Experimental investigation of removal of flue gas emissions exhaust from municipal solid waste incinerator using photovoltaic-based electrostatic precipitator

For the past decades, the flue gas emitted from municipal solid waste incinerator, power plant, and various industries is a permanent problem for the environment and has been affecting human life. Many flue gas filtration devices have been emerging out over the years. Although the electrostatic precipitator was an appropriate device due to high filtration efficiency and little pressure drop and energy efficiency, the cost and design of the electrostatic precipitator is a major restriction for manufacturers and end-users. With recent advances in technology, designing a cost-effective and less complex electrostatic precipitator has become mandatory. This article aims to design and develop a solar-powered cost-effective needle-plate type electrostatic precipitator which includes a static power converters and high-voltage transformer-rectifier (T-R) set with an input voltage as 230V AC, output voltage as 80-kV direct current (DC), and output current of 40mA for mitigation of flue gas emissions exhaust from municipal solid waste incinerator. The analysis of flue gas at ESP inlet and outlet has been performed using Ecotech stack sampler and flue gas analyzer. The obtained experimental results are validated with emission standards provided in the Solid Waste Management rules book, India, 2016.


Introduction
Municipal Solid Waste (MSW) Management, a decisive factor in the direction of sustainable urban growth, includes characterization, storage, treatment, segregation, collection, and disposal of solid waste to reduce its harmful impact on the atmosphere (Kumar et al., 2009). The quantity of MSW enlarges in corresponding to economic growth, owing to extreme consumption (USEPA, 2002). Solid waste is categorized into three types, namely degradable, partially degradable, and nondegradable materials, and it is generally collected from streets (Arvind et al., 2008). Recently, all developing countries are paying more attention to MSW management because most people do not have enough facilities to utilize MSW storage service (Schubeler, 1996). As a result, proper management of MSW is significant for human health and the environment. In several countries, MSW is treated in three ways, such as incineration, landfilling, and recycling (USEPA, 2012). The incineration method of treatment is cost-effective and ash produced from this treatment method can produce expensive materials (Huang et al., 2006). The primary advantage of incineration is the reduction of waste quantity; thereby, space required for landfills is decreased and provides additional energy sources from ignition (Ornebjerg et al., 2006). The incineration of MSW produces flue gases such as carbon dioxide (CO 2 ), carbon monoxide (CO), particulate matter (PM), HCl, sulfur dioxide (SO 2 ), nitrogen oxide (NO), total organic carbon, Hg, and its compounds (Alba et al., 1997;Quinaa et al., 2008). The burning of coal, coal processing waste, and coal derivatives has produced various gas emissions, especially nitrogen and sulfur oxides, which are harmful. The percentage of harmful gases depends on the composition and concentration of coal, water, and flammable liquid. The results indicate that coal mixed with water produces fewer gas emissions compared to coal (Margarita et al., 2018). Suppose the particulate matter in harmful gases is released into the atmosphere without a filtration system. In that case, it could cause climate change, affect human health, and also cause lung diseases and bronchitis. Generally, an electrostatic precipitator is one of the devices to purify the fine dust particles and smoke from flue gases. It has high collection efficiency, low energy consumption, and pressure drop (Kim et al., 2001;Remaoun et al., 2014;Patino et al., 2016). In ESP, the high-voltage power supply requires a range of 15 to 100kV for effective ultra-fine dust particle collection from flue gases (Grass et al., 2004;Shafiei et al., 2013;Slobodan et al., 2016). Researchers worldwide have published many articles pertaining to the augmentation of collection efficiency of ESP based on discharge electrode shape, material, inter-discharge electrode distance, the distance between collecting plate and discharge electrode, and other parameters. The filtration efficiency of ESP has been investigated for different discharge electrode positions using aerosol spectrometer GRIMM 1.109, which measures the particle concentration with a measuring range of 0.25 to 32 μm. The obtained results clearly indicate that position of the discharge electrode and polarity of DC voltage play a vital role in collection efficiency (Niewulis et al., 2011). The electrical characteristics of wire-plate ESP, which is a function of discharge electrode geometry, inter-discharge electrode distance, and the number of discharge wires, have been analyzed. The results have proved that electrical characteristics and ESP filtration efficiency are enhanced by reducing the collecting plate distance without increasing applied voltage (Khaled and  Eldein, 2013). The collection efficiency is the ratio of dust particles deposited on the collection electrode and total dust particles entered into the ESP. Design of ESP requires accurate calculation of collection efficiency, which in turn requires total collection area, gas flow rate, and dust particle migration velocity. The simulation for two types of ESP-(i) ESP with spike discharge electrode and (ii) ESP with wire discharge electrode-to investigate the particle collection efficiency and other geometries remains unchanged. High-voltage DC is applied to the spike electrode; thereby, corona is produced at the spike tips (S. Arif et al., 2016). Needle-plate electrostatic precipitator by changing design parameters such as radius of the needle tip, distance between needle and plate, applied voltage to the needle and temporal characteristics of trichel pulses. Needleplane distance varies from 6 mm to 3 cm, radius of the needle varies from 19 to 55 μm and applied voltage varied from −4 to −10kV. The DC corona current and frequency of trichel pulse increases by increasing applied voltage and decreases by increasing needle-plate distance (Peyman Dordizadeh et al., 2016). The ESP filtration efficiency is enhanced with the help of foam-covered collecting electrodes. Dust particles coming into the pores have not easily returned to the atmosphere, and also it reduces the pre-and post-filter in the ESP. The particle counter (Haltech HPC600) has been used to count the dust particles at ESP outlet, and the experiments have been carried out for different airflow velocities, i.e., 0.5 to 2.5 m/s. The results have been validated and found that foam-covered collecting plates in ESP have higher collection efficiency and moderate airflow velocity, suitable for capturing small dust particles (Wen et al., 2015). Collection efficiency of ESP has been analyzed based on the concentration of particles and found that high efficiency is achieved when inlet particle concentration ranges from 200 to 3600mg/Nm 3 (Arif et al., 2018). A wet dust removal system (WDRS) has been developed by using wet phase transition agglomerator (WPTA) and wet electrostatic precipitator (WESP). In a coal-fired power plant, the emission of particles has reduced to less than 5 mg/m 3 with the help of WDRS. The results clearly indicate that the filtration efficiency of the WESP has been increased by increasing the applied voltage (Cao et al., 2017). The removal of flue gas emission from incinerator has  been carried out in two ways: (i) using after burners, (ii) using a combined system. The experimental results clearly show that the concentration of dioxins (PCDDs+PCDFs) at inlet and outlet of after burner is 66.1ng-TEQ/Nm 3 and 0.213 ng-TEQ/Nm 3 , respectively. Further, the efficient dioxin decomposition has been achieved with the gas temperature greater than 850°C. The combined wet type ESP system, scrubber, and radiator successfully remove gaseous pollutants and dusts such as dioxins (PCDDs+ PCDFs) and HCl for a large range of initial concentrations. Besides, this system achieves more than 90% efficiency in any case of initial concentration (Kim et al., 2000). The elimination of PM 2.5 particles has been done by using lab-scale two-stage electrostatic precipitator with precharge and parallel collecting plates. The analysis of collection efficiency for two-stage ESP is conducted with alternating current (AC) and DC voltages and found that >96% and >90% efficiency has been attained when the precharger was energized by DC and AC voltages, respectively (Jaworek et al., 2017). The flue gas cleaning method has been conducted using eight different air pollution control (APC) technologies from no treatment to the most modern APC technology and found that wet flue gas cleaning, semidry flue gas cleaning, and dioxin filter followed by flue gas condensation and selective catalytic reduction (SCR) have provided the finest flue gas emission control mechanism. Apart from this, electrostatic precipitator and fabric filter provide better removal efficiency (>95%) for small and large particle size (μm) compared with cyclone and low-pressure venture scrubber (Vehlow and Dalager, 2010). The removal of particulate matter from a small waste incinerator's exhaust gas has been achieved with the pilotscale ESP with the sizing of 1000mm*10000mm*1000mm. The results clearly indicate that 100% collection efficiency has been achieved with the particles larger than 400 nm (Intra et al., 2014). Hence, considering the above facts, the intention of this paper is to design a cost-effective photovoltaic-powered needle- plate electrostatic precipitator with dimensions 900mm*250mm to filter the fine dust particles and smoke from flue gases produced by incineration of MSW.

Materials and methodology
The needle-plate ESP has been preferred in this work because it can handle a higher gas flow rate and produce high electric field strength around the discharge electrode compared to other ESP types. The schematic diagram of the entire experimental setup is depicted in Fig. 1. The experimental setup consists of three parts such as (i) power supply to the electrostatic precipitator, (ii) electrostatic precipitator, and (iii) municipal solid waste incinerator. In the power supply part, four 250-Wp PV modules with maximum power point voltage (V mpp ) of 74.3V and maximum power point current of 14.58A were given to the solar power conditioning unit, which consists of DC-DC boost converter and voltage source inverter.
The DC voltage from the solar panel was stepped up and regulated by DC-DC boost converter, and the FGA25N120AN IGBT has been utilized in the inverter circuit to convert DC link voltage into AC with the switching time of 20 ms. The 230V AC power supply is given as input to the T-R set, which converts 230V AC into 5-to 80-kV variable DC with the help of an electronic controller which is designed to adjust the output voltage and current of T-R set. The electronic controller circuit consists of DSPIC33FJ128GP710A microcontroller, antiparallel thyristors, voltage sensor, and current sensor. By adjusting thyristors' duty cycle, the primary winding voltage is varied, and thereby, secondary winding voltage is changed accordingly (Fig. 2). The negative terminal of the T-R set is connected to the discharge electrode of the electrostatic precipitator, whereas the positive terminal is connected to the collecting plate, and it is grounded. The dimension of the proposed needle-plate electrostatic precipitator is given in Table 1.
The discharge electrode is made up of stainless steel (SS 302), which has higher carbon than other grades and does not corrode easily. The collecting plates, carcass of ESP, inlet, and outlet duct are designed using mild steel. The collecting plates are connected to the outer structure of the electrostatic precipitator, and it is earthed. The discharge electrodes are connected to the high-voltage power supply, and it is isolated from the ESP body using a Teflon insulator. High-voltage DC is slightly varied from 5kV, and the negative gradient of the electric field is also increased accordingly. When applied voltage attains the corona onset voltage, corona current starts flowing from discharge electrode to collecting plate which ionizes the flue gases produced within a specially designed incinerator by burning MSW. Liquefied petroleum gas (LPG) has been used as fuel in the incinerator. Exhaust fan is located downstream of the ESP to collect the flue gas from incinerator through electrostatic precipitator. The experimental setup entire system is depicted in Fig. 3. Flue gas analysis has been performed at ESP inlet and outlet using stack sampler and flue gas analyzer. The sampling process has been conducted for 30 min. The sampling porthole has a diameter of 76.2 mm, which helps to place the gas analyzer  Fig. 4.

Results and its discussion
The open circuit test of the transformer-rectifier set has been performed (Table 2) and the corresponding values have been tabulated. The magnitude of VDC limit has been manually set in the electronic controller. It clearly indicates that ESP voltage is varied based on the transformer primary winding voltage.
The magnitude of the V DC limit has been manually set in the electronic controller. It clearly indicates that ESP voltage is varied based on the transformer primary winding voltage. The transformer-rectifier's output voltage has been reached 78.9-kV DC for the transformer primary winding voltage of 199-V AC. The current-voltage characteristic of the developed electrostatic precipitator is depicted in Fig. 5.
The flow chart depicts the system boundary for the filtration efficiency of ESP analysis (Fig. 6). The DC voltage applied to the electrostatic precipitator can be controlled from the electronic controller of the transformer-rectifier (TR) set.
When the applied voltage is −45 to −90kV, the distribution of charge density is depicted in Fig. 7a-d. The distribution of charge density is modeled using peeks law at the needle and collecting plates. From the results, we can conclude that the magnitude of space charge is high at needle and it decreases as we move away from the needle towards the collecting plate. Further, the magnitude of space charge is directly proportional to the applied voltage.
A stack sampler is used to collect the gas from the inlet and ESP outlet to measure the PM concentration. After the gas is collected, PM concentration is obtained by using the weight of a sampling thimble. The mathematical calculation of PM concentration is represented in Equations (1) and (2) where A is the initial weight of the thimble, B is the final weight of the thimble, C is the air volume in cubic meters, E is the volume of liters per minute (LPM), D is the volume of air in liters, and T is the sampling time. The flue gas analyzer measures the CO and NO in parts per million. The conversion of parts per million into mg/Nm 3 is given in Equation (3) where PPM is the parts per million, M is the molecular weight, P is the sea level pressure, T is the stack temperature, the standard pressure (P 0 ) is 760 mm Hg, and the standard temperature (T 0 ) is 273.15 K. Several countries have framed rule books enclosing (   Fig. 9. a and b Polyester filter samples before and after the ESP, c collecting plate after gas treatment in ESP The flue gas emission test has been done for the concentration of particulate matter (PM2.5 and PM10) at electrostatic precipitator outlet. The obtained results were 28 mg/Nm 3 and 31 mg/Nm 3 (Fig. 8), which are found to be a little amount, and the permissible emission standard provided in the Bio-Medical Waste Management rule book is 50mg/Nm 3 .
Further, the filtration efficiency of ESP has been validated with the polyester filter samples, which are fixed upstream and downstream of the ESP, and it is depicted in Fig. 9a and b. As anticipated, the color of the filter sample used to capture the smoke particles alters depending on their location. The filter sample has been recovered a large quantity of dust particles before the ESP, which considerably changes its color. After the ESP, the filter sample color is not changed because the dust particles are deposited in the collecting plate of ESP. The collecting plate after gas treatment is shown in Fig. 9c. It is noticed that a large amount of smoke particles are deposited in the plate after the ESP operation.
In addition to the experimental investigation of ESP, a CFD modeling has been done to analyze the dust particle charging and its collection process in electrostatic precipitator. Numerical calculations were conducted for various applied voltages in the range of 45-90 kV and various particle diameters in the range of 2-5 μm. The temperature and pressure were 293 K and 1 atmosphere, respectively. The dust particles with different diameters are entered into the needle-plate ESP inlet, and then it is charged and deposited on the collecting plates by an electrostatic field. In this case, 180 dust particles are entered, and then the performance of needle-plate ESP is analyzed for different applied voltage and particle diameters.
With the help of Table 4, we can conclude that the performance of needle-plate ESP is excellent for large particle diameter with high applied voltage. The collection efficiency of the proposed needle-plate type electrostatic precipitator is high compared to the other two methods. Compared with existing methods, the proposed method utilizes renewable energy sources; thereby, no external power supply is required.

Energy and cost savings in comparison to conventional system
The emissions or waste from the electricity produced by solar panels would be zero (Table 5). In contrast to fossil fuels and power plants, solar panel can produce clean and renewable energy without locating, excavation, transportation, or combustion. It is a simpler, cheaper, cleaner, and all-around better energy solution.

Conclusion
This proposed article has identified the potential ways to design and implement cost-effective solar-powered electrostatic precipitators for the filtration of flue gases from municipal solid waste incinerators. The ESP has been energized by solar energy with power electronic converters and a high-voltage T-R set. The flue gas analysis of ESP has been carried out with a stack sampler and flue gas analyzer. The obtained results for particulate matter, carbon monoxide, and other flue gases have been validated and found that it falls within the required emission standards. Further, the collection efficiency of ESP has Rs.262800 Total amount spent for 1000-Wp solar power with four backup batteries and solar power conditioning unit: Rs. 1,72,000 Total savings: Rs.90800 (Rs.262800-Rs.1,72,000)

The human element
The availability of fossil fuels has been decreasing over the years. It can lead to labor strikes, price volatility, and even war The availability of solar energy is abundant and it will be accessible for another 5 billion years been validated with a polyester filter sample. Near future, we planned to conduct flue gas analysis for all the major pollutants released from the incineration of municipal solid waste (MSW) with the help of pilot-scale ESP with the dimensions of 2740 mm × 1310 mm.