DOI: 10.3303/CET24111111 Paper Received: 29 April 2024; Revised: 15 May 2024; Accepted: 05 September 2024 Please cite this article as: Das B., K Reji A., Ray T.K., Boral E., 2024, Impact of Compressed Natural Gas in Changing the Air Quality of Agartala City with reference to CO2 Emissions, Chemical Engineering Transactions, 111, 661-666 DOI:10.3303/CET24111111 CHEMICAL ENGINEERING TRANSACTIONS VOL. 111, 2024 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Valerio Cozzani, Bruno Fabiano, Genserik Reniers Copyright © 2024, AIDIC Servizi S.r.l. ISBN 979-12-81206-11-3; ISSN 2283-9216 Impact of Compressed Natural Gas in Changing the Air Quality of Agartala City with reference to CO2 Emissions Bulti Dasa, Ashish K Rejia, Tuhin Kanti Rayb, Eshita Borala* aDepartment of Geography and Disaster Management, Tripura University, Suryamaninagar, Tripura 799022, India bDepartment of Geography, Vidyasagar College, Kolkata, West Bengal 700006, India e.boral@gmail.com Transport sector is one of the very significant sources of air pollution. The environmental effects of transport are huge because transport sectors are the major users of energy, and it burns most of the world's petroleum. Within the transport sector, road transport is the largest contributor to air pollution. Carbon emissions from conventional fuels such as petrol and diesel contribute significantly to this pollution. Sustainable approach to urban transport addresses the environmental concerns with the need to provide affordable mobility choices to the rapidly growing population of urban areas. The aim is to validate Compressed Natural Gas (CNG) as a safe alternative vehicular fuel to reduce CO2 emission in Agartala city. For that reason, an attempt to conversion of vehicles running on harmful conventional fuels into CNG fuel in the Agartala Municipal Corporation (AMC) area under three intervention scenarios has been undertaken and analyzed. The result reveals a huge change in environmental status, which makes it appropriate to conclude that the use of CNG as a vehicular fuel is the best sustainable approach in urban surface transport to significantly reduce CO2 emission. 1. Introduction In the transportation sector, diesel accounts for two-thirds of all fuel consumption (Aggarwal, 1999). Diesel exhaust is harmful to health due to its various substances. To address this, the transportation sector is adopting alternative fuels (Ramli et al., 2017). Compressed natural gas (CNG), which is one of these alternative fuels, can meet the demands of countries worldwide (Khan et al., 2015). CNG is introduced as a clean-burning alternative fuel for automobiles (Lejda et al., 2021) the use of CNG fuel as an alternative to conventional fuel in public and private vehicles helps by lowering air pollutants (Kaushik et al., 2019). Carbon dioxide (CO2) emissions, as a greenhouse gas, play a pivotal role in driving climate change posing a significant global threat (Qin et al., 2023). The CO2 emissions from CNG vehicles are lower than those from conventional vehicles because natural gas has a lower carbon intensity than petrol or diesel (Iqbal et al., 2011), CNG emits about 25% less CO2 (Wakdikar, 2002) than diesel fuel for the same amount of energy and thus makes an important contribution to the reduction of CO2 and pollutants (Milojević et al., 2011). Compared to diesel vehicles, CNG vehicles have a lower impact on climate change (Rose et al., 2013) as CNG brings profit of 20 to 25 % compared to gasoline and 10 to 15 % compared to diesel in emissions of Greenhouse Gas on the overall chain of use (Amrouche et al., 2012). Therefore, Compressed Natural Gas may undoubtedly be considered to be an environmentally clean alternative to these fuels (Fattah et al., 2023). At the same time, one of the major advantages of CNG is that it offers a cheap source of energy. To run with expensive fuels such as gasoline and diesel, the low-cost CNG offers a glimmer of hope (Khan et al., 2016). Compressed natural gas (CNG) has been considered one of the finest alternatives to fossil fuels due to its global availability, inherent capacity to burn cleanly, affordability as a fuel, and adaptability with both petrol and diesel engines (Shamsapour et al., 2021). Therefore, to balance the supply and demand of energy and to limit the overall release of greenhouse gases, an appropriate energy policy must be implemented for the transportation sector. The major objective of the study is to assess the environmental impacts of the use of CNG as a vehicular fuel. It reveals current emission levels of CO2 in the Business As Usual (BAU) scenario and the impact it has on the environment by converting all the liquid fuel-based vehicle engines into the natural gas- 661 mailto:e.boral@gmail.com based engine, which may be referred to as an environment-friendly fuel, in the city. The results show that the replacement of CNG fuel with high-emission fuels can have a significant effect on reducing CO2 emissions. 2. Study Area Agartala is the capital of Tripura and is an important city in the North-East India. Agartala is situated in West Tripura District. AMC area has a total population of 4,00,004 (Census 2011) with a total area of 76.5 km2 (City Development Plan, Agartala, 2006). Agartala city is the main hub of business, government, industry, education, and health services in Tripura. There are transport systems that connect the city via rail, air, and road. For the present study 31 road traffic intersections connecting almost all the major roads, have been considered all over the municipality which accounts for a significant amount of CO2 and other greenhouse gas emissions because of its heavy traffic flow. 3. Methods and Methodology This research work is largely dependent on primary data which has been collected from 31 intersections for fulfilling the objectives of the study. The data that has been collected from the intersection were the number of CNG and non-CNG vehicles running on the roads during peak hours. The emission rate of per kg CO2 emitted per kg/litre of the vehicular fuel used An emissions factor is a measure of the amount of a pollutant that is released into the atmosphere because of a particular activity (Agyemang-Bonsu et al., 2010). The emission factor for vehicles depends on various factors such as fuel type, vehicle type, vehicle age, engine type, etc. There is limited availability of age-wise data for various vehicle categories running on diesel, gasoline, and CNG at the city level. Therefore, for this study, the CO2 emission factor of vehicles using different types of fuel has been taken into account. Using the emission values of these fuels and the number of vehicles running on these fuels, 3 different intervention scenarios have been performed to assess the CO2 emissions of the use of CNG as a vehicular fuel. Figure 1 shows the different what-if intervention scenarios (conversion of non- CNG vehicles into CNG vehicles) explored in the current study. The default scenario shows the Business As Usual (BAU) scenario. It comprises the present usage of fuel for different categories of vehicles. Two-wheelers and cars use petrol; diesel is used by LCVs, buses, and trucks whereas auto rickshaws and buses in Agartala operate on CNG. Emission levels of different fuel-used vehicles in each traffic intersection were calculated through equations 1, 2, and 3. ELP= ⅀NP (i+j) × EFCP (1) ELD= ⅀ND (k+l)× EFCD (2) ELNG= ⅀NNG (m+n) × EFCNG (3) Where, N is the number of vehicles, P, D, and NG are fuel types petrol, diesel, and CNG, vehicle types bike (i), car (j), LCV (k), bus (l), truck (m), and auto rickshaw (n) denotes respectively, EFC is the emission factor. The following equation (4) has been utilized to calculate the total emission of carbon TEC= ELP + ELD+ ELNG (4) Figure 1: The schematic illustration of the three intervention scenarios 662 Scenario I In this scenario conversion of possible diesel vehicles has been considered. Conversion of buses and trucks running on diesel is not feasible as their engine and make need to be changed completely. However, LCVs are considered in this category to be converted into CNG. Next to, calculate the reduced carbon emission of each intersection the following formula has been used. RELCScI = ELNkD- ELNkNG (5) Where, RELCScI is the reduced level of carbon in scenario I, EL is the emission level of N number of vehicles, D, NG denotes fuel types and k is the vehicle types. Scenario II In scenario II the conversion of possible petrol vehicles, i.e. private cars to CNG vehicles has been considered. Conversion of 2-wheelers into CNG is not possible as 2-wheelers cannot accommodate the cylinder required for the CNG fuel. Though cars are running on CNG in the city there are considerable numbers of cars running on petrol hence based on an assumption that all the cars are operating on petrol this scenario has been made workable. In scenario II using the equation to calculate the reduced level of carbon emissions are RELCScII = ELNjP- ELNjNG (6) Here, RELCScII is the reduced level of carbon in scenario II, EL is the emission level of N number of vehicles, P, NG denotes fuel types and j is the vehicle types. Scenario III Scenario III is the combination of both the above-mentioned scenarios. Diesel-run LCVs and petrol-run four- wheelers are considered in this category to be converted into CNG. The primary motive behind the performance of these scenarios is to determine the change in the level of CO2 emission in each of the successive scenarios and at last how much total pollution can be reduced when it reaches the last scenario. Equation 7 was utilized to estimate the reduced level of carbon emission in each of the selected nodes of the study area. RELCScIII= RELCScI + RELCScII (7) The emission rate after conversion has been calculated through equation 8. ER= TECBAU – TECScenario III (8) Here, ER is the emission rate after the conversion of petrol and diesel vehicles to CNG, TECBAU is the total emission of carbon in the business-as-usual scenario and TECScenario III is the total emission of carbon in scenario III. 4. Results and Discussion 4.1. Emission level after conversion of petrol and diesel-powered vehicles to CNG powered vehicle The present scenario of the usage of the fuels for the vehicles in the city of Agartala has been studied and the rate of emission from vehicles running on each fuel type, i.e. diesel, petrol, and CNG respectively has been calculated. In addition, three "what-if" intervention scenarios were taken into account to determine the extent of improvement of air quality due to the reduction in the primary emission of air pollutants. Table 1 : Emission factor of the CO2 kg/ kilometres (km) of the fuel Fuel Emission (Kg/kilometres) Petrol 0.22 Diesel 0.21 CNG 0.163 Source: Iqbal et al, (2011), Verma and De (2001) 663 Table 2: Level of CO2 in different scenarios for all the intersections in Agartala Intersection TEc BAU Scenario I Scenario II Scenario III Nagerjala 556.77 550.801 536.934 530.965 Battala 566.03 559.685 548.873 542.528 Amtali Bypass 419.758 413.977 409.726 403.945 Hapania 369.545 365.503 357.689 353.647 ONGC 395.776 391.264 384.49 379.978 Badharghat 391.514 387.66 379.202 375.348 Drop Gate 477.956 473.679 461.654 456.717 Camper Bazar 216.785 214.388 209.888 207.491 Fire Service Chowmuhani 501.277 496.718 483.493 478.934 Ker Chowmuhani 440.291 437.894 425.072 422.675 Durga Chowmuhani 433.97 431.056 421.715 418.801 Paradise Chowmuhani 502.147 499.703 484.078 481.634 Kaman Chowmuhani 494.19 491.041 476.691 473.542 IGM Crossing 429.9 427.503 417.531 415.134 Orient Chowmuhani 432.366 429.499 417.033 414.166 Bidurkarta Chowmuhani 362.619 359.893 349.566 346.84 Rajbari 462.352 459.955 447.589 445.192 Ganaraj Chowmuhani 411.427 409.171 397.69 395.434 Colonel Chowmuhani 459.049 456.323 444.628 441.902 North Gate 453.092 449.896 438.386 435.19 Bhagaban Thakur 393.348 390.575 381.036 378.263 Math Chowmuhani 419.622 417.319 405.6 403.297 Khayerpur 253.073 250.441 243.782 241.15 Gurkhabasti 500.285 497.089 480.221 477.025 Secretariate House 451.867 448.718 435.679 432.53 Albert Ekka 453.053 448.87 437.036 432.853 Usha Bazar 264.611 262.073 253.325 250.787 Narsinghar 251.446 249.002 240.217 237.773 GB Bazar 469.794 467.115 456.285 453.606 Nandan Nagar 253.471 251.027 242.869 240.425 Jogendra Nagar 263.452 260.726 253.078 250.352 Source: Calculated by the authors based on primary data and emission factor Through this detailed study, it has been possible to identify which of these intersections are in a more vulnerable state and which one is comparatively having a low presence of CO2. The result of the performed calculation shows that the emission level is reduced at all of the intersections because of the conversion which will undoubtedly ensure environmental security by reducing the vehicular pollution of the city. This will benefit the people of Agartala to breathe clean and will keep the people free of many diseases as well which is associated with the inhaling of unclean air. Table 2 depicts that, among all the intersections, Nagerjala has the highest CO2 emission (556.77kg/km) in the Business As Usual scenario because it is the busiest terminal; and supports all the inter-city bus services which usually run on diesel as well as other vehicles like private cars and bikes which run on petrol. This intersection has been depicted to reduce the highest CO2 emission (25.805 kg/km) after the possible conversion of vehicles. Battala intersection, which is the busiest intersection and has a significant commercial area around which greatly increased traffic volume and vehicular emission has reflected a reduction of 23.502 kg of CO2. Gurkhabasti which is home to several government offices experiences the large number of vehicles running on harmful fuels in this area. After conversion, this intersection has reduced CO2 (23.26 kg) after Nagerjala and Battala. It is noteworthy to emphasize that due to the inherent risk, ministers' automobiles and ambulances are not equipped with compressed natural gas (CNG). After converting possible petrol cars and diesel-running LCVs to CNG, 532.712 kg/km CO2 emission has been reduced for the whole of Agartala city. Camper Bazar has the lowest CO2 presence which is 216.785 kg mainly because this is a minor intersection 664 with fewer vehicles plying through it and after conversion of fuel the emission at this intersection will be reduced by 9.294 kg/km. Contrarily at major intersections, the difference in reduction is lesser between the BAU scenario and scenario III as because large number of auto-rickshaws, which are already operating in CNG and E-rickshaws ply through these intersections. Table 3: The difference in the emission rate of CO2 in the whole of Agartala by the conversion Source: Calculated by the authors based on Primary data When looked at from a macro level, in the whole of Agartala, CO2 emitted by the vehicles is calculated and be 12750.84 kg/km. After the conversion of possible diesel vehicles, i.e., LCVs, the emissions were reduced to 12648.56 kg/km (reduced by 102.272 kg/km). If the feasible petrol vehicles are converted into CNG, from the present scenario, the emission is reduced to 12321.06 kg/km (reduced by 429.78 kg/km). With the combination of both of these scenarios, the total emission of CO2 will reduce to 12218.12 kg which is 532.712 kg less than the original state. However, it should be mentioned that buses for intra-city travel are not popular in a medium-sized city like Agartala, and whatever few routes in intercity travel traverse through the city run on CNG. Another aspect is that LCVs running in the city cannot be directly converted to CNG, its make need to be rebuilt before the conversion. Intersection Emission in Business As Usual Scenario Emission After Conversion Reduced CO2 Nagerjala 556.77 530.965 25.805 Battala 566.03 542.528 23.502 Amtali Bypass 419.758 403.945 15.813 Hapania 369.545 353.647 15.898 ONGC 395.776 379.978 15.798 Badharghat 391.514 375.348 16.166 Drop Gate 477.956 456.717 21.239 Camper Bazar 216.785 207.491 9.294 Fire Service Chowmuhani 501.277 478.934 22.343 Ker Chowmuhani 440.291 422.675 17.616 Durga Chowmuhani 433.97 418.801 15.169 Paradise Chowmuhani 502.147 481.634 20.513 Kaman Chowmuhani 494.19 473.542 20.648 IGM Crossing 429.9 415.134 14.766 Orient Chowmuhani 432.366 414.166 18.2 Bidurkarta Chowmuhani 362.619 346.84 15.779 Rajbari 462.352 445.192 17.16 Ganaraj Chowmuhani 411.427 395.434 15.993 Colonel Chowmuhani 459.049 441.902 17.147 North Gate 453.092 435.19 17.902 Bhagaban Thakur 393.348 378.263 15.085 Math Chowmuhani 419.622 403.297 16.325 Khayerpur 253.073 241.15 11.923 Gurkhabasti 500.285 477.025 23.26 Secretariate House 451.867 432.53 19.337 Albert Ekka 453.053 432.853 20.2 Usha Bazar 264.611 250.787 13.824 Narsinghar 251.446 237.773 13.673 GB Bazar 469.794 453.606 16.188 Nandan Nagar 253.471 240.425 13.046 Jogendra Nagar 263.452 250.352 13.1 Total 12750.84 12218.12 532.712 665 5. Conclusion The most effective way to reduce the amount of CO2 is to burn less liquid fuels and replace them with CNG fuel. Due to its numerous benefits, the Indian government has targeted to replace all conventional fuel vehicles with CNG vehicles. With the implementation of CNG as an alternative to diesel or petrol-fuelled vehicles, the concentration of CO2 in the city significantly decreased. With the combination of both of the scenarios performed in this paper, the total emission of CO2 will reduce to 12218.12 Kg/km from 12750.84 Kg/km, which is 532.712 kg less than the present scenario. Since Agartala is a comparatively small municipal corporation with vehicular movement much less than many other major cities, vehicular emission is within satisfactory standards in most of the intersections. The vehicular numbers will be increasing with the growth of the city and so will its vehicular pollution levels. 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