Combustion of Coal Mine Ventilation Air Methane in Thermal Reverse-flow Reactor
ZHENG Bin  LIU Yong-qi  LIU Rui-xiang College of Traffic and Vehicle Engineering Shandong University of Technology
Zibo City, China
zhengbin@sdut.edu
JI Li-xia
College of Foreign Languages and Literature
Shandong University
Jinan City, China
jilixia888@tom
Abstract—Combustion of coal mine ventilation air meth ane (VAM) was investigated in a th ermal reverse-flow reactor. Effects of CH4 concentration, VAM flow and temperature were studied. T    e results
s ow t at combustion of coal mine ventilation air methane be achieved for methane concentration of 0.2%~0.8% in reactor. The conversion rates of CH4 are all above 99%.Some h eat could be recovered. With th e increase of CH4 concentration and VAM flow, th e temperature of middle area increase, th e volume of h igh temperature increase. It is to improve conversion rate of CH4. T    e lowest combustion temperature is 880ć.
Keywords- coal mine ventilation air methane; thermal reverse-flow reactor; combustion; conversion rate
I.I NTRODUCTION
Worldwide coal mine methane (CMM) emissions make up approximately 8% of the world’s anthropogenic methane emissions, the quantity of methane emissions from coal mining alone was over 25 million ton every year. Approximately 70% (90% in China) of methane emissions are from coal mine ventilation air methane (VAM). Ventilation air methane is not only a greenhouse gas but also a wasted energy resource if not utilized. The net calorific power of CH4 emission in VAM every year is equal to that of 33.7 million ton standard coal. As a greenhouse gas, CH4 is over 21 times more effective in trapping heat in the atmosphere than carbon dioxide over a 100-year period. CH4 (17%) is
the second largest contributor to global warming after CO2 (55%). Thus recovering and utilizing CH4 properly in VAM is significant in both energy-saving and environment protection [1-4].
CH4 concentration in ventilation air methane is usually below 1%. The inflammability limit concentration of CH4 is 4.5%~15%. When the concentration is below 4.5%, it can’t be ignited or keep burning. So CH4 in VAM is hard to utilize. There are two main utilization techniques. One is CFRR. It employs catalyst to decrease the autoignition temperature of CH4 and makes CH4 oxidized. The reaction temperature is reduced in this technique, but catalyst is expensive and its reactive activity is greatly influenced by temperature. The processing is complex. The other is TFRR. The heat retainer is heated to the autoignition temperature of CH4 and CH4 is oxidized. The reaction temperature is a little higher in this technology, but the conversion rate of CH4 is higher. Simple making and low cost favors its large-scale implementation. Now only a few foreign scholars have made some study on utilization [5-8]. Nearly no relevant studies have been published in china. Combustion of VAM was investigated in a thermal reverse-flow reactor and effects of operating parameters were studied in this paper.
II.E XPERIMENTS
The thermal reverse-flow reactor shown by Figure 1 consists of a combustion reactor, four valves and
a heat output system. The body dimension of the combustion reactor is 2m×1m×1m. The inner part of reactor is honeycomb ceramics heat retainer and the inner surface of the reactor is ceramics fabric insulation, which makes heat dissipation from reactor surface impossible. An electric heater is in the middle of the reactor. Twelve thermocouples are laid on the reactor axis. The inner part of the two heat-exchangers is called middle area and four thermocouples are laid here. The outboard of the exchangers is called preheating area, and there are four temperature measuring points each. The temperature signals are transmitted a computer momentarily. The change of air flow is controlled by four solenoid valves, two forming a group. When valve group 1 opens, valve group 2 closes. The flow direction is from left to right. After a half cyclic period, valve group 1 closes and valve group 2 opens. The direction is from right to left. It ensures symmetry of the temperature profile in reactor. The heat output system consists of a drum
and two heat-exchangers which are fixed symmetrically.
Figure 1. Schematic diagram of the experimental apparatus (1-electric heater; 2-ceramic heat accumulator; 3-heater exchanger)
(No.Y2006F63), Zibo Research Programme (No.20062502).
978-1-4244-2487-0/09/$25.00 ©2009 IEEE
T e m p e r a t u r e  (䛐)
A xis (m)
Figure 2. Temperature profiles in reactor  (experimental condition: C CH4˙
0.4%; L VAM =90m 3·h -1; t=180s)
Operating process: The reactor is preheated by electric heater and when the temperature in the middle area is above 950ć, VAM  is added. VAM  is rapidly combusted in the middle high temperature area. The heat of combustion is transferred to ceramics heat retainer and heat-exchanger. Simulated ventilation air methane is produced by natural gas and air. CH 4concentration of natural gas is 99.9%. CH 4concentration of simulated ventilation air methane (C CH4) is 0.2%~0.8%. The flow of simulated ventilation air methane (L VAM ) is 90 m 3·h -1~180 m 3·h
-1. The cyclic period (t) is 60s~180s
III.
R ESULTS AND DISCUSSION
A.Temperature Profile Characteristics in Reactor
Fig.2 shows the temperature profile in thermal reverse-flow reactor of the two change direction times in one cyclic period. It is clear that the temperature profile is symmetrical, with middle high and two ends low. The temperature of the middle area is higher and stable, which ensures combustion. The
temperature of the two ends changes little, with no more than 20ć difference. It shows the heat of flue gas loss is low and the heat of combustion is recovered. The temperature profile,
O u t l e t  C H 4 c o n c e n t r a t i o n  (%)
Figure 3. Variations of combustion at various temperatures of middle area
(experimental condition: L VAM =90 m 3·h -1; t=90s)
T e m p e r a t u r e  (䛐)
A xis (m )
Figure 4. Variations of temperature profile at various CH 4 concentrations
(experimental condition: L VAM =90 m 3·h -1; t=120s)
orthokinetic changing with the flow direction, is always dynamic balance. Temperature is unstable in the edge of the middle area. The temperature difference could be as high as 450ć~500ć. This is becau
se the heat-exchangers are set in the edge of the middle area. The heat of combustion is transferred to heat exchangers. If cyclic period time is too long, great amount of heat will is transferred to heat exchanger. This could cause the temperature in the middle area lower than the lowest temperature of combustion and then “flameout” will occur in reactor. Thus, it is important to choose proper cyclic period to ensure the combustion going stably.
B.Effect of Temperature on Combustion
Temperature is an important condition in the realization of VAM combustion [9]. Fig.3 shows variations of combustion at various temperatures of middle area. It is obvious that when the temperature is below 870ć, the concentration of methane at the outlet is the same with that at the inlet. This shows that methane can’t be combusted at such temperature. When above 880ć, the concentration of methane is nearly zero at the outlet, which that this temperature ensures the complete combustion of methane. The lowest temperature of combustion is 880ć,and the concentration of methane has no effect on it.
S a t u r a t i o n  w a t e r  t e m p e r a t u r e  (䛐)
CH 4 concentration (%)
Figure 5. Variations of saturation water temperature at various CH 4concentrations (experimental condition: L VAM =90 m 3·h -1; t=120s)
TABLE I. CH4 CONVERSION RATE AT DIFFERENT REACTION
CONDITIONS
CH4 concentration
(%)
VAM
flow
(m3·h-1)
cyclic
period
(s)
CH4
conversion
rate (%)
0.2 90 120 99.2
0.2 180 120 99.4
0.4 90 120 99.2
0.4 115 120 99.3
0.4 130 180 99.3
0.4 180 180 99.4
0.6 90 90 99.1reactor technology
0.6 180 90 99.4
0.8 90 60 99.0
0.8 115 60 99.2
0.8 130 180 99.4
0.8 180 120 99.6
C.Effect of CH4 Concentration on Combustion
Fig.4 shows variations of temperature in reactor at various CH4 concentrations. It is clear that with the increase of concentration, the temperature in the middle area rises, the high temperature area becomes large and combustion area becomes wide, which favor combustion reaction. The temperature of the preheating areas changes little and the difference between temperature at inlet and outlet changes slightly, showing that flue gas loss hardly changes and that CH4 concentration variation has no effect on it. The extra heat of combustion with the increase of CH4 concentration is recovered totally by the heat output system. The higher the concentration is, the higher the temperature of saturation water in the drum, as is shown by Figure 5. When the concentration is 0.8%, the temperature of saturation water rises to 140ć. The heat could be exported in the form of steam.
When CH4 concentration is 0.2%, the temperature in the middle area is about 1000ć, significantly higher than the lowest combustion temperature. The conversion rate of CH4 is 99.2% (Table ĉ). It shows that almost total combustion of methane can be achieved, and the additional heat isn’t added in reactor. Combustion could be maintained at low concentration of CH4.
Tab. ĉ shows CH4 conversion rate at different reaction conditions. It is obvious that when CH4 concentration is 0.2%~0.8%, the rates are all above 99%. It shows that at experimental conditions, VAM could maintain combustion reaction in reactor.
D.Effect of VAM Flow on Combustion
Fig.6 shows the variations of temperature profile at various VAM flows. It is clear that with the increase of VAM flow, the temperature in the middle area rises and the high temperature area becomes wide. When the flow rises to 180 m3·h-1 from 90 m3·h-1, the highest temperature rises by 50ć. The reason is that the increase of VAM flow makes amount of CH4 combustion increase in one unit of time and more heat is released from the reaction. But the difference of temperature at inlet and that at outlet becomes high. With the increase of flow, the heat that is discharged increase. Flue gas loss increases. The difference between temperature at the inlet and that at the outlet could be reduced by choosing proper cyclic period and heat loss could be reduced at the same.
IV.C ONCLUSIONS
When CH4 concentration is 0.2%, the temperature in the middle area is about 1000ć. Almost total combustion of VAM can be achieved, and the additional heat isn’t added in reactor. CH4 conversion rate is as high as 99.2%. Temperature is an important condition. The lowest temperature of combustion reaction is 880ć, and the concentration has no effect on it. Choosing proper cyclic period is important to maintain combustion reaction at low concentration.
When CH4 concentration is 0.2%~0.8%, with the increase of CH4 concentration, the temperature rises in the middle area, the area of high temperature becomes large and combustion zone becomes wide, which favors combustion reaction. CH4 conversion rates are all above 99%. Heat of combustion could be recovered. When CH4 concentration is 0.8%, the temperature of saturation water in the drum can rise to 140ć. With the increase of VAM flow, the temperature in the middle area rises and the high temperature area becomes wide. But flue gas loss increase.
Thermal reverse-flow combustion technology is a better method to decrease the pollution of ventilation air methane, and recover heat.
A CKNOWLEDGMENT
This research was supported by Shandong Natural Science Foundation (No.Y2006F63) and Zibo Research Programme (No.20062502).
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