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Abstract

Emissions of NO_X from automobile causes various environmental impacts. In order to satisfy the emission norms and standards, it is necessary to reduce the toxic gases from emission. This paper concentrates on designing a packed column and analysis on the selection of an effective absorbent to reduce NO_X emission from diesel engines. NO_X emission is taken from the exhaust of diesel engine (Kirloskar TV1 Diesel engine). Flue gas inlet to the packed column and the treated gas at the outlet of the packed column are measured using NO_X inlet and outlet gas sensors, respectively. Absorbent flow is carried out in a lab-scale packed column experimental setup. Various parameters of the packed column such as liquid: gas ratio, diameter, total height and packing height are determined based on the mathematical modeling. The NO_X emission was experimentally measured with various combinations of different absorbents with different flow rate. Based on the experiments, effective absorbents are chosen as NaClO₂ and NaOH. Experimental results show chemical absorbent combination with 0.5 M concentration of sodium chlorite and 0.3 M concentration of sodium hydroxide at 80% data acquisition card opening condition is absorbing NO_X effectively.

Keywords

Diesel engine exhaust emission packed column design chemical absorbents

Introduction

Emission of toxic gases like NO_X causes acid rain and other forms of pollution. NO_X emissions are produced due to the combustion of diesel engines in the transport sector and other industries.¹ Diesel engines are also used in agriculture applications; emissions from diesel exhaust cause more impact on environmental pollution.² Increased NO_X emissions cause human health hazards particularly for the respiratory system. The major emissions of pollutants from coal combustion at thermal power plants, transport sector and other industries are carbon monoxide (CO), Nitrogen oxide (NO_X), Sulfur oxide (SO_X), Chlorofluorocarbons (CFC) and Suspended Particulate Matters (SPM), such as fly ash, soot and other trace gas species.¹ Industries located in Asian countries lead to rapid increase in energy usage and as a consequence, emission of NO_X is increased.³ Even the vehicles with some of the biodiesel produce NO_X emissions.⁴ Various changes and technological advancements in diesel engine combustion systems are required to satisfy the emission standards and norms.⁵ In case of diesel engines nowadays, there is a necessity toward increasing the performance of the engine and to satisfy the emission norms specifically toward NO_X emissions.⁶ Compared to that of other fuels, diesel fuel emission exhausts more NO_X.⁷ Considering the harmful emissions of engine exhaust, researchers derived various solutions like adding oxidation catalysts, anti-oxidation catalysts and so on.⁸ NO_X and other emission reduction techniques were proposed by various researchers,^9–11 and they proposed different devices such as packed column,¹² budding jet reactor,¹³ combined packed and spray tower absorber,¹⁴ wetted wall column,¹⁵ cable bundle wet scrubber¹⁶ and so on. For efficient removal of NO_X, wet scrubbing method is practiced in a majority of industries. It is one of the cost-efficient techniques.¹⁷ Selection of absorbents for wet scrubbing process is one of the key factor to be considered for producing maximum removal efficiency. Mostly used absorbents are hydrogen peroxide as the scrubbing liquid, NaClO₂,^15,16 sodium chlorite,¹⁸ NaClO₂ and NAOH.¹⁹

Experimental conditions

Experiments were carried out with diesel engine exhaust under 25% of loading conditions with normal temperature.²⁰ For this specified diesel engine, mechanical loading was carried out through the mechanical loading set up. Maximum loading that can be carried out for this set up is 100% (12 kg). In this experiment, exhaust gas was obtained at 25% (3 kg) of mechanical loading.²⁰ Table 1 shows the average values of engine exhaust readings. These readings were obtained by using engine gas analyzer. From Table 1, it is found that exhaust emissions of NO_X is more compared to that of other exhaust gases. Exhaust emission is almost constant. The readings were taken with an interval 1 h for 12 h.²⁰

Table 1.

Diesel engine exhaust emission under various load conditions.

Diesel engine exhaust (in ppm)
% of loading/exhaustemission	0%	25%(3 kg)	50%(6 kg)	75%(9 kg)	100%(12 kg)
CO	0.12	0.1	0.07	0.06	0.07
HC	35	36	37	39	51
CO₂	2.6	3.7	4.9	6.2	7.3
O₂	18.31	16.75	15.4	13.6	11.66
NO	135	344	678	962	1101

The present research work aims toward the effective removal of NO_X in two steps. The first step is to determine packed column parameters with reference to two film gas liquid absorption theory such as diameter, packed height and liquid: gas (L/G) ratio. Second step for removal of NO_X is to carry out experiments with different absorbents such as KMnO₄, NaOH and NaClO₂ with different combinations and concentrations in a lab-scale emission control setup.

Physical modeling of structured packed column

The absorption process efficiency is based on solubility of contaminant gas in liquid phase. Packed column system is the commonly used method for absorption. In a packed column system, absorption rate is increased by increasing interfacial area between gas and liquid.²¹ Packed column performance depends on following parameters like liquid and gas (L_m/G_m) ratio, diameter, packing height and total height. All the above parameters ensure better liquid and gas distribution in the entire surface area. The packed column is designed in such a way that it must produce maximum absorption as possible. Figure 1 shows two-dimensional model of the packed column.

Figure 1.

Design of packed column.

Determination of L_m/G_m ratio

In a packed column design, liquid flow rate and the amount of gas entering the packed column are to be determined initially. In this work, sodium chlorite (NaClO₂) is considered as an adsorbent for removal of NO_X. During the mass transfer between the gas (NO_X) and NaClO₂, NaClO₂ becomes aqueous sodium chlorite. The solubility of gas (NOx) in aqueous sodium chlorite determines the quantity of pollutant to be absorbed from gas phase to liquid phase. The mole fraction of NO_X in aqueous sodium chlorite is determined by dividing the moles of NO_X dissolved into the liquid by the total moles of aqueous sodium chlorite. In the present work, NaClO₂ is considered as an absorbent liquid (L) and NO_X gas is considered as gas (G) and L/G ratio is calculated by the following relation, shown in Equation (1)

\frac{L_{m}}{G_{m}} = \frac{H}{K_{4}}

(1)

where L_m is minimum liquid flow rate, H is Henry’s constant, G_m is minimum gas flow rate and K₄ is flooding factor. Flooding factor is identified for the corresponding pressure drop in the packed column by Sherwood correlation and it is used for calculating the column diameter. By considering G_m = 1 m³/h, flooding factor is 0.7 and Henry constant is 0.0014²²

L_{m} = 0.002285 kg / s

Determination of diameter

Diameter of the packed column influences directly over the velocity of gas and liquid. Flow rate of gas inside the packed column gradually increases with decrease in packed column diameter, hence it produces flooding.²³ Therfore, the column diameter is calculated for 70% of the flooding velocity to achieve the highest economical pressure drop and good liquid and gas distribution²¹ based on Sherwood correlation. The flooding factor (K₄) is identified for the corresponding pressure drop (F_LV) and it is represented as

Abscissa (F_{LV}) = \frac{L_{m}}{G_{m}} \sqrt{\frac{ρ_{G}}{ρ_{L}}}

(2)

By substituting the molar flow rate of liquid, gas and also their corresponding densities, the calculated pressure drop (F_LV) is 0.1724. For determining liquid flow rate inside the packed column, flooding is another important factor to be considered. Flooding factor gives the gas velocity at which the liquid droplets become entrained in the exiting gas stream and it is shown in Equation (2). From the flooding factor, the gas mass flow rate/column cross-sectional area $(V_{w}^{*})$ is determined and it is expressed as in Equation (3) where ρ_G is gas density (1.21 kg/m³), ρ_L is aqueous NaClO₂ density (1857 kg/m³), G_m is molar flow rate of gas (0.0003383 kg/s), µ_L is liquid viscosity (0.00348 N s/m²), L_m is flow rate of liquid (0.002285 kg/s) and F_p is packing factor

V_{w}^{*} = {[\frac{K_{4} ρ_{G} (ρ_{L} - ρ_{G})}{13.1 F_{P} {(\frac{μ_{L}}{ρ_{L}})}^{0.1}}]}^{0.5}

(3)

V_{w} = 1.7415 kg / m^{2} s

The area of the packed column (A) is calculated as shown in Equation (4)

A = \frac{G_{m}}{V_{w}}

(4)

where G_m is gas flow rate and V_w is gas mass flow rate/column cross-sectional area

A = 1.573 \times 10^{- 3} m^{2}

D = \sqrt{\frac{4 \times A}{π}}

(5)

Based on Equation (5), substituting the area value and the diameter obtained is

D = 0.0475 \approx 0.050 m

Hence the area of the packed column is determined as A = 0.001573 m² and the diameter of the packed column is D = 0.050 m.

Determination of packing height

Liquid hold up time for the packed column is determined by packing height. Better removal efficiency is obtained by higher liquid hold up time. Computation of packed height (Z) of the column is expressed as shown in Equation (6)

z = H_{oG} N_{oG}

(6)

where N_OG is the number of transfer units and H_OG is height of transfer units (0.3–1.2 m). The packed column height at different flow rate was estimated.²¹N_OG is calculated as

N_{OG} = \ln \frac{y_{1}}{y_{2}}

(7)

where y₁ is mole fraction of mixture gas entering the column (5000 ppm); y₂ is mole fraction of gas leaving the column (assumed as 50 ppm) and the calculated N_OG is 4.6. Based on Equation (7), packed height is determined as 0.4224 m.

Experimental setup

Experimental work was carried out in the laboratory scale emission control setup as shown in Figure 2 and its functional block is shown in Figure 4. Inlet NO_X emission is taken from the exhaust of diesel engine (Kirloskar TV1 Diesel engine). NO_X from the chamber enters at the bottom of packed column.

Figure 2.

Photographic view of experimental setup.

In the packed column, liquid enters at the top, by using liquid distributor evenly spread out using packing arrangement. Gas enters at the co-current section (bottom) of the packed column and both are blended together in the packed area. Based on the mass transfer occurred between gas and NaClO₂, polluted gases gets absorbed into the liquid. The concentration of NO_X at the inlet and outlet of the packed column is measured by NO_X gas sensors.

The output from the PC-based controller is directed to actuate the peristaltic pump which functions like a final control element. Based on the NO_X concentration in the untreated gases, the flow rate of the NaClO₂ is manipulated through the peristaltic pump. DAQ is Data acquisition card used for acquiring sensor (NO_X) data and transfers control signal for the final control element (peristaltic pump). A specially designed ADuC841 microcontroller-based VMATDAQ unit is used for monitoring and controlling the NO_X emission control process (M/s V-I Microsystems, Chennai, India).

Experimental data were obtained from a real-time lab-scale experimental setup as shown in Figure 3. Data were recorded automatically by using DAQ. The front end SIMULINK/MATLAB platform as shown in Figure 3 is created²⁴ to store the sensor data. While obtaining the data using MATLAB interface–based DAQ, it is possible to control data acquisition system both by manual mode and automatic mode. Based on the measured data, a graph was drawn and shown in Figure 5. Initially, the data were obtained by considering a time interval of 100 s for each absorbent. But no major changes occur up to 1000 s. The changes happened approximately at the interval of 1000 s. Hence, the graph is plotted for an interval of 1000 s. Experimentation was carried out for maximum of 5000 s to all the absorbents.

Figure 3.

MATLAB - Simulink model for NOx emission control system.

Figure 4.

Function block of experimental setup.

Result and discussion on optimization of absorbents for NO_X absorption

Various absorbents such as NaClO₂, KMnO₄, H₂O₂, H₂O, NaOH and so on are used by the researchers for NO_X removal process. It was analyzed by conducting the experiments in the laboratory-based emission control experimental setup.

Water (H₂O) as absorbent

From Figure 5, it is observed that the removal efficiency obtained from water absorbent is around 20% to 25% at various NO_X inlet concentrations, respectively. Although water is the mostly used absorbent in industrial practice, the fractional absorption occurred during continuous runs. H₂O reaction is not enough to influence the mass transfer since the response time was very rare. NO_X gases need strong oxidant. Hence percentage of removal was very minimum. Hence, it is not considered in the present study.

Figure 5.

Performance analysis of various absorbent.

Sodium Hydroxide (NaOH) as absorbent

In presence of NaOH solution, it acts as an effective oxidant of nitrogen oxides. The absorption rates of NO slightly exceed gas-film controlled rates. This suggests that the presence of NO may consume more NaOH in the solution. Figure 5 shows the 35% of removal efficiency.

Potassium Permanganate (KMnO₄) with NaOH solution as absorbent

For 0.01 M KMnO₄ and 0.1 M NaOH absorption rate over NO concentration is trivial. There is a significant decrease of NO absorption rate with increasing concentration of other pollutants due to chemical reactions. The chemical reaction pointed out in Equations (8) and (9), in which NaClO₂ would be decomposed in an acid solution are as follows

NO + Mn O_{4} + 2 O H^{-} \to 2 N O_{2} + Mn O_{4} + H_{2} O

(8)

3 N O_{2} + 2 Mn O_{4} + H_{2} O \to 3 N O_{3} + 2 Mn O_{4} + 2 O H^{-}

(9)

From Figure 5, it is seen that it can obtain the 35% of NO_X removal as an absorber in a packed column.

Sodium Chlorite (NaClO₂) with NaOH solution as absorbent

When the NO concentrations in the gas stream ranges between 0.5% and 15% and the experiments were carried out at temperature of 25°C, the chemical reaction shown in the Equations (10) and (11) is NaClO₂ would be decomposed in an acid solution as follows

NaCl O_{2} + H_{2} O \to HCl O_{2} + NaOH

(10)

2 NaCl O_{2} + C l_{2} \to 2 NaCl + 2 Cl O_{2}

(11)

Some ClO₂ may evolve from the solution into the gas phase and oxidize NO_X to help the absorption of NO_X in the system as shown in Equation (12)

4 NO + 3 NaCl O_{2} + 4 NaOH \to 4 NaN O_{3} + 3 NaCl

(12)

Hence from Figure 5, it is observed that high removal percentage for the absorbent NaClO₂ gives above 70% of removal by using sodium chlorite. Figure 6 shows that NaOH solution does not produce resistance during the gas and liquid reaction, 0.1 M NaOH produces around 35% for various inlets of NO_X. NaOH shows that the removal efficiency gradient is less compared to NaClO₂ but it is effective oxidant which is to improve the removal amount combined with NaClO₂. It shows that NaClO₂ is a powerful oxidant and behaves as good absorbing medium leading to the formation of NO₃. Hence NaClO₂ with NaOH is taken as an adsorbent for removal of NO_X gases.

Figure 6.

Effect of absorbent flow rate with various percentage of DAQ opening with outlet concentration of NO_X.

The pure NaClO₂ and NaOH (0.5 + 0.3 M) is taken as an absorbent to attain better removal efficiency. The effect on NO_X removal at various flow rates of the absorbent is recorded. In this analysis, the flow rate is varied between 40% and 100% DAQ opening. The higher and optimal removal efficiency is obtained at 80% DAQ opening of the NaClO₂ solution and the corresponding outlet concentration. The various flow rate of NaClO₂ is obtained by the VDPID-03 Digital controllers.

The developed by-product with reference to Equation (12) after the reaction of NO+Na ClO₂ + NaOH is 2NaNO₃ (sodium nitrate) and NaCl (sodium chloride) solution is stored in a container. After the experiments, it is not reused. Thus, the by-products generated after NO_X removal from these experiments were very less. By-products will be maximum to the range of 300–350 ppm of NO_X along with absorbent solution to a maximum of 3 L/h.

Experimental readings show the liquid–gas interaction nature and desired maximum removal efficiency obtained at the 60%–100% of DAQ opening because oxidation reaction and mass transfer that occur between sodium chlorite and sodium hydroxide at that point are very high. This ensures that depending on the flow rate, outlet concentration of emission gases NO_X is continually reduced. This is shown in Table 2 and Figure 6.

Table 2.

Effect of the flow rate at the outlet concentration of NO_X.

% DAQopening	The flow rate of NaClO₂ withNaOH(L/h)	NO_X outlet concentration(ppm)
20	0.5	–
40	1	307
60	1.6	278
80	2.3	156
100	3	148

DAQ: data acquisition card.

Conclusion

The experiments were carried out in the lab-scale experimental setup using various absorbents such as water, NaOH and KMnO₄ with different combinations and concentrations of NaClO₂. Based on the experimental results, it is found that for diesel engine exhaust gas de-nitrification process, the 0.5 M concentration of sodium chlorite with a 0.3 M concentration of sodium hydroxide was an attractive alternative to increase the simultaneous removal efficiency. This combination created 65% of nitrogen oxides removal efficiency, the experimental results conclude that the increase in sodium chlorite increases the NO_X removal efficiency. From the NO_X removal analysis, the greatest NO_X removal efficiencies are in the range of 36%–65%. Control action is essential to regulate the flow rate of the absorbent NaClO₂ since it has the direct influence on the NO_X removal efficiency.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The authors wish to acknowledge the University Grants Commission under Major Research Project F.No. 39-883/2010 dated 12 January 2011 for providing the financial support to develop this experimental setup.

ORCID iD

V Arunkumar

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