YUAN Wei-jun   ZHANG Fu-liang  WANG-hui  HAN Ming-ru

(Tangshan Keyuan Environmental Protection Technology & Equipment Co., Ltd.

Tangshan Hebei  063020)

Abstract: This paper briefly explains the reasons to the continue existing and development of coal gasifier gasification technology from the perspective of fuel gas demand in processing industry, the scale of enterprise investment, diversified coal gasification technology as well as the international status and marketing demands of Chinese coal gasifier gasification technology. Also makes a system analysis on the environmental and safety problems of present gasification technology and some deficiencies on system automation and large scale of gas supply, as well as raises some technical measures and suggestions to settle the problems and overcome shortages, then points out the development direction of atmospheric fixed bed gas generator technology.

Key words: coal gasification technology; gasification technology of coal gasifier; reasons of development; environmental protection ; safety; automation degree; large scale of gas supply

1.Introduction

In China, coal storage accounted for 92.94% of fossil energy reserves, while oil and natural gas are relative deprivation. In 2010, the consumption ratio of Chinese coal 、oil、natural gas and renewable energy sources are respectively 71.9%、20.0%、4.6% and 3.5%, it can be seen from the above that coal accounting for an important status in China's energy structure, it is estimated that the consumption proportion of coal in Chinese energy will still remain at around 50% by the middle of the century. Pollutant emissions control is particularly important in the coal application process as energy consumption, while coal gasification technology belongs to the category of clean coal technology that can effectively reduce the emissions of pollutants. Through the data in《CHINA ENERGY STATISTICAL YEARBOOK》（2012）: as for China’s coal consumption in 2011, in addition to 50% of coal consumption in power industry, the coal application as fuel in manufacture industry is also nearly 23%. Try to energetically develop coal gasification technology which matched IGCC power generation system and gasification technology suitable for the gas scale in manufacturing industry are effective methods to settle the problem of energy diversification and environmental pollution.     

In the numerous coal gasification technology, the coal gasifier as representative gasifier of atmospheric fixed bed gasification technology in industrial enterprise of fuel gas supply unit is widely used in a lot of burn up enterprises, but due to many factors, the application of coal gasifier is constrained and restricted increasingly. Some specialists even think atmospheric fixed bed gasification technology is backward technology and on the brink of elimination. This paper makes an analysis and description on the reason to the development & existing of coal gasification technology and present existing problems as well as its future developing direction, then aim to determine whether the gasification technology should exist and how to develop healthily.  

2. Reasons to the development of gasification technology of coal gasifier

2.1 Requirement of fuel gas demand in manufacturing industry

The gas consumption scale of fuel gas widely applied in manufacturing industry which including building material, building materials, chemical industry, metallurgy and machinery industry converted into heat is generally around 10×10³ - 30×10⁴MJ/h and equivalent to 16-380t/d of coal consumption, while the coal consumption for most of enterprises are about 100-200t/d. There are various specifications of coal gasifier, the minimum coal consumption of each gasifier is about 10t/d and the largest coal consumption can be achieved at 70-80t/d.Meanwhile, each set of gasifier can be combined together into one separate gas supply unit. As it was showed in ChartⅠ, the coal consumption of other fluidized bed or airflow bed gasifier is generally about hundreds of tons and even up to thousands of tons per day, that’s not conform to the requirements of manufacturing scale of fuel gas. 

ChartⅠ Domestic developing coal gasifier and coal consumption of each gasifier

2.2 Investment strength requirements of processing and manufacturing enterprises

In terms of equipment investment, take same scale of gas station(daily coal consumption is around 300-400t) as an example, the investment of domestic pulverized coal gasification or pressurized fixed bed gasification equipment is usually several times of the investment in atmospheric fixed bed gasification equipment, generally the equipment investment calculated on a hundred million yuan, it is hard for general processing and manufacturing enterprises to construct gas station in this intensity of investment in equipment. The gasification technology displayed in ChartⅠare generally belong to the large scale of coal-chemical industry such as IGCC combined power generation or synthetic ammonia and methanol synthesis, etc. If  generate 100GJ of coal gas per hour, the coal gas station investment in equipment is about 5-5.5 million yuan, that is suitable for the investment strength requirement of general processing and manufacturing enterprises.

2.3 The requirement of coexistence of lump coal gasification technology and pulverized coal gasification technology

There are various kinds of coal gasification technology. As for the granularity status of appliance materials, it generally includes the application of pulverized coal gasification technology（such as pulverized coal gasification technology, CWS gasification technology and multiple slurry gasification technology etc..）and lump coal gasification technology(such as pressurized fixed bed gasification technology and atmospheric fixed bed gasification technology). Gasification technology of coal gasifier belongs to the category of atmospheric fixed bed gasification technology which needs to take a fixed particle size of coal as gasifying material. Regarding the coal supply structure, due to the mechanization adoption and depth mining technology, the production of pulverized coal is bigger, so the pulverized coal price has always been lower than the price of lump coal. Historical data showed that when the lump coal price up to around RMB1000/t, its price difference with pulverized coal is about RMB200-300/t. This is mainly because the present application amount of pulverized coal is mismatch with its production. Considering keeping the supply and demand balance of pulverized coal and lump coal and in terms of the development of coal gasification technology, maintaining a diversification development of coal gasification technology means keeping the development of lump coal gasification technology at a proper level at the same time of developing the pulverized coal gasification technology is quite necessary.

2.4 International status and market requirements of Chinese gasification technology of coal gasifier

Since from the 1950 s, China has developed the single-stage coal gasifier on the basic of US WG and Д type of coal gasifier from former Soviet Union. Then in the 1980s, China started to introduce the gasification technology of double-stage coal gasifier from England, America, Italy and France to absorb their essence. The research, development and application of coal gasifier technology have been lasted for more than 60 years, it has already formed a full set of design, manufacture and application system.With the exploration of cheap oil and gas in the 1960s, some countries with advanced gasifier technology gradually gave up the development of this technology, when the oil crisis happened in 1972, some countries like America redeveloped the FW-Stoic double-stage coal gasifier, but gave up again with the relief of crisis. While China has always kept improving and innovation based on the double-stage coal gasifier from Occident and has owned the most perfect atmospheric fixed bed gasification technology in the world so far, it is one of the few coal gasification technology which not involves with international intellectual property dispute as well. Around the year of 2000, with the constant rise of international oil and natural gas price, as well as gradually pressure increase of world’s energy, a lot of countries include Vietnam, Thailand, Indonesia, Malaysia, India, Ukraine, Australia, Japan and South Korea came to seek for gasification technology and equipment of coal gasifier. Due to the international status and market demand of Chinese gasification technology of coal gasifier, it is necessary for us to make further development and improvement on this technology.

3. Existing problems and developing direction of gasification technology

3.1 Environment Problems

3.1.1 Phenol water problems

Phenol water problems have always been an environmental problem distressed the development of coal gasifier gasification technology. After long-term research and discussion, many experts carried out long times of application experiments on dozens of phenol water treatment includes steam chemical dephenolizing method, steam dephenolizing method, incineration method, solvent extraction dephenolizing, resin dephenolizing, sulfonated coal adsorption and biochemical process, but all ended in failure. In recent years, the gas station phenol water treatment had tried many new technology and achieved some results. Many ceramic factory adopted the technology introduced in literature^【1】, it makes the phenol water from gas station and pulverized coal into coal water slurry(CWS) then used as fuel of spray drying tower, this technology is more thoroughly on the treatment of phenol water, but due to the odor and other problems of phenol water CWS, the slurry has to be generated on the spot and applied nearby, that limited the wide spread of this technology. Phenol water treatment technology introduced in literature^【2】 which utilizing sensible heat of coal gas from lower section of double-stage coal gasifier, then gasifying the phenol waster water and used as gasifying agent has been used in many gas stations, but as the proportional control requirements in upper section and lower section of double-stage coal gasifier, the sensible heat of coal gas from lower section is not enough to deal with all the phenol wastewater generated by coal gas station, normally only 50-80% of phenol water can be treated. The phenol water treatment technology in literature^【3,4】 applied all the sensible heat of coal gas and gasified the phenol water to used as gasifying agent. This technology can deal with all the phenol water with low boiling point from gas station, however the high boiling point of residual phenol wastewater can not be treated thoroughly inside the gas station.           

Based on the principal idea of phenol water treatment by evaporation concentration method introduced in literature^【3】and combine the characteristics of gas station system to further develop and improve the equipment and processing technology, it can totally deal with the phenol wastewater thoroughly. A tar deep processing enterprise from Laiwu Shandong province adopts the process of pyrolysis clean gas station and utilizes the gasification and structure characteristics of KM5Q pyrolysis coal gasifier as well as combines the evaporation concentration method introduced in literature^【3,4】 ,finally this technology can not only completely deal with all the phenol waste water from gas station itself, but also effective solved part of coking wastewater generated during the tar processing process. The practical running data shows that when the production of coal gas station is around 6000-7000Nm³/h, the gas station can deal with about 15t/d of extra coking wastewater.      

3.1.2 Tar problem

The main problem of by-product tar is with high content of pulverized coal and moisture, especially the moisture in tar is around 20-30%. A lot of gas stations applying the heating evaporation of steam to aqueous tar in pool before selling the tar, in order to decrease the tar moisture content, so the environment of gas station will be seriously polluted.  

Increasing the outlet gas temperature above the tar condensation temperature and setting ash separator at the exit of coal gasifier. After all the pulverized coal in coal gas being removed, decreasing the temperature of coal gas and removing tar can effectively decrease the pulverized coal content in tar. As the density of gasifier by-product tar is close to water, it is difficult to separate oil and water by gravity separation technology, so consider to set up a special oil-water evaporation separator, then heat oil -water mixture with steam for evaporation separation, transport the evaporated steam to the bottom of coal gasifier as gasifying agent, in this way, oil-water separation and phenol water treatment process can combine effectively.           

3.1.3  Foul odour problem

Odor pollution is a kind of serious feeling public nuisance. The main odor pollutants of coal gas station are volatile matter such as hydrocarbon, alkene, aromatic hydrocarbons, benzene and phenol etc.. At present, the majority of coal gas stations do not have effective treatment facilities, that caused a serious pollution to the surroundings.  

The literature^【5】 introduced several common treatment methods on foul odor in coking industry, including washing method, filtration method, adsorption method, burning method and nitrogen sealed method etc.. The coal gas station in a factory of POSCO collected the foul odor gas, then transported to the bottom of gasifier and mixed with gasifying agent, the effect of burning technology in coal gasifier is obvious. Combine the characteristics of gasification technology of coal gasifier itself and reference the treatment technology of coking and other related industries, the foul odor of gas station can be effective controlled.      

3.1.4 Reduction of CO₂ emission

A literature^【6】 made a comprehensive consideration on the gas combustion of CO₂ emissions, as well as the indirect CO₂ emissions generated during the gas production, purification, transportation and auxiliary combustion system of power consumption etc.. Please find the chart of CO₂ emission in the application process of commonly used industrial fuel gas (Chart Ⅱ), it indicates that the CO₂ intensity is relatively large during the application process of generator gas.

Chart Ⅱ CO2 emission in the application process of commonly used industrial fuel gas（GJ^-1 coal gas）^【6】

The literature^【7】 introduced a kind of coal gasifying technology which apply “oxygen enriched air—CO₂ exhaust gas—steam”as gasifying agent and use CO₂ in parts of exhaust gas from kiln reaction gas taking part in reduction reaction in coal gasification process, then react with carbon in coal to produce CO combustible gas. This technology is especially suitable for the industry which has bigger CO₂ emissions intensity. Coal gasifier with this gasification technology can not only save coal resources but also effective reduced the cycle of CO2 emissions.  

3.2 Security problem

3.2.1 Security problem of steam generating system

The steam generating system of coal gasifier is mainly concerning the water jacket and vapor drum, explosion accident caused by this system is usually much more serious than the explosion of low pressure steam boiler, it always accompanied by casualties. At present, the safety awareness of steam generating system in gas stations is not strong enough: the application of water for water jacket is irregular; there is no automatic water adding facilities in vapor drum etc., so the safety hazards is greatly. With reference to the relevant specification of steam boiler design, manufacture and management, strictly supervise the steam generating system of coal gasifier can effectively avoid the system safety accidents.  

3.2.2 Security problem of gas system

Gas system of coal gasifier including gas generating system and pressurized gas purification system, safety accident is generally divided into explosion, burning and CO poisoning, the malignancy grade of this safety accident is usually lower than the steam generating system. China has strictly standard and specifications on generator gas station and its safety design, the normative design of equipment and process system are particularly important to the security assurance of gas generating system. In the 1980 s-1990 s, the process and equipment design of gas station are totally completed by several major subordinate design institutes，the strictly enforcement of regulations effective decreased the accident rate. At present the design of technology are totally completed by equipment suppliers, so the irregularity quality of design team leads to the lower safety coefficient in gas station. If the relevant government departments establish a full set of perfect system concerning project design and examination approval system for supervisory control, it can effectively control the system safety accidents.

3.3 Automation degree

Comparing with other similar equipments, the present automation degree of coal gas station is relatively lower that have to appoint more personnel in gas station, meanwhile, the frequency of man-made accident will increase. With the increase of labor costs, environmental protection and safety pressure, the demand of gas station’s automation for enterprise have increasingly increased. At present, fire probing must be operated artificially to grasp the situation of material layer in coal gasifier so that to adjust the operation of gasifier, that restricted the gas station’s automation space for improvement that has always been the bottleneck for gas station to realize its comprehensive automation control. On the basic of developing automatic detection and adjustment system、designing fault diagnosis expert system^【10】 for gas station, developing perfect DCS control system can effectively improve the automation degree of gas station.    

3.4 Large-scale gas supply problems

At present, most of coal gas stations applied in Chinese processing enterprises are constructed as a single-unit enterprise and with relatively small scale, but considering the requirement on environment, safety and efficiently management, multiple approaches enterprises or industrial park building concentrate gas station for gas supply, that will be the development direction of industrial enterprises’ fuel gas supply form in future. However, the current production capacity of each coal gasifier is smaller. Take 3.6m of hearth diameter as an example, its largest gas production is normally 11000Nm³/h(Calorific value of coal gas is about 6270-6688KJ/Nm³）which can be convert into about 68.97-73.57GJ/h of heat supply, that will need more than ten and even twenty sets of coal gasifiers supply in parallel for large scale of gas supply, therefore, it will occupy large building area and not convenient to manage. So in order to adapt to the requirement of large scale gas supply, the production capacity of each coal gasifier must be enhanced.         

Regarding the improvement of each gasifier production capacity, it can be solved from two aspects. Firstly, through increasing hearth diameter to increase gas production; Secondly, taking corresponding measures to enhance gasification intensity of gasifier and calorific value of gas. The gas production of gasifier is generally in direct proportion to gasifier’s sectional area, the increase of hearth diameter makes limited contribution to the increase of gas production, moreover, subject to the limited conditions of equipment transportation, this method can only as auxiliary measures to improve the productivity of each gasifier. It is introduced in literature^【11】that taking “oxygen-enriched air + steam” as gasifying agent instead of “air+steam” to produce coal gas can increase combustible component（CO+H₂）content, the calorific value of coal gas will increase as well, meanwhile, it also contributed to the coal oxidation degree as well as improved the gasification efficiency and intensity. It can be seen from the comparison data in Chart Ⅲ that, using oxygen-enriched air (50% of oxygen content) as gasifying agent instead of air, the calorific value of coal gas and gasification intensity respectively increased 56% and 83%, the comprehensive considerations in production capacity for each oxygen-enriched gasifier increased 139%, that means by using oxygen-enriched technology and with 50% of oxygen content , the production capacity of each gasifier is equivalent to the 2.4 sets of original gasifier. In conclusion, based on the oxygen-enriched technology, proper increase the hearth diameter is development direction for coal gasifier to adapt to the requirement of large scale gas supply.

Chart Ⅲ Comparison of atmospheric fixed bed continuous air gasification and oxygen-enriched gasification

4. Conclusion

Atmospheric fixed bed gasifier gasification technology has been developed for decades in China. Considering the requirement of coal gas demand scale of manufacturing fuel, business investment intensity and diversified demand of coal gasification technology as well as the international status and international market demand of Chinese coal gasification technology, this gasification technology has its reasons to the continue existing and development. But we must face up to the present environmental and safety issues on atmospheric fixed bed gasifier gasification technology and deficiency in the aspect of system automation and large scale gas supply. On this basis, we should study further, solve problems and overcome shortcomings, only in this way, atmospheric fixed bed gasifier gasification technology can step into a healthy development way.  

Bibliography

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First Author

Name: Yuan Weijun（1968-）, Male, senior engineer and master of engineering from Bazhou, Hebei province, Working content: Coal gasification industry   Research direction: Coal gasification technology and equipment  

Tel.:13703243469

E-mail：2329081462@qq.com

Contact person: Yuan Weijun

Address: NO.10 Yunhe East Road, Fengnan Economic Development Zone, Tangshan, Hebei, P.R. China

Code: 063300

Tel.：13703243469