Under normal operation, the pressure in the reaction furnace is between 0. 04 – 0. 054Mpa and the temperature is between I 100 – 1400℃.
Generally, the acid gas in the reactor can reach its thermal equilibrium, and there is no need to add extra combustion fuel.
The temperature of the furnace is governed by H2S contained in the fuel. Normally, 62%～70% of the Sx is created in the reactor, and the remaining is generated in the catalyzing reactor.
The heat reaction furnace is the main equipment in the sulfur recovery unit. It plays various roles in different areas including ignition, automatic operation, deep reaction, and high-temperature edulcoration. So the reliable running of the heat reaction furnace is important for all units.
Operating character of the heat reaction furnace
- There is a high running temperature in the furnace. Normally between 1100-1400℃. While in an accident situation, it can up to 1600 ℃ in certain minutes or even higher.
- H2S、S02 and S03 and other corrode materials are contained in the furnace. So there is a strict limitation on the impurities contents of refractory materials.
- Any fluctuations in the combustion and emergency shutdown conditions are thermal shocks to the furnace liner. Therefore, the furnace liner must have good thermal shock resistance.
- In order to resist dew point corrosion, there is a certain limit on the temperature of the outer metal shell of the heat reaction furnace, and the lining material is required to have outstanding resistance to heat vibration.
- The diameter of the reaction furnace of a large-size sulfur recovery unit is too big. So, there is a strict requirement for the high temperature strength of the furnace liner.
- In order to avoid acid corrosion and protect the liner, there must be a rain cover out of the furnace.
The key points to furnace design
“3T” of the reactor furnace plays an important role in the design and reliable running of the equipment. “3T” includes time residence, temperature, and turbulence mixing.
The high reaction temperature and turbulent mixing degree can shorten the residence time of flue gas accordingly. The residence time is unusually between 0.7-1.2 seconds. Overlong residence time may lead to a waste of equipment or even cause a reversible negative reaction. The higher the reaction temperature in the furnace is, the more favourable for the full reaction, but it should not exceed 1400 ℃. Excessive temperature may place an enormous burden on the material and heat preservation of the furnace and also affect the long-term operation of the equipment. At the same time, it will bring some difficulty to the operation and control if the reaction degree is promoted by supplementing fuel and increasing furnace temperature. Also, the inconsumable hydrocarbon and oxygen may give an adverse impact on the quality of downstream catalyst beds and sulfur.
The degree of turbulent mixing of the reaction components in the furnace is guaranteed by three factors, including the hydro cyclone of the acid gas burner located at the end of the furnace, the mixing speed of the acid gas and air, and the configuration of the gas turbulence in the furnace.
The velocity vector caused by part of the pressure drop is the main power for the turbulent flow in the furnace. As the main part of the burner, the cyclone converts the axial supply air into a certain radial rotation vector. The torsional flow coefficient is generally greater than 0.65, which not only strengthens the mixing degree of acidic gas and combustion air but also strengthens the flue gas disturbance in the furnace. It also provides strong support for the depth of the flue gas target reaction. The throttle ring and firewall are set in the furnace to facilitate the solid flow of flue gas and even mixing.
About furnace liner
At present, there are two types of furnace liner: one is insulated castable with refractory plastic, and another is insulation castable with multi-tenon slot connection corundum brick.
Two forms have their own advantages and disadvantages.
The former type
The advantages: it is integrally moulded, has good thermal shock resistance and resistance to thermal stress, and is relatively inexpensive. It can be pre-formed at the manufacturing plant, dried, and transported directly to the site.
The disadvantages: the requirements for construction and drying are relatively high, and the continuity and consistency of the construction must be ensured. At the same time, use the high temperature flue gas of the hot blast to dry the reactor, and the temperature difference can’t be too large. If the construction and drying quality is low, the liner is prone to penetrating cracks, resulting in local over-temperature during operation and even lining off during severe conditions, which affects the normal operation of the device.
The latter type
The advantages: good resistance to high temperatures and corrosion and is relatively expensive.
The disadvantages: the lining bricks must be built before the equipment is in place. The structure of the lining brick is limited by the diameter of the equipment. If the diameter is too large, the curvature is small, and the strength of the overall structure is reduced. If there is no suitable protective measure, the upper lining brick is likely to fall off under strong thermal shock or vibration.
About the rain cover
The rain cover extends the life of the heat reaction furnace by protecting the carbon steel shell from the thermal embrittlement of the lining material. Thermal embrittlement can happen easily when meeting with heavy rain or cold wind. The rain cover should at least cover the range of thermal reactor shells above 270 °, the perfect design is a mobile, easy-tear-open outfit. It provides an insulated air gap to allow air convection. The insulating air layer stabilizes the shell temperature and prevents the supercooling of the shell and the acid condensation and corrosion caused by it.