Experiment Research on Gasification Character of Pulverized Coal at Medium Temperature

In this paper, effect of gasification temperature and residence time on gasification characteristics of typical pulverized coals under atmosphere pressure were studied on a one-dimensional electric heating drop tube furnace system to evaluate gasification characteristics of pulverized coal at medium temperature. The results show that the gasification rate increases with the raising of the gasification temperature under the temperature among 1150°C and 1450°C, and as the temperature is above 1350°C, the increasing rate become slower. Cold gas efficiency and carbon conversion meet the maximum value when the temperature approaches the ash fusion point of coal. With the further rise of the temperature, ash on the surface of the char starts to be melted, the gasification reaction rate decreases, due to the hindrance of the effective contact between coal char and gasification agent. The gasification efficiency would become steady when the residence time exceeds 8s.


Introduction
Issues on natural resource and environment become increasingly severely in China, due to the long-term use of traditional coal combustion.Integrated gasification combined cycle (IGCC) takes advantage on high efficient and environmentally-friendly, is a new power generation technology.Gasified is one of the key devices of IGCC power generation system.Compared to the other gasification technologies, entrained flow gasification technology get rapid development for its advantages of high gasification intensity, high carbon conversion and be easily practiced on a large scale.Influencing factors on gasification efficiency of entrained flow gasified are many, effect of gasification temperature and residence time on gasification characteristics of typical pulverized coals were studied on a one-dimensional electric heating drop tube furnace system under the conditions of atmosphere pressure and medium temperature from 1150℃ to 1450℃ [1][2][3].

Experimental System
Figure 1 is the picture of experimental system, which is consisted of high temperature tube electric heating fur-nace, ash collected system and syngas cleaned and detected system.The furnace is made of corundum tube(2.1 m in length, 60 mm in inner diameter), the adjust temperature is up to 1550℃, the gas analysis equipment is a infrared gas analyzer produced by Wuhan Cubic Optoelectronics Technology Co., Ltd., which can proceed real time measurement on CO, CO 2 , H 2 , CH 4 and O 2 .Copyright © 2013 SciRes.EPE

Work Conditions
Datong bituminous coal, a typical bituminous coal in China was selected for the experiment on gasification characteristic of pulverized coal at medium temperature, and effect of gasification temperature and residence time were mainly considered.The average particle size of pulverized coal is more or less than 100 µm (under 150 µm) and the feeding rate is about 0.2 kg/h.The proximate analysis and the elemental analysis of the coal is in Table 1, the experimental operating conditions are in Table 2.

Experimental Data Processing Methods
It's relatively difficult to measure the syngas directly as its high temperature.So the total amount of dry gas could be obtained by the calculation of N 2 balance, the divided relationship between the N 2 amount in the inlet and the N 2 percentage of dry gas in the outlet:  is the gas volume fraction of N 2 in the dry gas.
As the experiment proceeded in one-dimensional drop tube furnace, residence time of pulverized coal staying in the furnace is equal to the residence time of reactivity gas: thereinto, L is the length of the furnace, r is the inner diameter of the furnace, T is the temperature of the furnace; and is the flow of O 2 in the inlet.

, in O
Oxygen element of the gas products is mainly from pulverized coal and the injected O 2 in the inlet, such as CO, CO 2 and H 2 O. Therefore, the amount of vapor in the syngas can be calculated by the balance of O 2 element, the amount of vapor in the syngas(mol) = oxygen amount of the injected O 2 + oxygen amount of coal -the amount of the oxygen element from carbon dioxide and carbon monoxide in the gas, the calculation formula is Thereinto, α is the gas volume fraction of in the dry gas.
Carbon conversion η is the index to calculate how much carbon is transferred to the content in gas during the gasification process: 12 thereinto, V is gas volume, M is the amount of coal; and C  is the amount of carbon in the coal.
Cold gas efficiency η d is the heat value proportion of fuel and gas products, the calculation formula are: 2 4 , 100% thereinto, h is the gas heat value, , f coal h is the heat value of coal.

Experimental Results and Discussions
In an actually running entrained flow gasified, the controllable parameter is the feeding rate of coal and O 2 , residence time and the amount of steam.Commonly, gasification temperature and the atmosphere inside the furnace can be adjusted by changing the rate of O/C.This adjustment would change the processes of gasification reactivity and the content of syngas, which would definitely change the residence time of gasification agent in the furnace.Therefore, research of gasification temperature and residence time on the gasification characteristic of coal has a significant meaning for the design and following operation of the gasified [4].

Effect of Temperature and Residence Time on the Composition of Syngas
The method of changing injecting rate of N 2 to change the residence time of pulverized coal staying in the furnace was selected in this paper, the range is from 3s to 10s. Figure 2 shows the effect of gasification temperature and residence time on the composition of syngas [5].As the figure shows that the composition of th changes with residence time under different gasification temperature conditions, the content of CO increases with the increasing of residence time.When the temperature is higher than 1300℃, the increasing rate becomes smaller and the content of H 2 slightly decreases with the increasing of the residence time, while the content of CO 2 generally remains steady.When the gasification temperature is lower than 1300℃, the increasing rate of CO is relatively bigger, and the fluctuation of H 2 and CO 2 is quite drastic as well.The content of CO 2 falls down with the increase of residence time.With the increase of residence time, the content of H 2 would increase first and then decrease while the temperature is 1250℃.This is mainly caused by the reactivity of H 2 O-C is higher than CO 2 -C.As the residence time decreases to a certain extent, combustion reactions are mainly happened in the furnace and most of coal char would escape from the furnace directly without reacting with H 2 O and CO 2 .So, most of H 2 are mainly produced by the paralysis of coal at this condition.With the increase of residence time, the reacting time between H 2 O-C and CO 2 -C would be extended correspondingly.Followed by the large products of CO 2 and CO, the volume proportion of H 2 in the syngas would be decreased.Due to the reactivity of H 2 O-C is bigger than CO 2 -C, the volume proportion of H 2 in the syngas increases again with the further increasing of residence time.It can also be seen from the Figure that the volume proportion of CH 4 gets down gradually with the rise of the temperature.When the temperature is above than 1350℃, the volume proportion of CH 4 drops to 0, which indicates that CH 4 is mainly from the volatile components, and the higher temperature of gasification reaction is, the easier CH 4 be decomposed [6,7].

Time on the Carbon Conversion and Cold
Gas Efficiency re 3 shows the effect time on carbon conversion and cold gas efficiency of Datong coal.It can be seen from Figure that the carbon conversion and cold gas efficiency increases with the increase of residence time.When it is lower than 1350℃, carbon conversion and cold gas efficiency increases with the increasing of gasification temperature.When it is higher than 1450℃, carbon conversion and cold gas efficiency almost remains the same as that at 1350℃, even slightly lower.The main reason is that the ash would be appeared and accumulated on the surface of coal char particles with the proceeding of gasification reactions, when the gasification temperature is higher than the ash fusion temperature, it would be transferred to liquid phase.It tends to shrink as globular material on the surface

Conclusions
re is one of the most important facto me also has a great influence on gasi temperature approaches to the coal ash fu

Figure 1 .
Figure 1.The schematic picture of the experimental process.

Figure 2 .The effect idence time on e syngas 3 . 2 .
Figure 2.The effect idence time on

Figure 3 .
Figure 3.Effect of Temperature and Residence Time on the