Keiichi Otsuka

Performance Characteristics of Pilot-Scale Indirect Plasma and Chemical System Used for the Removal of

The process of removing NO_x from the flue gas emitted from a smoke-tube boiler was investigated using an ozonizer, which is used for carrying out the indirect oxidation of NO, and a Na₂SO₃ chemical scrubber. The flow rate of the flue gas was in the range of 410 - 1480 N·m³/h, its temperature was in the range of 185°C- 325°C, and the concentration of NO_x was approximately 40 ppm during the combustion of city gas. Operational and performance data were obtained from the experiments. The efficiency of the NO_x removal process was clearly dependent on the oxidation reduction potential (ORP), and it increased with a decrease in the ORP of the scrubbing solution. To effectively remove the requisite amount of NO_x, it is essential to maintain the ORP by controlling the injection rates of the Na₂SO₃ solution and maintaining an ORP of less than 0 mV and the rate of change in ORP to less than 0 mV/min. More than 4 kg/h of Na₂SO₃ must be added to the scrubber in order to sustain the aforementioned ORP condition of the scrubbing solution. The amount of NO_x emitted from the flue gas was maintained as less than 4 ppm at a flue gas flow rate of 410 N ·m³/ h during a 300-min continuous system operation by maintaining an ORP of -30 mV and a pH of more than 7.8 for the scrubbing solution.

\hbox{NO}_{\rm x}

NO_x removal from the flue gas of a smoke tube boiler using an ozonizer for NO oxidation and a Na₂SO₃ aqueous solution scrubber (diameter: 0.6 m; height: 3.7 m) was investigated. In this paper, in order to determine the correlation NO_x removal performance, the height of the packing material layer in the scrubber was varied from 2.1 to 0 m. The performance tests were carried out using both a compressed natural gas (CNG) and a heavy oil. The flue gas flow rates were in the range of 455-1440 Nm³/h under CNG firing and 675-1330 Nm³/h under heavy oil firing, and the boiler NO_x emission concentrations were approximately 40 and 100 ppm, respectively. A total packing height of 0.75 m resulted in NO_x removal efficiencies of 70% and 60% for CNG and heavy oil, respectively. Moreover, the Na₂SO₃ consumption rate at 0.75 m was 10%-20% less than that at 1.2 m, although the NO_x removal efficiency was nearly the same. The correlation between the NO_x removal performance and the gas retention time in the scrubber was obtained, providing the design information of the scrubber for industrial applications.

From Boiler Emission

A pilot-scale low-emission boiler system consisting of a bio-fuel boiler and a plasma-chemical hybrid NO_x removal system is investigated. This system can achieve carbon neutrality because the bio-fuel boiler uses waste vegetable oil (WVO) as one of the fuels. The plasma-chemical hybrid NO_x removal system has two processes: NO oxidation by ozone produced from plasma ozonizers and NO₂ removal using a Na₂SO₃ chemical scrubber. Test demonstrations of the system were carried out for mixed oils (mixture of heavy oil and WVO). A stable combustion was achieved for the mixed oil (20%-50% WVO). The properties of flue gas-e.g., O₂, CO₂, and NO_x -when firing mixed oils were nearly the same as those when firing heavy oil for an average flue gas flow rate of 1000 Nm³/h. The NO_x concentrations at the boiler outlet were 90-95 ppm. Furthermore, during a 300-min continuous operation when firing 20% mixed oil, a NO_x removal efficiency of more than 90% was confirmed. This is equivalent to less than 10 ppm at the scrubber outlet when the flue gas flow rate was 870 Nm³/h. In addition, CO₂ reduction when heavy oil was replaced with WVO was estimated. The system comparison is described between the plasma-chemical hybrid NO_x removal and the conventional NO_x removal.

Improvement in

NO x removal from a smoke tube boiler flue gas was investigated using a commercial ozonizer for indirect NO oxidation and a Na2SO3 chemical scrubber. The flue gas flow rate was in the range of 410 Nm3/h-1480 Nm3/h, the gas temperature of 185°C–325°C, and NO x concentration of around 40 ppm in city gas firing. Operational and performance data were obtained. The NO x removal efficiency was clearly dependent on the ORP and increased inversely as the oxidation reduction potential (ORP) in the liquid decreased. To keep the specified NO x removal performance, it is essential to maintain the ORP properly by controlling additional Na2SO3 and NaOH solution injection. NO x emission of less than 4 ppm was attained at 410 Nm3/h for a 300 minutes system continuous operation by maintaining ORP of-30 mV and pH of more than 7.8.

\hbox{NO}_{\rm x}

The number of small boilers using city natural gas, heavy oil (HO), and waste oils has been increasing annually in Japan, and more stringent regulations for nitrogen oxides (NOx) emission are being anticipated to reduce environmental NOx concentration. Taking this into consideration, it is envisioned that a suitable flue gas treatment system for small boilers will be required. The author proposed a plasma–chemical hybrid clean technology, consisting of an indirect nonthermal plasma process and a wet-chemical treatment. The tested flue tube boiler has an original rotary burner for gas and/or oil and is operated using city natural gas, biomass oil, or HO. The boiler has a steam generation rate of 2.5 ton/h, and for the clean technology, two sets of silent discharge-type plasma ozonizers are employed to generate ozone. Using the combination of ozone injection and chemical scrubber for flue gas, NOx can be effectively decomposed to nitrogen and oxygen, thus purifying the resulting effluent. The amount of nitrogen monoxide (NO) removed is almost the same as the amount of the corresponding ozone required to oxidize NO to nitrogen dioxide (NO2) (1:1 stoichiometric ratio). Previously reported experimental data are also discussed in the paper. A NOx removal efficiency of more than 85% was achieved over an operating time of 23 h using city natural gas as fuel. Based on the concept of carbon neutrality, ~80% carbon dioxide (CO2) reduction or fuel saving is also possible using waste vegetable oil (WVO)/HO mixed fuels. This low-emission boiler system can be used in industry.

Removal Performance of the Pilot-Scale Boiler Emission Control System Using an Indirect Plasma–Chemical Process

A pilot-scale low emission boiler plant which consists of a multi-fuel boiler and a plasma-chemical hybrid NOx removal system was investigated. This plant can achieve carbon neutrality because the multi-fuel boiler uses waste　vegetable oil as one of fuels. The plasma-chemical hybrid NOx removal system has two processes; one is NO oxidation by ozone produced from plasma ozonizers, and the other is NO2 removal using a Na2SO3 chemical scrubber. The operation tests of the plant were carried out for mixed oils (mixture of heavy oil and waste vegetable oil). Stable combustion was achieved for the mixed oil (20%~50% vegetable oil). The flue gas properties, such as O2, CO2 and NOx in firing mixed oils, were nearly the same as those in firing heavy oil when the flue gas flow rate was 1000 Nm3/h on average. NOx concentrations at boiler outlet were 90~95 ppm. Furthermore, it is confirmed that during a 300 min continuous operation in firing 20% mixed oil NOx removal efficiency was attained more than 90%, equivalent to less than 10 ppm at scrubber outlet when the flue gas flow rate was 870 Nm3/h. In addition, the effect of replacing heavy oil by waste vegetable oil on CO2 reduction was estimated.

Development of Low-Emission Bio-Fuel Boiler System With Plasma-Chemical Hybrid

NOx removal from a smoke tube boiler flue gas was investigated using a commercial ozonizer for indirect NO oxidation and a Na2SO3 chemical scrubber. The flue gas flow rate was in the range of 410~1,480 Nm3/h, the gas temperature of 185~325degC, and NOx concentration of around 40 ppm in city gas firing. Operational and performance data were obtained. The NOx removal efficiency was clearly dependent on the ORP, and increased inversely as the oxidation reduction potential (ORP) in the liquid decreased. To keep the specified NOx removal performance, it is essential to maintain the ORP properly by controlling additional Na2SO3 and NaOH solution injection, and to keep the ORP of less than 0 mV and the ORP change rate of less than 0 mV/min. At least, more than 4 kg/h of Na2SO3 added to the scrubber should be necessary to sustain the condition. NOx emission of less than 4 ppm was attained at 410 Nm3/h for a 300 minutes system continuous operation by maintaining ORP of -30 mV and pH of more than 7.8.