2.2. ELECTRICAL DISCAHARGE AND HYBRID PLASMA SYSTEMS FOR GAS TREATMENT
DESCRIPTION
The subchapter presents the techologies wich used “Electrical discharge and Hybrid process” in three aplications: NOx removal from synthetic gas diesel flue gas; Cleaning of incinerator flue gas and Cleaning of indoor air. For diesel flue it has been demonstrated that the combination of discharge with TiO2 catalyst (hybrid system) was a very efficient method for NOx removal. For incineration flue gas treatment the Application of the wet-type plasma reactor reduced significantly Dioxin, Dust and HCl. For the indoor air cleaning, it has been found that the combination of pulsed discharge and TiO2 catalyst is effective in acetaldehyde decomposition.
CONTENT
Non-thermal plasma processes have been intensively investigated during the past ten years by many research groups for air pollution control applications. Various gaseous pollutants including SOx, NOx, odors and VOCs have been tested and promising results have been obtained [1-7]. Recently the application field of non-thermal plasma processing is expanding for various purposes. In addition to the gaseous pollutant removal , several new approaches have been reported including methanol synthesis, sterilization, surface treatment, and water treatment . Some of them already appeared in the commercial market.
In the application of De-SOx and De-NOx, reduction of electrical energy consumption has been pointed as critical problem. Hybrid plasma system can solve this problem. As will be discussed in later, the combination of discharge plasma with photocatalyst (TiO2) can enhance the performance. Titanium dioxide, well known as photocatalyst, is a n-type semiconductor having a band gap energy of 3.2 eV. This catalyst is activated by irradiating UV light having energy larger than the ban gap energy ( <380nm), and then various chemical reactions are induced on its surface. In this work, we used pulsed discharge plasma instead of UV light as excitation energy source. High energy particles in the discharge plasma can transfer their energy by collision with TiO2 surface. The recombination problem, which has been pointed out in photocatalyst processes, could be reduced by applied electric field in this combined system. In this combination, catalytic oxidation process might be effective on the surface of TiO2 and removal efficiency of gaseous pollutants might be improved.
Dioxins (PCDDs+PCDFs) from municipal waste incinerators are becoming urgent environmental problem worldwide including Japan. Among dioxin congeners, particularly, tetrachloride dibenzo-p-dioxin (2,3,7,8 -TCDD) has the highest toxic, as known to as an intense carcinogenic agent [9]. Classifying the chlorine containing plastics prior to combustion process is one possible method to reduce the formation of dioxin. However all the organic matter actually contributes to the generation of dioxin, and it is impossible to perfectly separate d chlorine containing plastics from municipal waste and there is the demerit of the classifying energy costs. Therefore development of post-combustion treatment technology is required to cope with the dioxin problems. In 1996, a research squad of the Ministry of Health and Welfare showed that the present Tolerable Daily Intake (TDI) of dioxin is 10 pg/kg/day. This numerical value, however, is settled as the tentative deadline of emergency that aim at limits of the existing incinerator and the newly established incinerator of 1.0 and 0.1 ng TEQ/Nm3 [12].
Formation of dioxins in combustion process is known to come from several paths. Up to now, it is through that there are two main route for dioxin formation in the combustion processes. One is the condensation of chlorinated precursor such as chlorophenols, which comes from incomplete combustion or contained in waste. These precursors react with chlorine to give various species of dioxin. Isomer patters of dioxin depends on the type of this precursor. Another route is well-known de novo synthesis. In the de novo synthesis, fly ash in the exhaust gas play a key role in the formation of dioxin at temperature range of 250oC ~ 300oC. At this temperature range, fly ashes containing copper (e.g. CuCl2 and CuCl) produce Cl2 by Deacon reaction R(5). The overall reaction can be expressed as R(1) ~ R(5). Produced Cl2 participate in the dioxin formation via substitution reactions and surface reactions on fly ash.
2CuCl2 + ½ O2 ® Cu2OCl2 + Cl2 (1)
Cu2OCl2 + 2HCl ®2CuCl2 + H2O (2)
2CuCl2 + ½ O2 ® Cu2OCl2 (3)
Cu2OCl2 + 2HC ® 2CuCl2 + H2O + Cl2 (4)
Net Reaction 2HCl + ½ O2 ® Cl2 + H2O (5)
Contamination of indoor environment related to SBS (Sick Building Syndrome) has been of concern because people spend approximately 93% of time in indoor space (EPA, 1989). Tabacco smoke is one of major pollutants and is a complex mixture of particles and gaseous pollutants including acetaldehyde, isoprene, formaldehyde, NO etc. Non-thermal plasma system can be used for the removal of both particulate matters and gaseous pollutants [13]-[18]. Previous work indicated a high the removal performance of particles and gaseous pollutants in the wet plasma reactor [18] and in plasma reactor combined with catalysts [19]-[20].
In this paper, we describe the experimental results of NOx and CH3CHO removal using the hybrid non-thermal plasma process, which combines discharge plasma with catalyst (TiO2). In addition to this, dioxin removal using the wet-type plasma reactor and NOx removal using the combination of discharge plasma with chemical scrubber also will be discussed.
The experimental systems for NOx removal are shown in Figure 2.2.1. A TiO2 pellet-packed plasma reactor was used for the simulated gas experiment. Anatase type of titanium dioxide (TiO2), well known as photocatalyst, was used. For the comparison test, the conventional wire-cylinder reactor and the alumina-pellet packed reactor were also used. The dimensions of the reactors (length – 200mm, inner diameter-19nn) and the pellet size (5mm in diameter) were the same for each case. The initial NO concentration and gas flow rate were kept constant at 400 ppm and 2 L/min, respectively.
In the diesel engine exhaust gas treatment (Figure 2.2.1(b), 16 cylindrical reactors were used in parallel. This reactor was made of ceramic tube of 16mm in diameter and 210 mm in length. A screw rod centered in the tube served as a ground electrode, and an aluminum tape wrapped on the outer surface was used for a high voltage electrode. Test gas was prepared from the 5kW diesel power generator. The chemical scrubber , using 2,5 wt % Na2SO3 solution, was connected to the outlet of the plasma reactor to remove NO2, which was oxidized from NO. The total exhaust gas from the diesel engine was 840 L/min (50,4 m3/hr) and the gas flow rate to the plasma reactor was controlled by adjusting the py-pass valve. The liquid supplied to the chemical scrubber was 2L/min and circulated after filtration. Voltage and current waveform were monitored using a digital oscilloscope (Tektronix TDS 350). The input power was measured with a digital power meter (HOIKI, 3186) at the input of the pulse generator. This value is the sum of the discharge power and the energy loss in the pulse generator. A NOx meter (Shimadzu NOA-305A) and FT-IR (Bio-Rad, FTS-30) were used for the NOx concentration and the by-product measurements.
Figure 2.2.1. - Schematic diagram of the experimental system
Fundamental experiments have been carried out to treat actual flue gas from an incinerator with capacity of 100kg per hour. Schematic diagram of the experimental system is shown in Figure 2.2.2. First, the flue gas from the incinerator was introduced into a scrubber system, where large dust particles in the flue gas were removed by water spray. Water was sprayed from nozzles to collect large dust, and the collected dust was separated from the water by a filter before circulation. Mists carried away from the scrubber by the flue gasses were collected by a separator before entering the wet type plasma reactor. In these experiments, the wet plasma rector consisted of five wire-cylindrical rectors in parallel. Figure 2.2.3 shows the details of the wet type plasma rector. In this experiment, negative DC.20 kV was applied between the electrodes using a high voltage generator. Water film is formed at inner surface of the rector by introducing water from top to bottom. The water film plays a role of prevention of dust reentrainment and of absorbing agent for gas components. The water used for the water film was also treated before recycling.
In this experiment, the concentrations of various species of dioxin were measured in the incinerator flue gas. The measurement was carried out by Japan Quality guaranteed Association (JQA). A waste of 15kg containing fragments of lumber, plastic and vinyl were burned for 15 minutes and a gas sample was taken during that time. The experiment was repeated four times and the data were averaged. Inlet and outlet measuring equipment were set between the fan and the scrubber system and at the exit of the plasma reactor. Sulfur dioxide (SO2), carbon monoxide (CO), hydrogen chloride (HCl), oxygen (O2) and dust were measured.
The methods for the measurements are based on the Japanese Industrial Standard (JIS) as follows:
· The concentration of dust is measured based on JIS Z 8808-1995 (Method of measuring dust concentration in flue gas).
· The concentration of HCl is measured using the Mercury (III) Thiocyanate absorptiometric method based on JIS K 0107 – 1995 (Method for determination of hydrogen chloride in flue gas).
· The concentration of O2 is measured using a zirconia analyzer based on JIS K 0301-1989 (Method for determination of oxygen in flue gas).
Figure 2.2.2.- Experimental system for the incinerator flue gas treatment
Figure 2.2.3. - Detail of the wet-type plasma reactor
· The concentration of CO is measured using an infrared gas analyzer based on JIS K 0301-1989 (Method for determination of oxygen in flue gas).
· The concentration of CO is measured using an infrared gas analyzer based on JIS K 0098-1988 (Method for determination of carbon monoxide in flue gas).
· The concentration of NOx is measured using an analyzer based on JIS K 0104-1984 (Method for determination of nitrogen oxides in flue gas).
· The concentration of SO2 is measured using an infrared gas analyzer based on JIS K 0103-1995 (Method for determination of sulfur dioxide in flue gas).
· The concentration of various species of dioxin is measured based on a manual for measurement and analysis of dioxin in waste disposal. This manual is provided by the Minister of Health and Welfare.
Figure 2.2.4 shows the plasma reactor used in indoor air cleaning experiment. The dimension was 300mm wide and 50 mm and 10mm thick. A set of parallel plate electrodes was used with a separation of 16 mm and a wire electrode with 0.1 mm in diameter was placed at the center. A TiO2 – coated aluminum mesh was set in the rear side of the wire–to-plate reactor.
The experiment was carried out at room temperature and atmospheric pressure . The velocity of airflow was 1m/sec and the pressure drop of the reactor was less than 1mm H2O. A light scattering particle counter (Dan Science 82-1200) was used for measuring particle concentration (0.5m size) at the inlet and outlet of the reactor.
Figure 2.2.4. - The plasma reactor combined with TiO2 catalyst
Acetaldehyde diluted with dry air was used as a test gas and its concentration was measured using GC-FID (Shimadzu GC-17A) with its carrier gas having more than 99.999 % purity. The initial concentration of CH3CHO was 1ppm for the one-pass test.
In the circulation test, the sample gas was passed continuously through the reactor placed in a vessel of 174 L volume. Fans were used to mix the test air, which contained 10ppm of acetaldehyde. The velocity of the sample air in the reactor was kept at 1.0 m/s. In order to minimize the loss by adsorption, inner surface of the vessel was covered with a Teflon sheet. Collection efficiency of particulate matter was evaluated by measuring the concentration at the inlet and the outlet of the reactor using a particle counter. Removal efficiency of acetaldehyde was evaluated by measuring the concentration at the inlet and the outlet of the reactor. Ozone , detrimental to human health especially for indoor environment, was measured with a gas detecting tube. Figure 2.2.5 shows the waveform of applied pulsed high voltage of 5.6 kV in peak voltage (2.6kV pulse voltage biased by a positive DC 3.0 kV) with frequency of 17kHz and rising time of about 3.0ms. The DC bias contributes to the collection of particulate matter by electrostatic precipitation. The pulsed high voltage was measured with a digital oscilloscope (Tektronix TDS 350) and high voltage divider (Tektronix P6015).
Figure 2.2.5. The pulse power source and its voltage waveform
Several types of hybrid plasma reactors were tested for removal of NOx, dioxin as well as indoor air cleaning , and the following conclusions are obtained.
(1) In the plasma reactor combined with TiO2, NO is removed through oxidation reactions to HNO3. The NOx removal efficiency increases significantly when hydrogen peroxide (H2O2) is injected in this system. From the comparison tests, it has been demonstrated that the combination of discharge with TiO2 catalyst was a very efficient method for NOx removal. In this system, the formation of inorganic byproducts, such as N2O and O3, was suppressed to a lower level as the operating voltage could be reduced.
(2) Real flue gas treatment test from a diesel power generator was also carried out. At the given gas composition, the NO removal rate is mainly dependent on the value of specific input power (J/L). Charging the gas flow rector, combined with the chemical not affect the performance. The hybrid plasma reactor, combined with the chemical scrubber containing Na2SO3 solution, NO2 converted from NO in the plasma reactor can be removed to N2 by reduction. Energy consumption to achieve 60% of NOx removal is 14 J/L, corresponding to 25 of the total output power from the diesel power generator. Further improvement of energy efficiency and reduction of NO2 at the output are possible by improving the scrubber, pulse generator, and reactor geometry.
(3) Application of the wet-type plasma reactor for the incinerator flue gas treatment has been carried out. Dioxin was reduced significantly from 160 to 13 ng-TEQ/Nm3. Dust and HCl concentration of the flue gas were reduced from 0.84 to 0.0008 g/Nm3 and from 670 to 12 mg/Nm3, respectively.
(4) Compact air filter for the indoor air cleaning has been developed. It has been found that the combination of pulsed discharge and TiO2 catalyst is effective in acetaldehyde decomposition. In one-pass test, with the short residence time of 10 msec, a high removal performance of 70% and 27% was achieved for 0.5 mm particulate matter and acetaldehyde (CH3CHO), respectively.
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