Application and exploration of SCR denitration in cement kiln

Date: 2019-06-17 Views:

The N emission standard of cement industry is more relaxed than that of thermal power standard, but with the continuous improvement of national emission standard, the cognition of haze harmfulness deepens. N, as the precursor of haze and acid rain, may be more stringent. At present, the denitrification technology commonly used in cement industry is low nitrogen combustion. In order to cope with the stricter emission standard, it is necessary to increase / 1~SCR denitrification on the basis of denitrification and realize SNCR-SCR combined denitrification. At present, there is little research on SCR denitrification technology in domestic cement industry, but SCR technology is widely used in thermal power industry. Although the smoke of both There is a great difference in the gas emission characteristics, especially the dust content in the cement industry is much higher than that of the thermal power industry. It is easy to block the ash deposit in the catalyst. However, by using the SCR experience of the thermal power industry, it is of great benefit to the SCR technology research in the cement industry.

2. Introduction of SCR Denitration Technology

Selective catalytic reduction is a selective catalytic reduction technique, and the present ammonia catalytic reduction method is the most widely used technology. The device has the advantages of no by-product, no secondary pollution, simple device structure, high removal efficiency (up to 90%), reliable operation, convenient maintenance and the like. the selectivity means that under the action of the catalyst and in the presence of oxygen, the nh3 preferentially and nox undergo a reduction and removal reaction to generate nitrogen and water without an oxidation reaction with the oxygen in the flue gas,

The main reaction forms are as follows:

1) 4NO 4NH3 O2 ≤ 4N2 6H2O

2) 2NO2 4NH3 O2 ≤ 3N2 6H2O

In the absence of catalyst, the above chemical reaction is only carried out in a very narrow temperature range(about 980℃). When using the catalyst, the reaction temperature can be controlled at 300400℃, which is equivalent to the flue gas temperature between the boiler economizer and air preheater. The above reaction is exothermic reaction. Since the concentration of NOx in the flue gas is low, the temperature rise of the catalyst caused by the reaction can be ignored.

Working principle of 3 SCR denitrification technology

Selective catalytic reduction (SCR) denitrification technology refers to the reaction of reductant with nitrogen oxides in flue gas to form harmless nitrogen and water under the action of catalyst, thus removing NOx. from flue gas. Selectivity refers to the reduction reaction between reductant NH3 and NOx in flue gas, but not with oxygen in flue gas.

4. Composition of SCR denitrification system

The SNCR denitration system mainly comprises an SCR reactor, a catalyst, a reactor shell and a steel structure, a flue system, a reducing agent storage preparation supply system, a reducing agent injection system and a soot blowing system.

1, SCR reactor

The section size of the reactor is designed according to the size of the boiler, and each denitration reactor is designed as a 2 + 1 layer catalyst arrangement mode, wherein the upper layer is a reserved layer. And the reactor is mainly composed of a catalyst arranged in the reactor, a catalyst support beam, a reactor shell, a sealing plate and the like. In the reactor, the reducing agent (liquid ammonia) reacts with the nitrogen oxides in the flue gas under the action of the catalyst to generate harmless nitrogen and water, so as to remove the NOx in the flue gas.

2. Catalyst

Catalysts are divided into precious metals and ordinary metals. Precious metal catalysts react with sulfur and are expensive, so they are not actually used. The catalytic efficiency of common catalysts is not very high, and the price is also relatively expensive. The reaction temperature range is required to be 300 ℃. The commonly used catalysts contain vanadium oxide and titanium oxide. SCR catalyst is composed of ceramic support and active components (vanadium oxide, titanium oxide, and sometimes tungsten). There are two main catalyst properties: honeycomb shape and strip shape. Using prefabricated honeycomb ceramics, the catalyst is filled in the honeycomb air or brushed on the matrix. When the shape of the plate is adopted, the support is in the process of being supported. The material is coated with catalyst. When the smoke contains dust, the absorption tower is generally arranged vertically, and the smoke flows from top to bottom. The catalyst is arranged on the catalyst bed of 2 to 4 layers (or groups). In order to make full use of the catalyst, three or four layers are generally arranged, and a standby catalytic bed is provided at the same time. When the activity of the catalyst decreases, the catalyst is installed in the standby layer. After continuous deactivation, the catalyst is replaced on the rotating pedestal, only one layer at a time, starting from the top layer, this method can make full use of the catalyst. The soot blower is arranged in the absorption tower. The soot is blown regularly and the dust deposited on the catalytic bed is blown away. The performance of SCR system is mainly due to the catalyst. The quality and reaction conditions are determined. The larger the volume of catalyst in SCR reactor, the higher the removal rate of NOx and the less ammonia escape. However, the cost of SCR process will also increase significantly.

3. Reactor shell and steel structure

The reactor shell is an external structure containing a catalyst, which is mainly formed by welding a frame steel structure and a steel plate to form a closed space. In order to prevent the heat dissipation of the flue gas, a heat insulating material is arranged between the inner and outer protective plates of the reactor. In order to support the catalyst, a support steel structure beam is arranged below each layer of catalyst, and the catalyst modules are arranged in rows on the support beams. A gas flow uniform distribution device is arranged at the inlet of the reactor, and anti-abrasion measures are arranged on the parts which are easy to wear inside the reactor. The internal various reinforcing plates and the supports are designed into a type that is not easy to deposit, and the compensation measures for thermal expansion will be taken into account. a door and a person for replacing the catalyst are arranged on the reactor shell, The hole of the hole door and the sound wave soot blower shall be installed.

4. Flue system

The flue is divided into two parts of a reactor inlet flue and an outlet flue. The inlet flue is from the outlet expansion joint of the boiler tail economizer to the inlet of the reactor. An ammonia injection device, a tubular mixer, a smoke guide plate and an ash bucket are arranged on the inlet flue. The outlet flue is from the reactor outlet to the boiler tail air preheater inlet. And a smoke guide plate and the like are arranged on the outlet flue.

5. Storage, preparation and supply system of reductant

The reductant is transported by can truck and stored in the reductant storage tank in liquid form. The reductant evaporates and vaporizes through the evaporator before injecting the flue gas into the SCR system and supplies the reactor for denitrification after the buffer tank is stabilized. This part of the work is done in the reductant storage and preparation supply system. The supply system of reductant storage and preparation is mainly composed of discharge compressor, reductant storage tank, reductant evaporation tank, reductant buffer tank and reductant transportation pipeline, and nitrogen purging system is provided.

6. Reductant injection system

The reductant injection system includes the process of mixing the vaporized reductant with diluted air and injecting the reducing agent grille into the flue gas upstream of the SCR reactor.

6.1 dilution fan

When the volume concentration of reductant in air reaches 16 ≤ 25%, II flammable explosive mixture will be formed. In order to ensure the safety of the mixture of reductant and air injected into the flue, not only the concentration of reductant in the mixer is controlled much lower than the lower limit of explosion, but also the ammonia is uniformly distributed in the mixer. The ammonia injected into the flue of the reactor is the mixed gas containing about 5% reductant after air dilution. The selected fan meets the requirement of removing NOx from flue gas and leaves a certain amount of allowance. The dilution fan is set according to 2 sets of 100% capacity (1 operation 1 standby). The air volume margin is not less than 10%, and the wind pressure margin is not low. At 20%.

6.2 Reducing agent/ air mixer

In order to ensure the uniform mixing of reductant and diluted air, two mixers are used for each boiler, and the mixer is a partition plate.

6.3 Reducing agent injection grid

The injection of reductant is grid type, and many nozzles are arranged on the pipe to ensure the uniform distribution and mixing of reductant and flue gas injected into the flue. A manual flow regulating valve is arranged on each area distribution pipe at the entrance of the jet grille to adjust the distribution of reductants in each area.

7. Soot blowing system

The SCR reactor adopts acoustic soot blower, and each reactor is equipped with a set of acoustic soot blowing system. Each layer of catalyst is equipped with 3 soot blowers, and a boiler is equipped with 12 acoustic soot blowers. Soot blowing control is included in the DCS system of the unit.

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