Balancing the Environmental Benefits of Removing NO2 from Road Tunnel Emissions with the Environmental Consequences of the Generation of Electricity for the Operation of the NO2 Removal Technologies by Fossil Fuel and Nuclear Driven Power Stations
1
Doctor Atsushi Katatani, Panasonic Ecology Systems Co., Ltd,
Professor Arnold Dix, Scientist and Lawyer, Tunnel advisor, University of West Sydney.
1
（May 2010)
1
Abstract:
With increased interest in environmental protection from road tunnel emissions NO2-denitrification (de-NO2 or NO2 removal)is gaining popularity in Japan (NO2; nitrogen dioxide). Initially it was thought that NOx (nitrogen oxides) removal(de-NOx) from tunnel air would be feasible however this concept has been abandoned inJapan because of its high cost. In Japan it is generally accepted that the ratio of NO2/NOx in tunnel ehaust is between several percent to 20 percent. Although most of NOx in tunnel gas is mainly NO (nitrogen monoxide), NO2 is more harmful to human health. While discussions about the benefits of NO2 removal from tunnel air have become increasingly common there has been little regard for the environmental consequences of the NOx and sulfur oxides (SOx) which are generated by the power stations used to generate the electricity for the denitrification plants for tunnels. This paper examines the net effect of these pollutants on the environment. Both a Japanese case study and a Chinese case study are simulated in order to determine the net effect on NOx and SOx. Examined are a tunnel operating normal exhaust fans only (type 1), a tunnel with electrostatic precipitators and normal exhaust fans (type 2), and a tunnel with ESPs, de-NO2 and traditional ventilation (type 3). And finally type 4 with de-NOx with ESPs and exhaust fans. The results demonstrate that the introduction of denitrification technology in Japanese road tunnels results in a reduction in total NOx and SOx but that the use of such technology in China increases the total amounts. The greenhouse gas implications of using the technology were not considered as part of this study.
Keywords: Tunnel exhaust; SPM; NOx; NO2; Denitrification
1
- Introduction
Tunnel ventilation systems routinely ensure that the regulated air quality is maintained in tunnels.1) In recent years some tunnel ventilation stations have been equipped with air purification technology such as ESPs (electrostatic precipitators) and NO2 removal technologies.2)-5) This paper examines the local environmental consequences of utilising air cleaning technologies by balancing the environmental benefits achieved at the tunnel ventilation point with the NOx and SOx emissions from power stations including thermalpower stations. An example of a Japanese case and a Chinese case are used to illustrate the issues.
- Removal of Contaminants from Tunnel Air
- SPM (suspended particle matter) and NOx
ESPs are used to remove suspended particles from tunnel air. The use of such technologies for external environmental purposes has occurred in Japan in excess of 10 years6). In Japan SPM is defined as particles of 10 micron metres or less (‘PM10’). Particles of 2.5 micron metres or less are known as PM2.5. The principle of ESPsis that corona discharge by high voltage in ionisers electrifies the suspended particles and in a strong electric field the particles are captured. The capture process of ionised particles is illustrated in Figure 1.
Fig.1 Typical structure of ESPs
The ESP technology has been widely applied to purifying tunnel air7),cleaning exhaust from thermal power plants8) and other processes in other industries. In recent years NO2 removal systems have enabled NO2 to be removed from tunnel exhaust air. These systems rely upon an efficient ESP to remove the particles prior to the NO2 removal. NO2 removal systemshave been installed on the Shinjuku line tunnel in the Central Circular Routein Tokyo. 5)
In Japan the principle of de-NO2 systems is that NO2 gas molecules in tunnel exhaust are adsorbed or absorbed as they pass through an adsorbent/absorbent material such as activated carbon. This process depends on the physical characteristics of adsorbing gas molecules2)(refer to figure 2). The de-NO2 systems are less expensive, require less space and use significantly less power 3)than de-NOx systems.
Fig.2 Theory of typical absorbent
There is no regulation of NOx or NO concentration in Japan. However there are environmental regulations with respect to the concentration of NO2. In Japan NO2 concentration must not exceed a zone between 0.04ppm and 0.06ppm as shown in Table 1.
Concentration of daily averageSPM / Not exceed 0.10 mg/m3
NO2 / Within the zone from 0.04 ppm to 0.06 ppm ,or below that zone
SO2 / Not exceed 0.04 ppm
Table 1 Environmental Quality Standards of Japan
In Japan it is considered that NO2 is more harmful to the human body than NO which ACGIH (American Conference of Government Industrial Hygienist) regulatesas the value of 25ppm or less.
In Japan it is generally considered that the ratio of NOx components in tunnel air is about 90% NO and about 10% NO23). A development policy on denitrification regarding tunnel air exhaust by MLIT (the Ministry of Land, Infrastructure, Transport and Tourism of Japan) is that de-NO2 systems have a better cost benefit ratio than de-NOx systems. Since 2004 MLIT has directed that only NO2 systems be used on the basis of their environmental performance from a cost/benefit perspective.
2.2.SOx
Because both motor vehicles and thermal power stations burn fossil fuels, in this sense, the main component of SOx is SO2in both sources. Furthermore NO2 adsorbent or absorbent for tunnel gas purificationcan also remove SOx.
Although removing sulfur from car fuels is common worldwide, a comparatively large amount of SOx is discharged from thermal power plants using coal fuel in some countries without fuel gas desulphurization.
- Conditions for Balance in Simulations
- Air quality in urban tunnels in Japan
Although the specific air quality characteristics of a tunnel may change from location to location, the air quality in Japanese road tunnels can generally be summarized as per table 2 below. 10)
Concentration of daily averageSPM (Suspended Particle Matter) / Approx. 0.2 mg/m3
NOx (Nitrogen Oxides) / Approx. 1.0 ppm
NO2 (Nitrogen Dioxide) / Approx. 0.1 ppm
NO(Nitrogen Mono-oxide) / Approx. 0.9 ppm
SOx(Sulfur Oxides) / Approx. 0.05 ppm
Table 2 Typical Air Quality in Urban Tunnels in Japan
3.2.Typical removal ratios on tunnel exhaust purifiers
The definition of removal ratio is as follows.
Removal Ratio = (1 – B/A)*100 %
A; The inlet concentration of a component
B; The outlet concentration of the same component
3.2.1.Removal of SPM only
SPM can be removed by ESPs whose specifications with removal ratios were issued by three companies and one government ministry as follows.
1)NEXCO (Nippon Expressway Company)
SPM removal ratio ; 90%
2)MEC (Metropolitan Expressway Company)
SPM removal ratio ; 80%
3)HEC (Hanshin Expressway Company)
SPM removal ratio ; 80%
4)MLIT
SPM removal ratio ; 80%
3.2.2.Removal of NO2
NO2 can be removed by de-NO2 systems including ESPs whose specifications with removal ratios were issued by MLIT, MEC. All of their specifications are the same as follows.
SPM removal ratio ; 80%
NO2 removal ratio ; 90%
SOx (SO2) removal ratio ; No requirement
3.2.3.Removal of NOx
NOx can be removed by de-NOx systems including ESPs. The only one specification of the de-NOx systems was issued by MLITaround 1997 as follows. De-NOx systems for cleaning tunnel exhaust shall not be used in Japan because of expensive cost.
SPM removal ratio ; 80%
NO2 removal ratio ; 80%
NO removal ratio ; 80%
SOx (SO2) removal ratio ; No requirement
3.3.Exhaust treatment options for tunnel ventilation stations
For the purposes of this analysis there are four ventilation station treatment options considered.
3.3.1.Type 1 with exhaust fans only
Type 1 is shown in Fig.3.
Fig.3 Type 1 ventilation station with ventilation
3.3.2.Type 2 ventilation station with additional ESP systems
Type 2 including fans and ESPs is shown in Fig.4.
Fig.4 Type 2 of ESP systems
3.3.3.Type 3 ventilation station with ESP and De-NO2 systems
Type 3 including fans, ESPs and absorbent (adsorbent) modules is shown in Fig.5.
Fig.5 Type 3 ofDe-NO2 systems
3.3.4.Type 4 ventilation station with ESP and De-NOx systems
Type 4 including fans, ESPs, absorbent (or adsorbent) modules, and ionisers is shown in Fig.6.
Fig.6 Type 4 of De-NOx systems
3.3.5.Initial costs for the four ventilation types
When the cost of Type 1 is based as 100%, the approximate costs of other three types are as follows.
( Type 1; 100 % as a base )
Type 2; 200 %
Type 3; 350 %
Type4; 550 % in Japan(In some cases, 700 % in other countries whose NOx concentration is much higher than that of Japan, which means the necessity for the larger power consumption on ozonisers for oxidizing NO into NO2.)
3.4.Volume of air exhausted into the atmosphere
In this paper the discharge volume of air exhausted into the atmosphere is used as 750,000 N m3/h (208 m3/s) for the convenience of calculations. All calculations in relation to energy consumption and the related absolute amount of SOx and NOx including NO2 and NO are based uponthat figure. It is generally considered a fair approximation to scale the results of this paper in order to calculate corresponding values for discharge rates at volumes other than 208 m3/s.
3.5.Power consumption for exhaust ventilation fans
The power consumption as fan motors input L [kW] is shown as follows1).
L = Q * P / (1,000 * ηf * ηm )
Q; Gas flow [m3/s] ( The value of 208 m3/s is used.)
P; Total pressure [Pa]
ηf ; Fan efficiency whose typical value of 0.8 is used
ηm ; Motor efficiency whose typical value of 0.9 is used
The atmospheric conditions for the purposes of these calculations are 101.3 [kPa] with a temperature of 20 degrees centigrade.
3.6.Power consumption for each type of purifier
Power consumption for each type (Type 1 to Type 4) is described in Table 3. These calculations are based upon data in the reference documents1,4,10,11). The power consumption of ESPs is derived from the NEXCO standard specifications for tunnel ESP systems11).
Although NO2removal systems do not need electricity because the process occurs by passing tunnel air through absorbent or adsorbent modules3,10), the NOx removal systems demand a large amount ofelectricity because NO gas molecules have to be oxidised into NO2.The oxidation process uses ozonisers or other oxidation equipment in order to promote adsorption in absorbent or adsorbent in case of de-NOx systems2,12). The power consumption 150kW on Type 4 (NOx removal systems) is obtained from Table 2, the reference 2) andthe reference 12). An experimental report as power consumption of 3.5kW per gas flow 1.94 m3/s under the concentration of NOx 2.5ppm is in the documents.This makes 0.72kW/(m3/s)/(ppm NOx) on oxidation power in theNOx removal type.
Type 1 / Type 2 / Type 3 / Type 4Fan / ESP / De-NO2 / De-NOx
Exhaust fans / Total pressure [Pa] / 340 / 690 / 1,140 / 1,340
Motor input [kW] / 98 / 200 / 330 / 388
Purifiers / ESPs [kW] / (None) / 23 / 23 / 23
Absorbent or adsorbent [kW] / (None) / (None) / 0 / 0
Oxidation [kW] / (None) / (None) / (None) / 150
Total power consumption [kW] / 98 / 223 / 353 / 561
Table 3 Power consumption for each type
3.7.Purification characteristics for each ventilation type
Removal ratios for tunnel exhaust from Type 1 to Type 4 are shown in Table 4. Type 3 of NO2removal indicates slight reduction of NO3) and a fair decrease of SO2. Type 4 of NOx removalperforms a high reduction of SO22).
Type 1 / Type 2 / Type 3 / Type 4Fan / ESP / De-NO2 / De-NOx
SPM [%] / 0 / 80 / 80 / 80
NOx / NO2 [%] / 0 / 0 / 90 / 80
NO[%] / 0 / 0 / 2 / 80
SOx / SO2 [%] / 0 / 0 / 50 / 90
Table 4 Removal ratios for each type
3.8.NOx and SOx generation at thermal power plants
NOx and SOx emissions are sometimes expressed with a unit of [g/kWh] called“unit of energy (or unit of power)”. The Federation of Electric Power Companies of Japan (F.E.P.C.J.) announced that an actual result of NOx and SOx generationat thermal power plants in Japan in 2005 was with 0.3 g/kWh(NOx) and 0.2 g/kWh(SOx). (It is normal that the fuel combustion processes in thermal power plants generates NOx and SOx.)
OECD (Organization for Economic Cooperation and Development) has reported the data of energy consumption in the world every year as “Energy Balances of OECD Countries” and “Environmental Data Compendium”. The F.E.P.C.J. has arranged the OECD data in order to clarify thecomparative position of differing countriesin the world. As a result Fig.7 of SOx and NOx emissions from thermal power plants has been obtained. This data was correct on or about the year 2000.
It is apparent that the values of NOx and SOx emissions in Japan are extremely smallcompared with the other countries noted. This is primarily because thermal power plants in Japan are equipped with SOx and NOx removal systems with high removal efficiency.
The second reason is that about 70% of thermal power plants in Japan are classified as LNG combustion plants which scarcely generate NOx and SOx. On the other hand where electricity is generated by burning coalor oil the combustion plants produce a much higher amount of NOx and SOxifgas purifiers are not used.
Fig.7SOx and NOx emissions at thermal power plants
The comparative contribution of differing types of power plants in Japan is summarized in Table 5. This summary is derived from OECD research. 13)
Power Source / Generation [TWh] / Ratio [%]Coal (Thermal) / 309 / 29
Petroleum (Thermal) / 146 / 13
LNG Gas (Thermal) / 231 / 21
Nuclear / 304 / 28
Hydro / 78 / 7
Others / 26 / 2
TOTAL / 1,094 / 100
Table 5 Classification of power plants in Japan
The contribution of thermal power plants to Japanese electricitywas 63% in the year 2005. Accordingly NOx and SOx emissions can be easily calculated as 0.19 g/kWh(NOx) and 0.13 g/kWh(SOx). It should be noted that most NOx generated by thermal power plants is composed of NO. (In thermal power plants there is very little NO2produced despite NO generation.)
- Contaminant balance simulations in case of Japan
A contaminantbalance calculation using the Japanese data noted above has been undertakenusing the data from the section 3.1 to 3.8. The results are shown in Table 6whichmeans decreases or increases in“gram per one hour” of contaminant substances. The negative values mean decreased amounts and the positive values mean increased amounts.The percent values in Table 6 mean the ratios of the initial costs for the three cases except Type 1(base).
Type 1 / Type 2 / Type 3 / Type 4Fan / ESP / De-NO2 / De-NOx
-100% / -200% / -350% / -550%
98kW / 223kW / 353kW / 561kW
Removal at / NO2 / 0 / 0 / -139 / -123
a tunnel site / NO / 0 / 0 / -18 / -723
[g/h] / NOx / 0 / 0 / -157 / -846
SOx / 0 / 0 / -54 / -96
NOx+SOx / 0 / 0 / -211 / -942
Generation atpower plants[g/h] / NO2 / 0 / 0 / 0 / 0
NO / 19 / 42 / 67 / 107
NOx / 19 / 42 / 67 / 107
SOx / 13 / 29 / 46 / 73
NOx+SOx / 32 / 71 / 113 / 180
RESULTS / NO2 / 0 / 0 / -139 / -123
Total / NO / 19 / 42 / 49 / -616
balances / NOx / 19 / 42 / -90 / -739
[g/h] / SOx / 13 / 29 / -8 / -23
NOx+SOx / 32 / 71 / -98 / -762
Table 6 Balances of contaminant substances in Japan
4.1.Result of Type 1 (exhaust fans only)
It is natural that removed amounts of NOx and SOx should be zero at a tunnel site because Type 1 has no purifiers. The electric power consumed is that of the exhaust fans and contributes to the generation of NOx and SOx at the power plants. As a result the net effect of NO, NOx and SOx production is positive(increased). NO2is not changed (zero) because NOx generated at power plants is all NO without NO2. The generated NOx+SOx of 32 grams an hour is the environmental cost of running normal tunnel ventilation.
4.2.Result of Type 2 with ESP systems
Type 2 ventilation has the standard ventilation configuration coupled with ESPs. It is natural that the outcome of a Type 2 ventilation system should be almost the same as that of a Type 1. The total balances of NO, NOx and SOx are positive and greater than those of Type 1 becauseESP systems including fans consume more electricity which has to be generated at power plants. The generated NOx+SOx of 71 grams an hour is the environmental cost due to particulate removal and ventilation system operation.
The ESP system is calculated at a rate of 208m3/s and removal ratio of 80% (on a mass basis) with a concentration of 0.2mg/m3. Such a system can collect 120 grams per hour of suspended particles, although the increased NOx+SOx of Type 2 is 40 g/h greater than that of Type 1.
4.3.Result of Type 3 (NO2 reduction) and Type 4 (NOx reduction)
Both Type 3 and Type 4 ventilation systems utilize ESPs and either NO2 removal or De-NOx technologies. Type 4 removes a large proportion of NOx+SOx.Understandably Type 3 indicates better purification of NO2 than Type 4.
MLIT has decided to choose Type 3 air cleaning technologies in Japanbecause of the lower initial cost and lower operational cost including the power consumption as noted in Table 6.
- A comparison with the situation inChina
It is often presumed that the successful use of a technology in one country can readily be transferred to another. In this paper a comparison is made between the various air treatment options for tunnels in Japan, and in this section the impact on China.
The results of the simulations for Chinese tunnels are significantly different to thoseof Japanese tunnels. Relying upon the OECD data published for 200513)a summary of the power plants in China is described. (See table 7.)
Power Source / Generation [GWh] / Ratio [%]Coal (Thermal) / 1,972,267 / 79
Petroleum (Thermal) / 60,634 / 2
LNG Gas (Thermal) / 11,931 / 1
Nuclear / 53,088 / 2
Hydro / 397,017 / 16
Others / 2,504 / 0
TOTAL / 2,497,441 / 100
Table 7 Classification of power plants in China
It is apparent from this data that coal fuel in China willgenerate much more NOx and SOx at thermal power plants without de-NOx and de-SOx than in Japan. Coal fuel utilisation ratio in Chinais extremely high(79%) as compared withJapanat only 29%.
The SOx and NOx emissions from thermal power plants in China are shown in Fig.7, SOx and NOx emissions for all power plants can easily be calculated as 3.28 g/kWh(NOx) and 4.01 g/kWh(SOx).
Although there is limited publishedinformation about the composition of tunnel exhaust in china field, a data is available tothe authors from a field test conducted in a large Chinese city. The data indicates that SPM concentration areabout thirty times higher than Japanese one and NOx/SOx concentration are around five times greater than the Japanese equivalent.
In the event that China were to seek NOx reduction technology, a muchgreater amount of power would be required to oxidize NO into NO2because of the significantly higher concentrations of NOxin the places greater demands upon the absorbent/adsorbent materials demanding greater volumes and a consequential highertotal pressure loss in NOx removal process.
The different conditions mentioned above are listed in Table 8and compared with the Japanese case.
Japan / ChinaPower / NOx emission [g/kWh] / 0.19 / 3.28
.Plant / SOx emission [g/kWh] / 0.13 / 4.01
SPM [mg/m3] / 0.2 / 6
NOx [ppm] / 1 / 5
Tunnel Site / NO2 [ppm] / 0.1 / 0.5
NO [ppm] / 0.9 / 4.5
SOx [ppm] / 0.05 / 0.25
Total pressure of Type 4 [Pa] / 1,340 / 1,540
Power of Type 4 (Oxidation)[kW] / 150 / 750
Power of Type 4(Fan motor)[kW] / 388 / 446
Initial cost of Type 4 [%] / 550 / 700
Table 8 Differentials between Japan and China