Heat Reaction Furnace
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 their thermal equilibrium and 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 are 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 heat reaction furnace
- There is a high running temperature in the furnace. Normally between 1100～1400℃. While in the accident situation, it can up to 1600 ℃ in certain minutes or even higher.
- H2S、S02 and S03 and other corrode material are contained in the furnace. So there is a strict limitation for the impurities contents of refractory materials.
- There is no hot taken body in the acid gas heat reaction furnace. 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 furnace liner. At present, there is 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 is integrally molded, which 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 disadvantage is that 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 has good resistance to high temperatures and corrosion and is relatively expensive. The disadvantage is that 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.
- In order to avoid acid corrosion and protect the liner, there must be a rain cover out of the furnace. 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.
The key to 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. 3T are interrelating and dispensable. 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 favorable for the full reaction, but it should not exceed 1400 ℃. excessive temperature may place an enormous burden on material and heat preservation of 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. 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, 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.
Acidic gas is separated from acidic water by steam stripping and liquid separation device, which is sent to the heat reactor of the sulfur recovery unit. According to different technological processes, acidic gas is generally divided into two types, namely, ammonia-containing acidic gas and ammonia-free acidic gas, which enter into the heat reaction furnace for combustion. Ammonia accounts for about 5 to 10 percent of the total acid gas, which is higher in special cases. The ammonia in the composition of the acidic gas must be completely dissolved in the heat reactor. In the process of decomposition, undecomposed ammonia gas will cause the blockage of the ammonium salt crystallization of the follow-up equipment, such as the condenser or pipeline, and affect the normal production and operation of the device. So, the heat reactor of sulfur recovery unit should not only achieve the maximum conversion rate of H2S to elemental SX, but also take on its function of complete incineration of ammonia gas. Therefore, the technical requirements for an acid gas burner and thermal reactor are very high.
There are mainly three reactions in the furnace according to the research of Canada Alberta Sculpture Research Ltd and the user experiences. There are three main reactions in the process of ammonia burning in the reactor.
2NH3+S02→2H20+ H2S + N2
3H2S + 2NH3 + 402→S2+ S02 + 6H20 + N2
Oxidation Reaction of ammonia: according to the test, the reaction speed of NH3 and O2 is slower than that of H2S and 02, and the oxidation-combustion reaction of heat reaction furnace is taken in the environment of anoxic. So, Therefore, the direct oxidation reaction of NH3 and 02 is not the main ammonia burning reaction in the reaction. Under the situation of sufficient oxygen at the 1200℃, 65% ammonia steam is oxidized. At 1300℃, the oxidation rate of ammonia reaches above 95%.
The test shows that the oxidation reaction of NH3 and O2 and reacts weakly under the environment of greater than 1200 ℃. At 400 ℃, H2S has an oxidizing reaction with O2, therefore under 1200 ℃, when the reaction rate of H2S and O2 is greater than that of NH3 and O2 before NH3 react with O2, H2S can react with O2 generating SO2, and then the reaction of NH3 and SO2. Next, we have the decomposition of NH3 itself.
Heat decomposition reaction of ammonia: this reaction directly affected by the temperature. Under the situation of no H2S and H20, at 1100℃, the decomposition ratio can reach 90%, while at the temperature of 1200℃，it can be reached to 100%, but the appeal of H2S and H20 can strongly restrain the reaction of the ammonia decomposition. Some research indicating: when the content ratio of H20 and NH3 reach 1:1, the heat decomposition ratio of ammonia is only less than 20%.
The reaction of ammonia and S02: this reaction happened all through the reaction in the furnace. According to the research, this reaction happened at the temperature of 700℃，but the reaction of NH3 and O2 happened at the temperature of 1200℃, So the main reaction in the furnace is NH3 and SO2.
All the test above is under the situation of a sufficient mixture of reactant. During the processing of design and using, the use efficiency is directly affected by the atmosphere uniformity in the furnace.
Due to the high temperature of the burner flame field, if the mixture in the furnace is uneven, it is impossible to restrict the reaction of NH3 and O2, which can be oxidized to NO2 and affects the use of downstream equipment.