ejector, and enters the upper and lower combustion chambers through the atomizer.

The heavy oil delivered by the heavy oil pump (22) is heated to 100°C by the heater (23), adjusted by the pressure regulating valve, atomized by steam jet, and burned in the burner (8) (12) together with the primary air (7) (13), and merged with the circulating gas in the furnace to become a high-temperature gas of 1100-1200°C, which enters the calcination zone.

2. Principle and structure

(1) Principle

In general, when limestone is calcined in the filling layer, it comes into contact with the combustion flame, causing local overheating and easy hard burning. Generally speaking, it is difficult to form a uniform temperature distribution in the furnace. Therefore, this furnace is designed with a combustion chamber to solve this problem, allowing the combustion exhaust gas with a certain temperature to enter the filling layer for calcination.

As shown in Figure 3-2, the preheating zone, calcining zone, and cooling zone of the furnace are continuous structures. The limestone thrown from the top of the furnace passes through each zone in sequence and is discharged from the bottom after calcination.

After the secondary air and the circulating gas temperature of three times the generated gas in the combustion chamber reaches 1150℃, they enter the T1 chamber. 97% of the hot air entering the T1 chamber passes through the 700mm thick lime layer on the entire surface of the T1 chamber and enters the T2 chamber.

During this period, the hot air gives the limestone a heat equivalent to 300℃, and the temperature itself becomes about 850℃. 25% of the gas entering the T2 chamber is introduced into the preheating zone by the four slide saddles installed at the top to preheat the limestone. The remaining 75% of the gas is sent back to the combustion chamber as circulating gas by the secondary air ejector. The 3% of hot gas entering the T1 chamber enters the cooling zone for aging.

The calcination zone is divided into a ratio of 2:1, and the low temperature side (T2) of the upper section becomes the high temperature side (T1 side) of the lower section. The upper and lower sections inject hot gas from opposite directions to stabilize the product quality.

The calcination of this furnace is a short-term heat exchange, which requires enough hot gas, so the circulation method with the help of the ejector is adopted. In order to ensure that the ejector has a large circulation ratio, the relationship between the ejector nozzle and the diameter of the air inlet section is crucial. It is generally taken as 1:8. Another problem is that the secondary air pressure and the pressure loss in the furnace affect the efficiency of the ejector, so the pressure loss is about 200par.

(2) Structure

Preheating zone

The preheating zone has a capacity of about 110m³. It preheats the raw stone and also serves as a furnace top silo. There are 4 lower slide saddles made of heat-resistant cast steel at the bottom of the preheating zone, and 2 upper slide saddles made of stainless steel are installed on the top, which uniformly absorb the exhaust gas. The gas from the outlet of the calcining zone preheats the limestone to 750-850℃ and then drops it to about 200℃, which is then extracted by the exhaust gas fan. The raw stone control in the preheating zone is carried out evenly and constantly by the furnace top shuttle conveyor interlocked with the material level detection device.

Calcination zone

As shown in Figure 3-3, the calcining zone has a square cross-section and is composed of 6 rows of 22 sections of grids made of refractory bricks. The upper calcining zone has 14 sections, with 1 section in the middle, and the lower calcining zone has 7 sections. The upper and lower calcining zones have their own independent combustion chambers and injectors, and the grid openings are unobstructed to allow hot gas to pass through.

Combustion chamber

There is a burner in each of the upper and lower sections, with the help of the internal mixing of air. There is an air intake section on the ceiling at the front end of the burner, forming a structure where secondary air and circulating gas flow in. The capacity of the burner is distributed in a ratio of 2:1 between the upper and lower sections, and the upper section consumes about 2/3 of the fuel.

Cooling belt

The cooling belt and the calcining belt are connected together. At the head end of the upper part of the cooling belt, a part of the hot air matures the lime. There are exhaust flues on both sides below the head end. The cooling air is sucked in from the air inlet at the bottom and heat-exchanged with the lime, and then introduced into the heat exchanger through the flue. 2-4 electromagnetic dischargers are installed at the bottom of the cooling belt, and the discharge volume is adjusted by the sliding line resistor voltage regulator.

Refractory materials

The furnace is built in a square steel shell with refractory brick lining to form various structures. Therefore, the consequences of its thermal expansion and contraction must be considered. In particular, the checker bricks, which are the heart of the furnace, should not only use refractory bricks with small expansion and contraction, but also use refractory bricks with load softening and good wear resistance.

Preheating zone: hard clay refractory bricks-insulating bricks.

Grid: high alumina refractory bricks.

Calcination zone side wall

T1 side: high alumina bricks-refractory insulation bricks-insulating bricks-insulating boards;

T2 side: refractory bricks-refractory insulation bricks-insulating bricks-insulating boards.

Cooling zone: high alumina bricks-refractory insulation bricks-insulating boards.

Combustion chamber: high alumina bricks-refractory bricks-refractory insulation bricks-insulating bricks-insulating boards.

3. Main machine specifications

The main machine specifications of the furnace are listed in Table 3-4