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Combined Reformation Technology of Low Nitrogen and Nitrogen Capacity in Circulating Fluidized Bed

(一)Technical background

On June 7, 2014, the General Office of the State Council issued the Circular of the General Office of the State Council on Printing and Distributing the Energy Action Plan for Energy Development (2014-2020) (No. 2014) No. 31), which calls for accelerating energy production and consumption revolution and enhance the level of clean and efficient coal power development.

December 11, 2015, the National Development and Reform Commission, the Ministry of Environmental Protection, the State Energy Bureau jointly issued a "full implementation of coal-fired power plant ultra-low emissions and energy-saving work program" (Central [2015] No. 164) notice.

The full implementation of ultra-low emissions and energy-saving coal-fired power plants, is to promote the use of coal clean, improve the quality of the atmospheric environment, mitigation of resource constraints an important measure. "Action Plan for Upgrading and Reforming Energy Saving and Emission Reduction of Coal and Electricity (2014-2020)" (hereinafter referred to as the "Action Plan").

The Action Plan request that till 2020, with all the coal-fired power plants in the country striving to achieve ultra-low emissions (ie, 6% of the baseline oxygen content, soot, sulfur dioxide and nitrogen oxides are not higher than 10,35 , 50 mg / cubic meter), the national conditional new coal-fired generating units must meet the ultra-low emission levels.

Nitrogen oxides and water in the air will eventually be converted into nitric acid and nitrate, nitric acid is one of the causes of acid rain; it's mixture with other pollutants under certain conditions can produce photochemical smog pollution; nitrogen oxides can stimulate the lungs, so that people are more difficult to resist respiratory diseases such as colds. Nitrogen oxide emissions caused great harm to the environment and human health.

二,the shortcomings and deficiencies of the existing technology

For previously designed and most of the existing coal-fired circulating fluidized bed (CFB) boiler, nitrogen oxide original emission concentration of 300 ~ 450mg / m3, can not meet directly discharge standards. Now the existing CFB boiler through the way of SNCR, SCR, SNCR + SCR denitrification to meet the ultra-low emission standards.

Both SNCR and SCR denitrification react selectively with NO x in flue gas under the typical reaction conditions of ammonia as reducing agent, and produce N 2 and H 2 O in the environment. The main reaction is:

4NH 3 + 4NO + O 2 =4N 2 + 6H 2 O     (1)

4NH 3 + 2NO 2 + O 2= 3N 2 + 6H 2 O   (2)

8NH 3 + 6NO 2 =7N 2 + 12H 2 O       (3)

2NH 3 + NO + NO 2 =2N 2 + 3H 2 O    (4)

Whether SNCR or SCR to spray a large number of ammonia or urea solution for denitrification, SNCR denitrification efficiency of 30 ~ 70%, in order to achieve the emission target, there will be a lot of ammonia escaping. SCR denitrification efficiency of 80% to 95%, but the catalyst is easy to fail poisoning, there will be a lot of ammonia escape. In the flue gas denitrification at the same time, the catalyst can also make part of the flue gas SO 2 oxidation generated SO 3, SO 3 and SCR denitrification process of unreacted ammonia (escaped ammonia) reaction to produce ammonium hydrogen sulfate. The reaction is::

N H 3 + SO 3 + H 2 O =NH 4 HSO 4     (5)

At normal operating temperatures, the dew point of ammonium bisulfate is 147 ° C, which accumulates in the form of a liquid on the surface of the object or in the form of droplets. Liquid ammonium bisulfate is a highly viscous material that is easily deposited on the surface of the air preheater and adsorbs fly ash from the flue gas, increasing air blockage and corrosion.

In order to meet environmental emissions requirements, the existing CFB boiler will increase a lot of operating costs, making the operating costs increase, lower product profit margins. And the original flue gas denitrification technology on the NOx removal rate is limited, making the CFB boiler environmental advantages are no longer prominent.

At present, most CFB boilers do not focus on nitrogen oxides (NOx) ultra-low emissions technology. Each boiler developed by the enterprise has its own advantages and disadvantages, but for most of the current operating boiler, the  thermal efficiency is lower than the design value and waste a lot of renewable energy which havn't make the most of CFB burning characteristics. Therefore, how to make the most of advantages of CFB,minimize NOx emission during combustion process through technical means is particularly important. Considering the environmental policy and running economy, reducing CFB boiler NOx emission and achieving high low emission now is the forefront of the current research.

三、coal-fired boiler nitrogen oxide generation mechanism

The NOx generated during the combustion of coal is mainly nitric oxide (NO) and nitrogen dioxide (NO2), both of which are collectively referred to as NOx, and also a small amount of nitrous oxide (N2O). In nitrogen oxides, NO accounts for more than 90%, nitrogen dioxide accounts for about 5% -10%.

The amount and amount of NOx generated during the combustion of coal are closely related to the burning conditions of coal combustion, coal, coal particle size, especially combustion temperature and excess air coefficient. In the process of coal combustion, there are three general ways to generate NOx:

(A) heat type

During combustion, the nitrogen in the air is oxidized at high temperatures, where the formation process is a non-branched chain reaction. The mechanism of its formation can be expressed by the Zeldovich reaction equation.

With the increase of reaction temperature T, the reaction rate is exponentially. When T <1500 ℃, the amount of NO generated is very small, and when T> 1500 ℃, T for each additional 100 ℃, the reaction rate increased 6-7 times.

Heat-type nitrogen oxides generation mechanism (Zeldovich reaction formula)

At the high temperature, the total generation is

(B) Instantaneous reaction type (fast type)

Rapid NOx was discovered in 1971 by Fenimore through experiments. In the case of hydrocarbon fuel combustion, when the fuel is too thick, NOx is rapidly generated in the vicinity of the reaction zone.

CH radicals generated by pyrolysis of hydrocarbons in the fuel volatiles can react with nitrogen in the air to form HCN and N, and then further with oxygen to produce at an extremely rapid rate. The formation time takes only 60ms, and the furnace The pressure of 0.5 power is proportional to the relationship with the temperature is not.

The above two kinds of nitrogen oxides are not the main part of NOx, nor the main source.

 (C) fuel type NOx

From the fuel in the nitrogen compounds in the oxidation of combustion. Since the thermal decomposition temperature of nitrogen in the fuel is lower than the pulverized coal combustion temperature, a fuel type is generated at 600-800 ° C, which accounts for 60-80% of the pulverized coal combustion products.

In the process of generating the fuel-type NOx, the first organic compound containing nitrogen is pyrolyzed to produce N, CN, HCN and the like intermediates and then oxidized to NOx. Since the combustion process of coal is composed of two stages: volatile combustion and coke combustion, the formation of fuel type is also composed of two parts: oxidation of vapor phase nitrogen (volatile matter) and oxidation of coke in coke (coke).

A description of the decomposition of nitrogen in the fuel into volatiles N and c

四 Low NOx combustion and capacity technology's main technical characteristics:

For coal-fired circulating fluidized bed boilers, low NOx combustion is used to reduce NOx generation opportunities.

1) In the combustion of higher volatile bituminous coal and lignite, the fuel type NOx content is more, fast type of NOx is very little. Fuel-type NOx is the reaction of oxygen in the air with the pyrolysis products of nitrogen in the coal to produce NOx. The nitrogen in the fuel is not all converted to NOx. It has a conversion rate, which reduces the conversion rate and controls the total amount of NOx emissions. Measures:

(1) reduce the excess air coefficient of combustion;

(2) control fuel and air pre-mixing;

(3) to improve the local fuel concentration into the furnace.

2) Thermodynamic NOx: N2 and O2 in the air at the time of combustion NOx is generated at high temperatures. The main condition is that the high combustion temperature allows the nitrogen molecules to increase free chemical activity. Then, the oxygen concentration is high, NOX generation, can take the following measures:

(1) reduce the maximum temperature range of combustion;

(2) reduce the peak temperature of the boiler combustion;

(3) reduce the excess air concentration and local oxygen concentration.

As a prerequisite, to ensure the safety of the boiler combustion of taking following measures to reduce the generation of nitrogen oxides.

2.1 low excess air burning

The combustion process closing to the theoretical amount of air conditions, with the excess oxygen in the flue gas can reduce the formation of NOx. This is one of the simplest ways to reduce NOx emissions. Generally can reduce NOx emissions 15 to 20%. However, if the oxygen concentration of the furnace outlet is too low (2.5% or less), it will increase the chemical incomplete heat loss, causing the carbon content of fly ash to increase the boiler combustion efficiency. Therefore, the design of 130 tons of CFB furnace outlet excess air coefficient of 1.15 (generally equal to 1.2), can reduce NOx emissions by about 20%.

Reasonable air rating into the furnace

The basic principle is to complete the fuel combustion process in stages, using inverted triangle style. During the first stage of the pre-combustion phase, the amount of air supplied from the primary chamber to the furnace is reduced (equivalent to 80% of the theoretical air volume) so that the fuel is first burned under anoxic fuel-rich combustion conditions. At this time, the excess air coefficient α <1 in the dense region reduces the combustion rate and the temperature level in the combustion zone. Therefore, not only the combustion process is delayed but also the NOx-generated reaction rate is reduced in the reducing atmosphere, and the amount of NOx generated in this combustion is suppressed. The second stage: the end of the burning stage, in order to complete the entire combustion process, the complete combustion of the remaining air through the layout in the upper part of the dense area and the transition zone of the special secondary air nozzle into the furnace, and the lower part of the " Oxygen combustion "under the conditions of the smoke generated by mixing, in the α> 1 under the conditions of the completion of the entire combustion process. This method makes up for the shortcomings of simple low excess air burning. The smaller the excess air coefficient in the dense phase, the better the effect of inhibiting the formation of NOx, but the more incomplete the combustion products, the lower the combustion efficiency and the greater the likelihood of slagging and corrosion. Therefore, in order to ensure both the reduction of NOx emissions and the safety and reliability of the boiler combustion, the air classification combustion process must be properly organized.

2.3 reasonable fuel classification burning

NO generated in the combustion of NO encountered hydrocarbon root CHi and incomplete combustion products CO, H2, C and CnHm, will occur NO reduction reaction, re-reduced to N2. With this principle, the main fuel is fed into the dense phase, burning and generating NOx under α> 1 conditions. The fuel fed into the dense phase is called primary fuel and the remaining 15 to 20% of the fuel is fed into the suspension zone at the upper part of the main burner and a strong reducing atmosphere is formed under the condition of α <1, The NOx generated in the zone is reduced to nitrogen molecules in the suspended zone (re-combustion zone), and the fuel fed into the suspended zone is also called secondary fuel, or refueling. In the reflow zone, not only the generated NOx is reduced but also the generation of new NOx is suppressed, and the NOx emission concentration can be further reduced.

 

2.4 Optimization of the material separation system

The old furnace type which running at present, separator separation efficiency is relatively low, the separator cutting particle size is relatively large, resulting in some fine ash can not be effectively separated. Circulating ash is an effective means of regulating the load and controlling the bed temperature. By adjusting the structure of the separator and the center cylinder, the separation efficiency of the separator is improved so that the circulating ash can effectively control the bed temperature, thereby reducing the NOx generation. Improve the separation efficiency of the separator, so that the furnace pressure can be increased, the furnace ash concentration increases, that is, to improve the furnace material filling degree, can further improve the boiler load.

2.5 increase in the furnace heating surface

The original boiler have problem of insufficient output, there are many reasons, the four main points are as below: First, the heating surface layout is insufficient; Second, the bed temperature and return feeder temperature is too high, difficult to transfer coal in; Third, the separator separation efficiency is relatively low , Furnace ash filled with very poor; Fourth, coal and combustion adjustment problem. On the one hand by increasing the separation efficiency of the separator to increase the load (previously mentioned), on the other hand by increasing the furnace heating surface in the furnace to improve the boiler load. In the furnace to increase the heating surface, not only can effectively control the furnace combustion temperature, improve the boiler load, control of NOx generation effectively. So the boiler capacity and low nitrogen transformation is complementary!

As per the circulating fluidized bed boiler combustion characteristics, through the implementation of a number of projects, our company summed up and develop the unique combination of low nitrogen capacity transformation technology. The low nitrogen capacity of this technology is very obvious, from the demonstration project, its nitrogen efficiency at least 55% or more, can achieve the vast majority of circulating fluidized bed boiler NOx emissions below 150 ~ 200 mg / Nm3, boiler load increase by 10% -20%

五cases

Project: Xiajin County Thermal Power Co., Ltd. 75t / h circulating fluidized bed boiler capacity and low nitrogen transformation

Before the boiler transformation problems below:

The maximum load of the boiler can only run to 65 tons, the initial concentration emission of nitrogen oxides 350 ~ 380mg / m3.

User requirements:

Users require the rated evaporation from 65t / h to 85t / h, nitrogen oxide original emission concentration of 150mg / m3.

Boiler transformation program:

1. Add four economizer screens;

2. Re-arrange fluidized bed and hood;

3. Reconstruction of the central drum and the return device;

4. rearrange the secondary air system;

After the boiler transformation effect as below:

1. Boiler normal operation load of 85t / h, the maximum load can reach 89t / h.

2. 85t / h load operation, a small amount of investment SNCR, the nitrogen oxide emission concentration of 78mg / m3.