Oxygen-enriched combustion technology refers to the use of oxygen-enriched air or gas with higher oxygen concentration than the air for combustion technology, which originated in the United States, China.In the 1980 s, it was used in the metallurgical industry and is now gradually maturing in metallurgy, glass and other industries. Oxygen-enriched combustion in China's cement industry began to explore in the 1990 s, but its development is relatively slow. With the gradual increase in the importance of energy conservation and emission reduction in the cement industry, the quality of fuels used has gradually declined. Inferior fuels and alternative fuels are gradually used in the cement industry, and oxygen-enriched combustion has attracted the attention of the cement industry.

The fuel combustion of clinker firing system in cement industry mainly occurs in two thermal equipment-- Rotary kiln and decomposition furnace, especially the combustion in rotary kiln, combustion rate, flame temperature, flame shape, radiation and other combustion properties play an important role in cement clinker calcination. The oxygen-enriched atmosphere in the rotary kiln can significantly improve the fuel combustion performance in the rotary kiln, and has a significant positive effect on improving the flame quality, improving the poor quality combustion and alternative fuel combustion performance, so the oxygen-enriched combustion is also known as "resource creative technology". In addition, nitrogen with high content in air cannot support combustion, and also reduces the contact area between oxygen and fuel, which affects the efficient combustion and burnout of fuel. NOx is also generated in high temperature environment. Oxygen-enriched combustion can obviously reduce the negative impact of nitrogen. Whether it is a rotary kiln or a decomposer, increasing oxygen content for oxygen-enriched combustion can reduce the amount of flue gas after combustion, which is beneficial to improving combustion performance and reducing nitrogen oxides.

 

According to the related arrangement of the decomposer and the rotary kiln, the decomposer has two forms: on-line decomposer and off-line decomposer. The main feature of the on-line decomposer is that the flue gas produced by the rotary kiln enters the decomposer (the kiln is connected in series), while the main feature of the off-line decomposer is that the flue gas produced by the rotary kiln does not enter the decomposer (the kiln is connected in parallel), oxygen-enriched combustion modes mainly include oxygen-enriched combustion in rotary kiln, oxygen-enriched combustion in decomposition furnace, oxygen-enriched combustion in the whole firing system and corresponding oxygen-enriched combustion mode with flue gas circulating and enriched carbon dioxide. According to whether there is circulating flue gas, oxygen-enriched combustion is divided into two categories: oxygen-enriched combustion without circulating flue gas and oxygen-enriched combustion with circulating flue gas, which are described in detail as follows.

1 Oxygen-enriched combustion without smoke circulation

On the basis of air or heated air as combustion gas, oxygen-enriched combustion of fuel is realized by increasing the oxygen concentration in the combustion air by increasing oxygen enrichment.

Oxygen-enriched combustion in 1.1 rotary kiln

In order to distinguish the whole firing system from the oxygen-enriched combustion in a separate decomposer, the oxygen-enriched combustion in the rotary kiln refers to the method of implementing fuel-enriched combustion in the rotary kiln, while the decomposer does not implement oxygen-enriched combustion. Specific implementation: the preparation of oxygen content is higher.21% oxygen-enriched air is introduced into the rotary kiln for oxygen-enriched combustion in the kiln. There are two ways to enter the kiln: oxygen-enriched gas can enter the kiln together with primary air (as shown in Figure 1) through the kiln head burner, and the combustion concentration and flame intensity can be improved by improving the combustion performance of the burner; Oxygen-enriched gas can also be sent into the rotary kiln through the oxygen-enriched channel alone, or as cooling air through the cooler and small kiln head cover, as oxygen-enriched secondary air (the tertiary air is not taken from the kiln head cover, but from the cooler shell behind the small kiln head cover), it enters the rotary kiln to carry out oxygen-enriched combustion of fuel in the rotary kiln to improve fuel combustion performance. Whether it is an off-line decomposing furnace or an on-line decomposing furnace, the (separate) oxygen-enriched combustion in the rotary kiln referred to in this article is not implemented in the decomposing furnace. Although the on-line decomposing furnace will be affected by the flue gas in the kiln, the oxygen content in the flue gas produced by the oxygen-enriched combustion of the rotary kiln is already very low (the oxygen content is generally controlled at 0.5 ~ 3%, and the oxygen content can be controlled by the excess air coefficient), the atmosphere in the decomposition furnace, especially the oxygen content, will not be greatly affected. The oxygen required for non-oxygen-enriched combustion in the decomposition furnace mainly comes from the tertiary air of the firing system (also can be said to be the hot air from the cooler). In addition, in the oxygen-enriched combustion of the rotary kiln without circulating flue gas, the oxygen concentration is too high, and there will be combustion safety hazards such as excessive flame temperature and deflagration, and it is difficult to achieve stable oxygen-enriched or full oxygen combustion with a higher oxygen concentration. Therefore, the increase in oxygen concentration will be subject to certain restrictions.

 

Figure1 Oxygen enrichment/total oxygen enters the rotary kiln process through primary air

Oxygen-enriched combustion in 1.2 decomposer

In order to distinguish the oxygen-enriched combustion of the whole firing system, the oxygen-enriched combustion in the decomposition furnace refers to the implementation of fuel oxygen-enriched combustion in the decomposition furnace, but not in the rotary kiln. Specific implementation: the preparation of oxygen content is higher.21% oxygen-enriched gas is introduced into the tertiary air pipe, and together with the tertiary air, it forms oxygen-enriched hot gas and is sent to the decomposer, as shown in Figure 2, for the fuel combustion of the decomposer to realize oxygen-enriched combustion in the decomposer; or the prepared oxygen-enriched gas is introduced into the decomposer separately by pipeline, as shown in Figure 2, so that the oxygen concentration before fuel combustion in the decomposer increases, an oxygen-enriched atmosphere is formed before the combustion of fuel in the calciner to carry out oxygen-enriched combustion in the calciner.

Figure2 Oxygen-enriched/full oxygen flow into tertiary air duct or decomposer

If the decomposing furnace is an off-line decomposing furnace, the oxygen-enriched combustion in the decomposing furnace will not be affected by the flue gas composition in the rotary kiln; if the decomposing furnace is an on-line decomposing furnace, the oxygen-enriched combustion in the decomposing furnace will be affected by the flue gas composition of the rotary kiln, especially the carbon dioxide content. Carbon dioxide has a certain negative effect on the decomposition of raw meal and fuel combustion in the decomposition furnace, because the volume percentage of carbon dioxide is small (the volume percentage of carbon dioxide in the pre-combustion flue gas mixed with the three winds in the kiln is generallyBelow 7%), the negative impact of carbon dioxide volume ratio below 7% is limited, while the use of oxygen-enriched combustion in the decomposition furnace improves fuel combustion, and the negative impact of carbon dioxide can be compensated by oxygen enrichment.

 

When oxygen-enriched combustion is carried out in the decomposition furnace, the flame requirement is not as high as that of the kiln head burner, and the negative impact of slightly high concentration of carbon dioxide on fuel combustion can be compensated by the oxygen enrichment degree. When the oxygen concentration increases, the carbon dioxide concentration in the flue gas will increase, which will affect the decomposition of raw meal to a certain extent. Therefore, the main difficulty faced by oxygen-enriched combustion in the decomposition furnace is the decomposition of raw meal under high concentration of carbon dioxide environment.

1.3 oxygen-enriched combustion of the whole firing system

Oxygen-enriched combustion of the whole firing system means that the rotary kiln and the decomposition furnace are fed with oxygen-enriched gas to increase the oxygen volume percentage of the combustion gas before fuel combustion. Specific implementation method: the prepared oxygen-enriched gas is introduced into the cooler through the cooler fan, and after exchanging heat with the clinker, it enters the rotary kiln and the decomposer respectively in the form of oxygen-enriched high-temperature secondary air and oxygen-enriched high-temperature tertiary air. Oxygen can also be introduced into the rotary kiln and the decomposer (or the tertiary air pipe) through a separate channel (including the primary air channel) to realize oxygen-enriched combustion of the fuel in the rotary kiln and decomposer.

Whether it is an off-line decomposer or an on-line decomposer, the oxygen-enriched combustion of the entire firing system can realize the fuel combustion of the decomposer and the rotary kiln under high concentration of oxygen, which has a positive effect on the fuel combustion and the reduction of flue gas. Due to the increase of oxygen concentration, the proportion of nitrogen in flue gas is reduced, and the volume percentage of carbon dioxide in the final flue gas can be increased to a certain extent.

2 Oxygen-enriched combustion with flue gas circulation

As oxygen-enriched combustion will bring about a reduction in the amount of flue gas, the higher the oxygen content of the pre-combustion gas, the lower the amount of flue gas produced, and the greater the material-gas ratio of the pre-heating pre-decomposition system. When the amount of flue gas cannot meet the carrying capacity of the decomposition furnace and preheater, additional external gas is required to supplement the amount of flue gas. It is a feasible technical route to circulate the flue gas at the end of the kiln to the firing system. Under the premise of ensuring the amount of oxygen used for fuel combustion, the flue gas at the end of the kiln can be circulated to increase the amount of flue gas in the system (mainly to increase the amount of non-combustion gas such as carbon dioxide) to reduce the material ratio in the system to meet the ability of the gas to carry materials. Flue gas recycling improves the concentration of carbon dioxide in the system gas to a certain extent, because carbon dioxide belongs to the fire extinguishing inert gas, and has a negative impact on the raw meal decomposition and fuel combustion of the firing system, which is a problem that needs to be paid attention to in the cement industry.

The advantages of the oxygen-enriched combustion process with flue gas circulation in cement industry are to greatly increase the concentration of carbon dioxide, reduce the amount of air and reduce the content of nitrogen to the greatest extent. The material-gas ratio of the decomposing furnace and preheater system can be adjusted through the flue gas circulation to adapt to the carrying capacity of flue gas. The disadvantage is that the carbon dioxide content in the gas composition before fuel combustion is large, which affects fuel combustion and raw material decomposition to a certain extent.

Oxygen-enriched combustion with recycled flue gas can be divided into three forms, which are described in detail as follows.

 

2.1 Oxygen-enriched Combustion in Rotary Kiln with Circulating Flue Gas

The flue gas at the kiln end is circulated to the rotary kiln separately, and the temperature of this part of the flue gas is lower than the secondary air temperature of the regular new dry-process cement production line, and needs to be preheated before entering the rotary kiln. According to the characteristics of the cement firing process, this part of the circulating flue gas is preheated by heat exchange with clinker, which is a more appropriate technical solution. In order to achieve oxygen-enriched combustion in the rotary kiln but not in the decomposition furnace, the firing kiln head should be designed as a small kiln head cover, and the circulating flue gas should be mixed with the prepared oxygen-enriched or total oxygen (the amount of oxygen meets the fuel combustion in the rotary kiln, and the gas entering the rotary kiln is rich in oxygen and carbon dioxide through the adjustment of the ratio of circulating flue gas to oxygen-enriched or total oxygen, and meets the requirements of clinker cooling and the wind speed of the new dry key thermal equipment), enter the front end of the clinker cooler, exchange heat with the clinker, and heat the combined flue gas1. Above 100 ℃ (this part of mixed gas preheated by clinker is called "kiln head oxygen-enriched combustion secondary air"), and then enters the rotary kiln through the small kiln head cover to realize oxygen-enriched combustion with circulating flue gas in the rotary kiln. The process flow is shown in Figure 3.

Figure3 CO2/O2 rich mixed flue gas through the cooling machine into the kiln process

The biggest difference between the oxygen-enriched combustion of rotary kiln with circulating flue gas and the oxygen-enriched combustion of rotary kiln without circulating flue gas is that the hot gas entering the rotary kiln contains a higher concentration of carbon dioxide (both before and after the combustion of the fuel are higher than that of the regular production line). The high concentration of carbon dioxide has a negative impact on the combustion rate, flame temperature and concentration of the fuel in the rotary kiln, and even affects the clinker calcination, it is necessary to adjust the fuel combustion performance through the coupling of oxygen enrichment and carbon dioxide and the development of special burners, which is one of the key contents of future research on oxygen-enriched combustion in rotary kilns with circulating flue gas.

 

Oxygen-enriched combustion with flue gas circulation is realized in a separate rotary kiln, which can form high-concentration carbon dioxide at the kiln tail. If this part of flue gas containing high-concentration carbon dioxide enters the decomposer, and air combustion is used in the decomposer, the flue gas generated in the decomposer is larger than that in the rotary kiln, and the concentration of carbon dioxide in the final flue gas is not too high. For example, the volume ratio of carbon dioxide in the rotary kiln is from regular to about15% to 50%, considering the mixed flue gas formed by certain oxygen-enriched and carbon dioxide-enriched gas as the gas before entering the kiln, the air volume is estimated at 0.3 Nm3/kg.cl, and the final kiln tail flue gas is calculated at 1.2 Nm3/kg.cl, the volume ratio of carbon dioxide in the final kiln tail flue gas is only about 40%, regular the concentration of carbon dioxide in the final kiln tail flue gas of the air combustion production line is about 30%, it will also increase by about 10%. If the decomposing furnace is an off-line decomposing furnace and the preheater is a double series preheater, the flue gas from the rotary kiln directly enters one preheater (generally called kiln preheater), the flue gas from the decomposing furnace enters another preheater (generally called furnace preheater), and the kiln preheater can form high-concentration carbon dioxide flue gas, but there is a problem: the proportion of internal combustion material in the decomposing furnace is large and carbonate in the furnace, theoretically larger than the kiln, it will lead to the difference of flue gas volume between the two preheaters, which will bring some difficulties to the development and operation of the preheater.

2.2 Oxygen-enriched Combustion in Decomposition Furnace with Circulating Flue Gas

The flue gas at the kiln tail circulates to the decomposer separately. The gas before fuel combustion in the decomposer is oxygen-rich and carbon dioxide-rich gas. The circulating flue gas is combined with oxygen-rich or total oxygen to enter the decomposer without entering the rotary kiln. There are two ways to realize: the first way is to mix the circulating flue gas with oxygen-rich or total oxygen into the tertiary air pipe, which is sent to the decomposer to realize oxygen-rich combustion, process flow diagram4; In the second way, the circulating flue gas is mixed with oxygen-enriched or total oxygen and then introduced into the clinker cooler to form a high-temperature mixed gas rich in oxygen and carbon dioxide after heat exchange with the clinker. The air outlet and the tertiary air pipe are separately taken by the cooler and sent to the decomposer to realize oxygen-enriched combustion with circulating flue gas in the decomposer. The process flow is shown in Figure 5, it should be noted that the gas composition entering the tertiary air duct after passing through the cooler will be affected by other hot air of the cooler.

Figure4 CO2/O2-rich mixed flue gas flow into the tertiary air duct

If the decomposer is an off-line decomposer, the flue gas of the rotary kiln does not enter the decomposer, and the oxygen-enriched combustion in the decomposer is not affected by the flue gas in the rotary kiln. The concentration of carbon dioxide in the off-line decomposer can be greatly increased by increasing the concentration of prepared oxygen and system air leakage control, the enrichment of carbon dioxide concentration is also affected by the composition of the rotary kiln flue gas. Separate on-line decomposition furnace oxygen-enriched combustion, rotary kiln non-oxygen-enriched combustion, the flue gas formed in the rotary kiln contains a large proportion of nitrogen, which will reduce the concentration of carbon dioxide in the final flue gas from the decomposition furnace. From the perspective of improving the concentration of carbon dioxide in the flue gas of the system, it provides technical support for the subsequent low energy consumption capture and purification of carbon dioxide, and it is reasonable to use the offline decomposition furnace to avoid the influence of the flue gas in the rotary kiln.

 

Figure5 CO2/O2-rich mixed flue gas flows into the tertiary air duct through the cooler

Considering the market demand and capture scale of carbon dioxide capture and purification, it is difficult to capture the whole cement production line. OneFor a cement clinker production line of 5,000 t/d, the annual carbon dioxide emission from kiln tail flue gas is over 1 million tons (the operation rate is calculated according to 80% ~ 85%). At present, it is difficult to capture, purify and utilize the carbon dioxide emitted from the whole kiln system in the market scale, and it is reasonable to partially enrich, capture and purify. According to the scale of carbon dioxide enrichment and capture and utilization, a new kiln system is built next to the kiln end of the original cement kiln production line. The self-contained decomposer adopts offline form. The flue gas circulation and oxygen-enriched or full-oxygen combustion of its own system are implemented in the newly-built offline decomposer. Through the improvement of oxygen concentration and strict control of system air leakage, high concentration enrichment of carbon dioxide in flue gas is realized, laying the foundation for low-cost capture, purification and utilization.

2.3 Oxygen-enriched combustion of the entire firing system with circulating flue gas

Part of the kiln tail flue gas of the whole firing system (called circulating flue gas) and the prepared oxygen are introduced into the rotary kiln and the decomposition furnace, and oxygen-rich and carbon dioxide-rich gas is formed before the fuel combustion in the rotary kiln and the decomposition furnace, which can realize the oxygen-rich combustion of the whole firing system with circulating flue gas. Since the temperature of the circulating flue gas and the prepared oxygen at the end of the kiln is far lower than the temperature of the secondary air and the tertiary air of the regular new dry-process cement production line, in order to further increase the temperature of the circulating flue gas and the prepared oxygen and reduce the heat consumption of the entire firing system, according to the process characteristics of the cement firing system, the circulating flue gas and the prepared oxygen are respectively introduced into the fixed end of the cooler and the front end of the movable bed, and the clinker, the formation of higher temperature oxygen-rich, carbon dioxide-rich gas, as the rotary kiln and decomposition furnace fuel combustion before gas (called."Secondary air and tertiary air of oxygen-enriched combustion firing system with circulating flue gas", the total oxygen amount of pre-combustion gas needs to meet the fuel combustion demand, and the volume percentage of carbon dioxide and oxygen before fuel combustion can be changed by adjusting the ratio of circulating flue gas to high-concentration oxygen). The oxygen amount entering the rotary kiln and the decomposition furnace needs to be distributed and designed according to the ratio of rotary kiln and decomposition furnace fuel; the circulating flue gas can also be mixed with the prepared oxygen to form an oxygen-rich and carbon dioxide-rich mixed gas, and then introduced into the cooler to exchange heat with the clinker to form a high-temperature mixed gas, which is used as the pre-combustion oxygen supply gas for the fuel in the rotary kiln and the decomposition furnace, in which the total oxygen content also needs to meet the total fuel combustion requirements, the oxygen content into the rotary kiln and the oxygen content into the decomposition furnace shall meet the combustion requirements of the fuel in the rotary kiln and the decomposition furnace respectively. The oxygen-enriched combustion process of the whole firing system is shown in Figure 6.

 

Figure6 Oxygen-enriched combustion process of the entire firing system

Through the mixture of flue gas circulation and prepared oxygen and heat exchange with clinker, high temperature oxygen-rich and carbon dioxide-rich gas is formed to realize the oxygen-rich combustion of the whole sintering system, and the enrichment and concentration of carbon dioxide in the flue gas of the whole sintering system are increased. The technical scheme can provide technical support for the low-cost capture and purification of the flue gas at the kiln end of the whole cement clinker production line, and also provide the possibility of zero carbon dioxide emission of the cement clinker firing system; through the adjustment and control of the flue gas circulation amount, the material gas ratio in the rotary kiln and the decomposition furnace can be made close to the regular new dry-process cement clinker firing system, especially to better meet the carrying capacity of the kiln tail flue gas and reduce the risk of system collapse and difficult material.

3 Development direction of cement industry oxygen-enriched combustion

The earliest oxygen-enriched combustion scheme in the cement industry is mainly designed from the perspectives of efficient burnout of fuel, improving the concentration of combustion flame, improving the output and quality of the cement firing system, and improving the combustion state of refractory fuel and alternative fuel. The oxygen concentration before fuel combustion is only appropriately increased, and the flue gas volume and carbon dioxide concentration are not changed too much. The material-gas ratio of the cement firing system, especially the kiln tail system, is only slightly increased, in addition, within the fluctuation range of the normal material-gas ratio, the main effects of oxygen-enriched combustion in the cement industry are: appropriately reducing the heat consumption of the firing system, increasing the output of the system clinker, and improving the combustion state of the flame-retardant fuel.

With the urgent global demand for carbon emission reduction, andWith the proposal of the goal of "carbon peak and carbon neutrality" and the response of various carbon emission reduction policies, the emission reduction and capture and utilization of carbon dioxide in the cement industry are gradually put on the agenda, among which it is a feasible technical route to achieve low-cost capture, purification and utilization of carbon dioxide by increasing the concentration of carbon dioxide in the flue gas of the cement industry. The oxygen-enriched combustion of cement industry is gradually promoted to full oxygen combustion (the proportion of oxygen in the oxygen-supplied combustion gas is generally above 85%), and the concentration of carbon dioxide in the flue gas of cement industry is increased to the greatest extent. By preparing full oxygen or even pure oxygen, combined with the recycling of the kiln tail flue gas, the nitrogen in the system (the hot air combustion of traditional cement industry will bring a large proportion of nitrogen content to the cement kiln tail flue gas) can be eliminated, through carbon dioxide capture and purification, high-concentration carbon dioxide can be obtained, and carbon dioxide products required for various industries, agriculture and life can be prepared.

Through flue gas circulation and oxygen matching, the cement industry realizes oxygen-enriched coupled combustion and calcination. The cement industry regular air combustion, and the main components in flue gas are mainly nitrogen, carbon dioxide, water vapor, residual excess oxygen (volume content listed from high to low), etc., of which nitrogen and carbon dioxide account for a relatively large proportion, and want to greatly increase the concentration of carbon dioxide (≥ 70%), nitrogen needs to be excluded or not introduced into the cement firing system. Due to the large amount of flue gas in the cement industry, it is costly to remove nitrogen from the flue gas after combustion. Separate oxygen and nitrogen in the air to prepare high-concentration oxygen, introduce the prepared oxygen into the cement firing system, and then circulate part of the flue gas to the cement firing system to form oxygen-rich and carbon dioxide-rich pre-combustion flue gas, so as to meet the process requirements of pre-heating and pre-decomposition of raw meal in the cement industry, clinker calcination and heat exchange with a cooler, etc., and can realize the generation of high-concentration carbon dioxide flue gas in the cement industry, the lower the cost of subsequent capture, purification, and utilization.

In summary, the cement industry with flue gas circulation oxygen-enriched combustion or full oxygen combustion is more in line with the process characteristics of the new dry process cement, is the future development direction of the cement industry firing system new process reengineering, but also cement science and technology workers in the field of carbon emission reduction technology focus on research direction. Due to the increase of carbon dioxide concentration in the pre-combustion gas, it brings difficulties to fuel combustion and raw material decomposition, which requires cement scientific and technological personnel to study and tackle key problems, break the technical bottleneck, provide core technical support for the enrichment and capture and utilization of carbon dioxide in the cement industry, and realize the early realization of the cement industry."Carbon peak, carbon neutral" and low-carbon green development to contribute.

4 Summary

According to the characteristics of the new dry process cement process, the oxygen-enriched combustion of the cement firing system is divided into oxygen-enriched combustion without circulating flue gas and oxygen-enriched combustion with flue gas circulation. By preparing oxygen in different addition positions in the cement firing system, the oxygen-enriched combustion in the rotary kiln, the oxygen-enriched combustion in the decomposition furnace and the oxygen-enriched combustion of the whole firing system can be realized, the new process of oxygen-enriched combustion or full oxygen combustion of cement firing system with circulating flue gas is the future realization of the cement industry.The important research and development direction of "carbon peak, carbon neutral" requires cement science and technology personnel to tackle key problems.