As a new energy technology, waste incineration is the best way to recycle, reduce and stabilize waste, and has been widely used in countries around the world. The key equipment of the waste incineration treatment system is the incinerator. Typical furnace types include circulating fluidized bed incinerators, mechanical grate incinerators, rotary incinerators, etc. At present, the mainstream furnace type for domestic waste incineration is the grate incinerator, which accounts for more than 70% of the market.

The grate incinerator recovers waste heat from waste incineration through the waste heat utilization system. The currently widely used waste heat utilization system is an integrated waste heat boiler, including a combustion chamber and a closed boiler furnace above the combustion chamber, also known as the first flue (or first channel). The operating temperature of the combustion chamber is 1100~1200℃. Since the pollutant dioxin produced by burning garbage can only be completely decomposed at above 800 ℃, DB31/768-2013 requires the flue gas temperature to be higher than 850 ℃ and the residence time to be greater than 2s.

Therefore, in order to ensure that environmental protection standards are met, the flue gas temperature at the outlet of the first channel of the grate incinerator is usually controlled above 850 ℃. The temperature of the flue gas at the outlet of the first channel is related to the amount of heat conducted away by the waste heat utilization system. The heat generated by burning garbage is transferred to the water-cooled pipe through the lining of the water-cooled wall of the first channel of the grate furnace and then converted into high-temperature steam to achieve waste heat recovery. The lining of the water-cooled wall generally selects high thermal conductivity SiC refractory materials.

There are great differences in the design of the water-cooled wall lining structure between domestic and foreign grate furnaces. The water-cooled wall lining structure of foreign grate furnaces is fin (flat steel) hanging nails + SiC hanging bricks + SiC self-flowing material filling and sealing, referred to as membrane wall structure; domestically, it is a water-cooled pipe anchor nail + SiC castable structure. Different lining materials and structures have a great influence on the heat transfer of the water-cooled wall lining. So far, there has been no report on the research of heat conduction of the water-cooled wall lining of the grate furnace.

In this work, the fluid module FUENT of the finite element analysis software ANSYS was used to conduct numerical simulation research on the heat conduction of two different water-cooled wall linings of the grate furnace, and the influence of the lining material and structure on the flue gas temperature at the outlet of the first channel of the grate furnace was analyzed.

I. Finite element analysis conditions

(I) Selection of calculation model

The fluid module FLUENT of the finite element analysis software ANSYS was used to conduct numerical simulation analysis on the temperature distribution inside the two water-cooled wall lining structures commonly used in grate furnaces at home and abroad. The calculation models of the two lining structures of membrane wall structure (hanging bricks + self-flowing material) and water-cooled pipe anchor nails + castables are shown in Figure 1.

(II) Simulation calculation boundary condition setting

During the simulation calculation, the actual working conditions were simplified and appropriate assumptions were made, and appropriate parameters were selected:

1. The water-cooling tube contains single-phase water, with a flow rate of 1m·s-1 (mass flow rate of 600 kg·m-2·s-1), a temperature of 280 ℃, and a pressure of 7 MPa;

2. The water-cooling tube is made of 20G carbon steel with a wall thickness of 5 mm;

3. The inner cavity size of the water-cooling tube is 5.94 mx8.91 mx14 m;

4. The flue gas flow rate is 3.4 m·s-1, and the furnace (one channel inlet) temperature is selected as 1100℃, ignoring the influence of different garbage calorific values ​​on the flue gas temperature;

5. The flue gas temperature distribution at the outlet of the first channel of the grate furnace is not uniform. The flue gas temperature is lower near the wall, and the temperature in the middle is higher. For the convenience of comparison, the outlet temperatures mentioned are all average temperatures.

(III) Physical parameters of materials

The thermal conductivity of the water-cooled wall lining needs to match the furnace conditions. It is necessary to conduct as much heat as possible to generate electricity, and to ensure that the outlet temperature of a channel is not lower than 850℃. The simulation calculation sets the thermal conductivity of the refractory material to be constant, and does not consider the influence of the composition change of the refractory material on the thermal conductivity during use and the influence of the ash accumulation on the thermal conductivity of the lining. It also does not consider the influence of the hanging nails and anchor nails on the thermal conductivity. When performing the simulation calculation, the thermal conductivity of each working layer of the water-cooled wall lining is selected as shown in Table 1.

(IV) Setting of the lining thickness

In order to explore the influence of the lining thickness on the flue gas temperature at the outlet of a channel, simulation calculations were performed on two water-cooled walls with different lining thicknesses. For the brick-hanging + self-flowing furnace lining, the SiC self-flowing material thickness is fixed at 5 mm, and the SiC brick thickness is changed to 20, 25, 30, 35, and 40.45 mm; for the SiC castable, the thickness is adjusted to 25, 30, 35, 40, 45, and 50 mm.

In order to understand the influence of the brick-hanging and self-flowing working layer thickness setting of the membrane fireplace lining structure on the thermal conductivity of the furnace lining in more detail: 1) The SiC brick thickness is set to 30 mm, and the SiC self-flowing material thickness is set to 5, 7.5, 10, 12.5, and 15 mm; 2) The total thickness of the furnace lining is 40 mm, and the thickness of the SiC brick-hanging + SiC self-flowing material is (35+5)(32.5+7.5)(30+10)(27.5+12.5)(25+15) mm respectively for thermal simulation calculation.

(V) Setting of thermal conductivity of SiC self-flowing material

The thickness of the furnace lining directly affects the effective volume in the furnace. Therefore, under the premise of meeting the demand, the thickness of the furnace lining should be as small as possible. Hanging bricks are usually fired products with relatively fixed thermal conductivity and small adjustable space, while the adjustable space of thermal conductivity of self-flowing material is relatively large. When the thickness of the furnace lining is constant, the thermal conductivity of the furnace lining and the flue gas temperature at the outlet of one channel can be adjusted by adjusting the thermal conductivity of the self-flowing material. During the simulation calculation, the furnace lining thickness is set to 40 mm (30 mm for hanging bricks and 10 mm for self-flowing material), the thermal conductivity of the hanging bricks is 25 W·m-1·K-1 (500 ℃), and the thermal conductivity of the self-flowing material is adjusted to 2, 4, 6, 8, and 10 W·m-1·K-1, respectively, to simulate its influence on the heat conduction of the furnace lining and the flue gas temperature at the outlet of one channel.

2. Results and discussion

The influence of furnace lining thickness on the flue gas temperature at the outlet of one channel The influence of the thickness of the two water-cooled furnace linings on the flue gas temperature at the outlet of one channel is shown in Figure 2. It can be seen that: when the thickness of the two linings changes from 25 to 50 mm, the flue gas temperature at the outlet of one channel is higher than 850 ℃; the two water-cooled wall lining structures show that the thicker the lining, the higher the flue gas temperature at the outlet of one channel; when the thickness of the two linings is the same, the flue gas temperature at the outlet of one channel of the castable lining is significantly higher than that of the one channel of the brick + self-flowing lining.

The influence of the thickness of the brick and self-flowing material on the flue gas temperature at the outlet of one channel The influence of the thickness of the brick and self-flowing material on the flue gas temperature at the outlet of one channel is shown in Figure 3. It can be seen from Figure 3 that when the thickness of the brick is fixed and unchanged, the greater the thickness of the self-flowing material, the higher the flue gas temperature at the outlet of one channel: for every 2.5 mm increase in the thickness of the self-flowing material, the flue gas temperature at the outlet of one channel increases by 8~15 ℃, and the rate of flue gas temperature increase decreases with the increase in the thickness of the self-flowing material; when the total thickness of the lining is fixed and unchanged, the thicker the brick and the thinner the self-flowing material layer, the lower the flue gas temperature at the outlet of one channel. When the total thickness of the furnace lining is 40 mm, by adjusting the thickness of the hanging bricks and the self-flowing material, the maximum difference in the flue gas temperature at the outlet of a channel reaches 20°C.

The influence of the thermal conductivity of the SiC self-flowing material on the flue gas temperature at the outlet of a channel Figure 4 shows the influence of the thermal conductivity of the self-flowing material on the flue gas temperature at the outlet of a channel. It can be seen that when the thickness of each working layer of the furnace lining is fixed, the flue gas temperature at the outlet of a channel decreases with the increase of the thermal conductivity of the self-flowing material. By adjusting the thermal conductivity of the self-flowing material, the maximum difference in the flue gas temperature at the outlet of a channel reaches 70°C.

III. Practical application effect

Based on the above simulation analysis results, combined with the actual working conditions of a domestic waste incineration power generation project, a practical application comparison of different lining designs was made on two grate furnaces with a daily processing capacity of 750t. The 2# furnace uses a water-cooled pipe anchor + SiC castable lining structure with a lining thickness of 40 mm: the 3# furnace uses a membrane-type fireplace lining structure of fin (flat steel) nails + SiC hanging bricks + SiC self-flowing materials, and the total lining thickness is also 40 mm (hanging brick thickness 32 mm, self-flowing material thickness 8 mm).

After the two equipment were put into operation for 7 months, the furnace was opened for routine maintenance. The 2# furnace channel castable lining had coking in all areas, and the overall coking was serious and vitrified. The thickest part could reach 1m, which was difficult or impossible to remove; about 80% of the brick hanging area of ​​the 3# furnace channel one had 1~2 mm floating ash, and the rest of the area had crusting, with the thickest thickness of 5mm, which was easy to remove.

After the two equipment were in operation for 11 months, energy efficiency tests were carried out. Figure 5 compares the outlet flue gas temperature of the boiler channel one under the conditions of 90% boiler maximum continuous evaporation (MCR) and 100% MCR. Figure 6 shows the comparison of the thermal efficiency of the two boilers under 100% MCR operating conditions. The results show that under 90% MCR operating conditions, the flue gas temperature at the outlet of the first channel of the 3# furnace is 135.9 ℃ lower than that of the 2# furnace. Under 100% MCR operating conditions, the flue gas temperature at the outlet of the first channel of the 3# furnace is 149.7 ℃ lower than that of the 2# furnace. The thermal efficiency of the 3# furnace is 0.64% higher than that of the 2# furnace. In comparison, the thermal efficiency of the waste heat boiler of the 3# furnace using a membrane fireplace lining is higher under the same load.

Conclusion

1. The thicker the water-cooled fireplace lining of the grate furnace, the higher the flue gas temperature at the outlet of the first channel. When the thickness is the same, the flue gas temperature at the outlet of the first channel of the castable furnace lining is significantly higher than that of the brick-hanging + self-flowing furnace lining.

2. For the brick-hanging + self-flowing lining, when the thickness of the bricks remains unchanged, the greater the thickness of the self-flowing material, the higher the flue gas temperature at the outlet of one channel; when the total thickness of the lining remains unchanged, the thicker the bricks and the thinner the self-flowing material layer, the lower the flue gas temperature at the outlet of one channel; when the thickness of each working layer of the lining is fixed, the flue gas temperature at the outlet of one channel decreases with the increase of the thermal conductivity of the self-flowing material. By adjusting the thermal conductivity of the self-flowing material, the maximum difference in the flue gas temperature at the outlet of one channel reaches 70 ℃.

3. Under the same operating conditions, the flue gas temperature at the outlet of one channel of the brick-hanging + self-flowing lining is lower than that of the castable lining, the thermal efficiency of the waste heat boiler is higher, and the anti-coking ability is stronger.