Issue 31/2012 - Technical selection and optimization of batch fixed-bed gasification processes

With the increasing shortage of resources such as coal, the era of low-cost high-quality anthracite coal is over, which brings great challenges
to nitrogen fertilizer companies with batch fixed-bed gasification processes.
In order to reduce the production cost of synthetic ammonia, improve economic benefits, and realize energy conservation and emission reduction of enterprises, the energy-saving technical transformation of the existing batch fixed bed gas furnace is particularly important
.
Anhui Jinmei Zhongneng Chemical Co.
, Ltd.
has three sets of synthetic ammonia production plants, with a total annual ammonia production capacity of 1 million tons
.
At present, the company has two sets of fixed bed intermittent fixed bed gas production equipment and 33 gas furnaces
.
Among them, the 1# synthetic ammonia system has 22 Φ2800mm gas furnaces and 20 normally open; The 2# synthetic ammonia system has 11 Φ2800mm gas furnaces, and 9 are
normally open.
Since the beginning of 2011, the company has increased the energy-saving technical transformation of the gas production system, and through a series of transformations, the current consumption of ammonia coal per ton is about 1135 kg, a year-on-year decrease of more than
50 kg.

First, increase the linear dehydration vibrating screen to reduce the pulverization content of the coal entering the furnace

Due to transportation, etc.
, the foam content in the coal entering the plant is large, the highest can reach more than 14%, and the foam content in the furnace coal can reach up to 3%~4%.

Excessive coal foam into the furnace causes the resistance of the inner bed of the gas furnace to rise, and the furnace condition deteriorates
in severe cases.
In order to improve the defoaming rate, the company decided to reduce the tilt angle of the elastic vibrating rod screen and extend the residence time
of the incoming coal on the vibrating screen.
A new linear dewatering vibrating screen is added after the elastic vibrating rod screen, and the coal into the furnace is flushed with water on the screen to ensure that the foam content of the coal in the furnace is less than
1%.
In order to recover the coal foam under the linear dehydration vibrating screen, two slag pickers were added to remove the coal foam under the screen and make it a raw material for processing briquette, realizing the comprehensive utilization
of the incoming block coal.

Second, the automatic measurement and control carbon layer device is adopted to stabilize the furnace condition

While transforming the original low-pressure water jacket into a medium-pressure water jacket, the automatic measurement and control device
of the carbon layer of the gas furnace is adopted.
Due to the manual measurement of the height of the carbon layer, human error is prone to human error, and the data is not timely
.
When the carbon layer in the furnace is too low, the resistance of raw materials becomes small and easy to blow, the thickness of the heat storage layer is thinned, the steam decomposition rate is reduced, and the gas quality and gas quantity are poor; When the carbon layer in the furnace is too high, although the heat storage layer becomes thicker, the resistance of the gasifier increases, which has a great impact on the gas generation of a
single furnace.
The automatic monitoring and control device of carbon layer height can realize the timing detection record and automatic control of the empty height of the gas furnace, and automatically adjust the carbonization time of the next cycle according to the empty height measured in the furnace in the previous cycle to keep the empty range stable
.
At the same time, it can also reduce the labor intensity of workers, stabilize the upward and downward temperature, and thus stabilize the condition of
the gas furnace.

Third, transform the oil circuit system of the gas furnace to improve the operating speed of the valve

Relevant information shows that each valve action (opening and closing) is 1 second faster, and a gas stove can save a total of 274,000 yuan
in coal costs throughout the year.
After several transformations, the diameter of each valve has been increased, and the maximum valve has DN800mm
.
However, the diameter of the solenoid valve of the oil circuit system is still Φ12mm, resulting in a slow running speed of the valve and a serious
phenomenon of valve waiting.
In winter, it is difficult to run in place within 8 seconds, and the second up-blowing time has to be extended, which affects the temperament and gas volume, and also poses a threat
to safety production.
To this end, the gas production oil circuit system was reconstructed
.
The diameter of the oil circuit main pipe was increased, and the original Φ57mm main pipe was changed to Φ89mm; The diameter of the solenoid valve controlling the large valve was changed from Φ12mm to Φ18mm; Measures such as changing the accumulator position and increasing the accumulator volume
.
After the modification, the valve operation time has been reduced from about 7 seconds to less than
3 seconds.

1.
Status quo
of fixed bed intermittent gasification technology The most widely used coal gasification technology in China's small and medium-sized nitrogen fertilizer industry is fixed bed intermittent gasification technology, at present, many nitrogen fertilizer companies have begun to pay attention to oxygen-rich technology, and many manufacturers intend to adopt this technology, especially nitrogen fertilizer production enterprises
using white coal as raw material.

(1) Oxygen enrichment and nitrogen adopts the traditional batch gasification process, and only the air nitrogen is changed to oxygen enrichment and nitrogen
.
Oxygen-enriched air with an oxygen concentration of 30%~35% is used instead of air into the gasifier
.
This improves and stabilizes the temperature of the gasifier layer, while shortening the blowing time, increasing the decomposition rate of the incoming steam, and reducing the residual carbon
in the slag.
The project investment is small, the gas temperature can be completely controlled, the oxygen consumption is small, and the gas temperament is also better
.

(2) Continuous upblowing oxygen enrichment to produce gas Oxygen and the air sent by the gas fan are mixed to form oxygen-enriched air with a volume fraction of 50%~60%, which enters the air-vapor mixer; The low-pressure steam also enters the air-vapor mixer after controlling the appropriate steam/gas ratio by flow regulation and is fully mixed with oxygen-enriched air; Then enter the bottom of the gas furnace and continuously blow up to take semi-water gas
.
The volume fraction of carbon dioxide in the coal gas generated by continuous gasification of oxygen enrichment is as high as 15%~20%, which increases the power consumption of the compressor, the high steam consumption of the transformation section, and the load of the decarburization section increases
.

(3) Single-furnace oxygen-enriched two-way continuous gasification Single-furnace oxygen-enriched two-way continuous gasification can be regarded as the extreme case of high oxygen and nitrogen, with the extension of the time of high oxygen and nitrogen addition, the increase of oxygen in the furnace in the nitrogen addition stage, the percentage of blowing air decreases accordingly, when the percentage drops to zero, the blowing stage is canceled, and the nitrox plus nitrogen becomes oxygen-enriched two-way continuous gasification
.
The total amount of oxygen required for this protocol is large
.

The purity of oxygen produced by the cryogenic air separation unit supporting HT-L pulverized coal pressurized gasification is 99.
9% and the pressure is 5.
1MPa, after meeting the needs of the gasification unit, the two sets of air separation units have a surplus of 2000 cubic meters / hour and a purity of 99.
9% oxygen, which is safely depressurized and diluted for use in oxygen enrichment and nitrogen production in the gas production device of the ammonia synthesis system
.

Based on the above situation, the company decided to use oxygen enrichment and nitrogen addition to nitrogen to carry out oxygen enrichment and nitrogen process transformation
of the existing fixed bed gas production system of the 2# synthetic ammonia system.

2.
Fixed bed intermittent gasification oxygen enrichment transformation
(1) Transformation plan
2# Synthetic ammonia gas production overview: a total of 11 Φ2800mm gas furnaces, 9 normally open gas furnaces, 3 C 600-1.
28 blowers, 1 C400-1.
35 blower for the rear section 3 sets of 6M50 compressors
.

The oxygen (purity 99.
9%, pressure 5.
1MPa) from the air separation oxygen buffer tank is 2000 cubic meters / hour, through the globe valve and the quick shut-off valve, through the first-stage pressure reducing valve to reduce the pressure, together with the pure carbon dioxide gas (3.
1 MPa) from the carbon dioxide compression post, into the first-stage buffer tank to mix, dilute the purity of oxygen to 85%~90%; Then it enters the secondary buffer tank through secondary depressurization to 0.
1 MPa to achieve the conditions before entering the gas furnace; Finally, it is sent to the oxygen-rich main pipe
in the gas production plant.

The purging nitrogen comes from the air separation device, added to the automatic shut-off valve of the oxygen pipeline and before the first-stage pressure reducing valve, the stainless steel pipeline is used, and the stop gas is controlled by remote distance, and the interlocking automatic switch
is realized.

(2)
After the transformation, the nitrogen and oxygen-enriched gas project was put into trial operation
in the 8#, 9#, and 10# furnaces of the 2# synthetic ammonia gas production system after a series of work such as pipeline blowing, degreasing, passivation and pressure test leak test.
After two days of exploration, the remaining two systems, a total of 9 gas furnaces, were all put into oxygen-enriched operation
.

(1) Process operation In the case of adding oxygen 1500 cubic meters / hour, the single furnace should reduce the blowing time by 3~4 seconds
.
Refer to the operating data of other companies to reduce the blowing time of each gas furnace by 4 seconds
.
The oxygen concentration of the secondary buffer tank is controlled at 85%~92%, and after mixing with nitrogen air, the oxygen concentration is 35%~40%.

Due to the reduction of the blowing stage time, the amount of upward displacement of the gasification layer in the blowing stage is reduced compared with the past, and the oxygen concentration of the furnace is increased when nitrogen is added, which reduces the rate of furnace temperature drop after the upblowing starts.
In order to ensure the stability of the gasification layer in the furnace, make full use of the higher carbon layer temperature in the furnace, increase the amount of up-blowing steam, increase the up-blowing time, reduce the amount of down-blowing steam, and shorten the down-blowing time
.

(2) Single furnace gas generation after transformation When the total gas flow at the outlet of the gas cabinet before the transformation is about 74,000 cubic meters / hour, the gas production system needs to run 9 gas furnaces to meet the production needs, and after the transformation, 8 gas furnaces can meet the production requirements
under the same gas volume.

(3) Coal consumption Because the gas of the company's two sets of gas production systems is connected at the inlet of the compressor section, there is a mutual gas conduction phenomenon, and the total amount of coal entering the furnace is related
to the gas consumption in the later section.
After the implementation of oxygen enrichment gas production, under basically the same conditions, the consumption of tons of ammonia alcohol coal decreased by about
40 kg.

(4) Power consumption Due to the reduction of the blowing time of each cycle cycle, the work time of the relative blower is reduced
.
Referring to the actual operation of the company's blowers, the air main was modified by taking advantage of the rear maintenance opportunity, and a 440kW blower was successfully shut down
.
After the transformation, the unit consumption of electricity consumption was reduced by 16.
8%, and the power consumption per ton of ammonia alcohol was reduced by about
10kW·h.

(3) Benefit accounting
because the use of the surplus capacity of the cryogenic air separation device to produce pure oxygen, on the basis of 16500 cubic meters / hour, when the amount of pure oxygen produced is 18500 cubic meters / hour, it consumes 3 tons more steam and 120kW
more electricity.
After calculation, the cost of pure oxygen per cubic meter is 0.
325 yuan, but the daily benefit is 20639 yuan
.
In addition, CO2 emissions were reduced by approximately 275 kg/t of
ammonia.

To sum up, from the above transformation effects, the effect of oxygen enrichment and nitrogen gas production is more obvious, but due to the company's cryogenic air separation oxygen production capacity, it is impossible to provide more oxygen, resulting in the 1# gas production system cannot be used
.
It is planned to add a new VPSA oxygen generator this year for use in
the #1 gas production system.

In the past two years, through the transformation of gas production, although the consumption of raw material coal has decreased to a certain extent, there is still a big gap compared with some advanced nitrogen fertilizer manufacturers, and the company will continue to explore and continue to improve
.