Introduction

     Coal gasification and rational utilization of coal conversion is one effective way, in line with China's development of clean coal technology, energy diversification strategy, China has both the current and long-term significance. In recent years, atmospheric fixed bed gasification technology as the main small-scale fuel gas supply unit, its scale of production, the cost of investment, construction period, which qualify for the majority of metallurgical, chemical, building materials and machinery industries such as gas demand application of the more extensive.
     After 1988, with the atmospheric fixed bed furnace a furnace and two continuous improvement and wide application of coal gasification national standard (GB9143-88), (GB9143-2001), (GB9143-2008), (GB50195 -94) followed successively promulgated standards for coal gasification adhesion, thermal stability, crushing strength, ash melting point and ash, sulfur grading dozen indicators were data constraints. However, at atmospheric pressure fixed bed coal gasifier, trial burn and production process, we found involved in addition to the standard dozen indicators, coal gasification reactivity of coal as a direct reflection of the reduction in the gasifier ability to respond to the strength characteristics of indicators, fixed bed coal gasification reaction with atmospheric effects can not be ignored, worthy of further analysis and discussion.
A coal gasification reactivity
     Coal gasification reactivity is an important feature, means and in a heated state coal gasifying agent (air + water vapor) effect the chemical reaction of the strength, that is, in a certain temperature conditions, with the coal in the reduction of CO2 ability of CO, CO2 reduction ratio is usually (α) that the size, the reduction ratio (α) and the size of the coal gasification temperature. Coal gasification reactivity, which represents the degree of difficulty for gasification of coal, gasification reactivity is stronger, the gasification reaction, the faster the same duration carbon conversion rate is higher, the time required for complete gasification of the more shorter.
     Scholars study found that over the years, coal gasification reactivity is affected by many factors, East China University of Xu Shen Kai et al [1] found in experiments the presence of alkali metal Na Coke can significantly reduce the degree of graphitization, improve coal gasification reactivity, while the presence of ash with an alkali metal has the same trends, but the impact is weak compared to the alkali metal. For coal gasification reactivity properties on the impact of the literature [2,3,4] were also reached the following conclusion: the greater the catalytic effect of minerals, coal gasification reactivity of the larger size of its catalytic coal metamorphism related to the higher degree of metamorphism of coal mineral catalysis smaller; enables the pore structure of coal pyrolysis rich, factors that increase the specific surface area, help to improve coal gasification reactivity; Coke coal gasification reactivity with the degree of metamorphism increases. East China University of Wu Jiaqi, etc. [5] experimental study was carried out through the following conclusion: Coke coal gasification reactivity with the increase in the fixed carbon content decreases, with the volatile content in coal and basic factor increases. Thus, by the coal itself the objective existence of a number of factors, different coal gasification reactivity of its different.
     Atmospheric fixed bed gasifier including coal gasification and CO2 responses and reactions of coal with steam, Kora, etc. [6] found through experiments, lignite and bituminous steam reaction rate of CO2 reaction rate 2 - 5 times; Zhang Xianlin, etc. [7] and 1193-1323K at atmospheric conditions, was studied using thermal gravimetric water vapor and CO2 Chinese anthracite chars gasification reactivity, experimental results show that the anthracite coke gasification reaction with CO2 activity was significantly less than with steam gasification reactivity of the latter is about 10 times larger. In [8,9] that the water vapor atmosphere, the gasification reactions occur in more than 0.6nm microporous surface, while in a CO2 atmosphere, the gasification reactions occur in the larger pores of the outer surface, is greater than 15nm in microporous surface fishes gasification play a role, so, coal and water vapor and CO2 reaction rate until there is such a big difference.
2 gasification reactivity for gasification ash melting point of impact indicators
     Table 1 for the gasification of coal ash melting point defined by the national standard indicators, the author believes that the definition of indicators in defining the scope and definition of classification there exist many unreasonable. (GB9143-2008) pointed out that the gasification of coal types include: lignite, long flame coal, non-caking coal, weakly caking coal, gas coal, lean coal, lean coal, lean coal and anthracite, and unify the ash melting point indicator is defined as Ad ≤ 18% when 1150 ℃ and Ad> 18% when 1250 ℃, so they put a lot of easy-gasification and ash melting point lower than 1150 ℃ excluded from the gasification of coal outside the ranks of coal, while, according to the standard We can understand that, if only one indicator of coal ash melting point evaluation, as long as the ash melting point greater than or equal to 1250 ℃ can be used on coal gasification, which is obviously unreasonable.
Table 1 national standard coal gasification ash melting point indicators defined

     In the choice of the actual trial burn coal gasification process found that some coal although not up to standard indicators specified in the ash melting point, but still can get a better effect of gasification, the reason is mainly affected by coal gasification Reactivity, gasification ash fusion temperature indicator should be changed as coal gasification reactivity varies, strong coal gasification reactivity, its ash melting point indicators can be appropriately reduced.
     Determining whether a coal gasification ash melting point meets the requirements, you first need to be determined gasification reactivity, and then according to the determination result of defining the coal for gasification ash melting point indicator, and finally the index compared with the ash melting point, If it is higher than the melting point of coal ash ash melting point indicator, then only in terms of ash melting point, which can be used in coal gasification. The specific definition is as follows: First, the coal samples taken from the CO2 reduction ratio (α) is determined, and its CO2 reduction ratio (α) corresponding to a percentage of A is defined based on the reaction temperature, the oxide layer considered furnace Temperature fluctuation may occur, based on the increase at this temperature is defined as the ash melting point B ℃ indicators. Where A value is equivalent to the CO2 reduction rate of gasification, gasification based on actual data estimates, the gasifier CO2 reduction rate (α) is generally 60%, so the ash melting point indicator A defining methods to take 60% more appropriate. When the coal gasification reactivity than 60%, the gasification reaction rate there was a substantial increase in the oxide layer at this time if the temperature rises, the resulting reduction intense endothermic reduction reaction will also increase, thereby to maintain a relatively stable temperature oxide layer, so the oxide layer temperature fluctuations generally does not appear, I believe that the B value in order to get 100 ℃ appropriate. Table 2 is based on the method described above for several typical coal ash melting point indicators defined data.
Table 2 atmospheric fixed bed gasification ash fusion temperature indicators defined

3 gasification reactivity effects on gasification furnace
3.1 pairs of oxidation and anoxic layers of
     Atmospheric fixed bed gasifier each functional layer is generally divided into dry layer, carbonization layer, reducing layer, the oxide layer and a layer of ash, coal furnace in the event of material runs from top to bottom, in the dry layer and the layer after drying and distillation distillation After a semi-focus state of the coal, as a gasifying agent, air and water vapor into the furnace from the bottom drum, in high temperature conditions, and into the gasification reaction layer (oxide layer and a reduction layer) in the state of semi-coke Coal redox reaction of CO and H2 gas as the main component, the reaction is more complex gasifier, the simplified pressure fixed bed gasifier with the main reaction (1) (2) (3) reaction approximate representation, where △ H for the reaction enthalpy of the chemical reaction.
     C + O2 = CO2; △ H = -409 KJ / mol (1)
     CO2 + C = 2CO; △ H = 162KJ/mol (2)
     C + H2O = CO + H2; △ H = 119 KJ / mol (3)
     Reaction (1) the oxide layer, the next step of the reduction reaction of the reactants to provide the necessary heat of reaction of CO2 and, (2) (3) reducing the reaction layer, the endothermic reaction, according to the actual atmospheric pressure fixed bed gasification data estimates, the gasifier CO2 reduction rate (α) of 60%, the gasification furnace is more complete. Normal gasification, the oxide layer is approximately equal to the temperature coal gasification CO2 reduction ratio (α) of 60% the reaction temperature, the strong reactivity of coal gasification, which in the gasifier achieve adequate gasification reaction of the corresponding lower temperature. For example, in Table 2 of the oxide layer Datong coal temperature of about about 1100 ℃, the temperature of the oxide layer Huolinhe lignite is only about 810 ℃ or so. Reduction zone endothermic reduction reaction faster, the temperature of the layer close to the oxide layer by the temperature begins, not gasified rapidly to reaction temperature.
3.2 gasification reactivity for distillation, drying, and a gas temperature of furnace
     Carried out within the gasifier coal distillation and drying, in some angles can be referred to as coal pyrolysis, the literature [10] describes the coal pyrolysis process can be divided into three stages: the first stage, the room temperature -300 ℃ as drying degassing phase, which did not change the basic shape of coal, lignite occurs above 200 ℃ decarboxylation reaction, about 300 ℃ around pyrolysis, bituminous coal and anthracite generally does not change, dehydration occurs mainly in the 120 ℃ ago , 200 ℃ complete CH4, CO2 and N2 gases such removal. The second phase ,300-600 ℃, at this stage in order to depolymerization and decomposition-based, semi-coke formation, generate a lot of gas and tar and other volatiles. Bituminous coal of about 350 ℃ begins to soften, melt, flow and expansion until cured to form a gas, liquid, solid-phase coexistence gelatinous body ,500-600 ℃ gelatinous body decomposition, condensation, curing semi-coke formation. Lignite colloid formation stage does not exist, only the dramatic decomposition precipitated a large gas and tar, powdered semi-coke formation. Phase III ,600-1000 ℃, mainly in the polycondensation reaction, into a semi-coke, coke, tar precipitation was minimal at this stage, the generated gas is mainly H2 and CH4.
     Dry distillation of coal in the gasifier and the dry layer within the layer of the heat transfer process, including the gas-solid phase, the solid phase between the different levels of conduction, convection and radiation heat transfer, etc., mainly in the gas-solid phase of the convective heat based. Strong coal gasification reactivity, the oxide layer in the gasifier temperature is lower, the reduction layer temperature and reduction layer resulting gas temperature is relatively low. Datong coal layers such as the reduction temperature range of approximately 800-1100 ℃, which reduced layer produced gas temperature is about 800 ℃, Huolinhe lignite reduced layer temperature range of about 650-810 ℃, which produce gas temperature reduction layer about 650 ℃. It can be seen that the two large differences gasification reactivity of coal dry distillation in the gasification furnace and the drying temperature of the heat transfer away from the heat source is large, distillation and drying effects are quite different.
     Gasifier and the gas temperature is reduced layer mainly produced distillation and drying gas temperature and the amount of heat-related processes, the stronger reactivity gasification of coal, the reduction zone in the gasifier temperature, the gas produced is low, and Generally, the higher reactivity of the coal gasification moisture and volatile content is high, the distillation of coal in the gasifier and drying processes the amount of heat is large, resulting in the furnace of the gas temperature is lower. For example in two gasification, pyrolysis of bituminous coal distillation section of the upper part of the first phase for the pyrolysis of the second stage distillation section of the lower portion, the third stage of pyrolysis layer near the restored parts of carbonization layer , the lower section of the furnace of the gas temperature is generally about 450-500 ℃. Lignite Pyrolysis Almost all of the first stage of distillation segment, while the second stage of pyrolysis layer near the restored parts of carbonization layer; pyrolysis layer and the third stage of the reduction reaction at the same time reducing carried out under paragraph furnace Gas temperature is generally about 200-250 ℃.
4 Conclusion
     (1) coal gasification reactivity is an important feature, mainly by the degree of graphitization of coal, fixed carbon content, volatile content and alkaline factors and other factors. At atmospheric pressure fixed bed coal gasifier coal gasification reactions include reactions with CO2 and water vapor react with coal, lignite and bituminous steam reaction rate is 2-5 times the CO2 reaction rate; anthracite steam CO2 reaction rate than the reaction rate about 10 times larger.
     (2) gasification ash fusion temperature indicators should be combined with its gasification reactivity OK to gasify coal CO2 reduction rate (α) of 60% plus 100 ℃ reaction temperature is defined as the ash fusion temperature indicators are more appropriate.
     (3) strong gasification reactivity of coal gasifier temperature is lower oxide layer, the reduction layer temperature and reduction layer resulting gas temperature is relatively low, carbonization temperature layers and dry layer is also low, But under normal circumstances, strong activity of the coal gasification and the volatile content of the water is high, the distillation of coal in the gasifier and drying processes the amount of heat is large, resulting in the furnace of the gas temperature is lower.