Report on Delivery and Operation Condition of Grate-Type (Stoker-Type) Waste-to-Energy Plants with High Energy-Efficiency Flue Gas Treatment System Corresponding to the EU Emission Standards in China (Songjiang and Fengxian, Shanghai)

We delivered stoker-type incineration facilities in the Songjiang and Fengxian Districts of Shanghai, China, conducted performance tests, and handed the facilities over to Special Purpose Companies (SPC) of Songjiang and Fengxian plant in November and early October 2016, respectively (Figure 1 and Figure 2).

SPC, which acquired a concession from Shanghai government, constructed these Waste-to-Energy plants, and the Ebara Group performed the basic design of waste incineration system and supplied the major equipment such as furnace and auxiliaries, core component of the flue gas treatment system and the automatic combustion control (ACC) system, etc.

Shanghai is the largest city in China, and one of the direct-controlled municipalities under the central government. It consists of 15 administrative divisions and covers a total area of 6620 km² , including the Pudong New Area, with a total population of approximately 24.20 million.

Shanghai has a northern subtropical climate and four distinct seasons, a good percentage of sunshine, and relatively high precipitation. Shanghai is situated at almost the same latitude as Kyushu, Japan, and has a short spring and autumn and a relatively long summer and winter.

The average yearly temperature is approximately 17.4 ℃, and 50 % of the city’s yearly precipitation is concentrated in May through September.

Shanghai’s location in Mainland China is shown in Figure 3.

Table 1 shows the lower calorific value and composition of waste in the Songjiang and Fengxian Districts of Shanghai, Figure 4 shows a flow diagram of the plants, and Table 2 shows the specifications of the units.

The emission control values are shown in Table 3. The O₂11% equivalent value is the flue gas standard value at the outlet of the stack, and the O₂12% equivalent value is the value converted into the standard oxygen concentration in the units used in Japan.

Unlike in Japan, construction of Waste-to-Energy plants in China, is often undertaken by the SPC itself who secures the project. Consequently, we are responsible for the basic design (including a part of detailed design) of the incineration system, delivering main equipment of incineration facilities (grate incinerator, hydraulic drive unit, burners, ACC system and waste hopper level sensor) and core component of flue gas treatment system, and dispatching supervisors. The guaranteed items are shown in Table 4.

The construction schedule is shown in Table 5. The period from the contract award to the turn over of the facility was 3 years and 3 months for Songjiang plant and 3 years and 2 months for Fengxian plant.

The features of the Songjiang and Fengxian plants include a wet flue gas treatment system with distinguished acid gas removal performance to comply with the highest flue gas regulation value in China (equivalent to the EU emissions standard), and a highly corrosion-resistant GGH (gas-gas heat exchanger) made of resin with the aim of reducing the amount of steam necessary for flue gas reheating and improving energy efficiency. They also adopt a dry flue gas treatment process using relatively low-cost calcium hydroxide injection to remove acid gases to some extent. This is to reduce the consumption of the wet flue gas treatment agent (caustic soda) and thereby the total cost of chemicals.

The Ebara Group proposed these processes to the SPC at the stage of basic design, and the SPC adopted them.

In the sections that follow, we explain these features in detail and report the performance of the incineration facility in Fengxian plant representing the two plants based on the operation results.

The dry sodium bicarbonate injection process described in the report before last (Ebara Engineering Review No. 252) is one of the approach to complying with the emission control value (hydrogen chloride concentration: 10 mg/m³ (NTP) or less) equivalent to the EU standard. However this plant adopted wet type flue gas treatment proses, which removes acid gases (HCl, SOx) by circulating water with neutralizer (caustic soda).

The wet flue gas treatment unit requires a relatively large amount of makeup water, but it was adopted without any difficulty because the Shanghai region has rich water resources.

The wet scrubber adopted for this plant is a perforated plate type. The combustion flue gas is rapidly cooled almost to the saturation temperature by the circulating water kept in the lower part of the scrubber, and acid gases are removed as the circulating water containing caustic soda and the flue gas are exposed sufficiently to each other by the three-layer perforated plate installed in the upper part of the scrubber.

To reduce the cost of the chemicals used for the wet treatment process, a dry treatment process (which uses relatively low-cost calcium hydroxide) is also adopted in the preceding stage.

In the dry treatment process, acid gases are removed by calcium hydroxide powder injected into the flue gas duct at the inlet of the bag filter.

The reaction between acid gases and calcium hydroxide is an exothermic reaction, and the temperature of the acid gases affects the removal efficiency. To improve the acid gas removal performance with calcium hydroxide, the outlet temperature of the flue gas cooler is generally adjusted to 150 to 160 ℃ .

At this plant, however, the outlet temperature of the flue gas cooler is set to 175 ℃ to indirectly heat the flue gas at the outlet of the wet type flue gas treatment unit, using the flue gas at its inlet via the GGH.

The injected amount of calcium hydroxide is set to an equivalent ratio ensuring the highest level of economy after a comprehensive evaluation of the cost of the chemicals used for the dry and wet treatment processes.

The flue gas treatment equipment in Fengxian plant has demonstrated stable performance since the facility began operation in June 2016. The performance of the dry and wet type flue gas treatment process of Fengxian plant is described below.

The changes in the HCl and SOx concentrations on the upstream and downstream sides of the dry type flue gas treatment unit are shown in Figure 5 and Figure 6, respectively.

The temperature of the flue gas at the time of measurement was 175 ℃ , and the average value of the HCl concentration decreased from 229.5 ppm to 65.3 ppm with the aid of the dry type flue gas treatment unit (O₂12% equivalent value). The average value of the SOx concentration decreased from 20.8 ppm to 10.8 ppm (O₂12% equivalent value).

The removal performance by dry calcium hydroxide injection declines as the temperature of the flue gas increases, but we confirmed the satisfactory performance of reducing the acid gas concentration at the inlet of the wet type flue gas treatment unit by roughly removing acid gases even when the temperature of the flue gas is 175 ℃ (Table 6).

The changes in the HCl and SOx concentrations on the upstream and downstream sides of the wet type flue gas treatment unit are shown in Figure 7 and Figure 8, respectively.

The average value of the HCl concentration decreased from 65.3 ppm to 1.9 ppm with the aid of the wet type flue gas treatment unit (O₂12% equivalent value). The average value of the SOx concentration decreased from 11.0 ppm to 3.1 ppm (O₂12% equivalent value).

The HCl and SOx removal rates are approximately 97 % and 85 %, respectively.

The pH of the circulating water directly affects the reaction efficiency, which rises as the pH increases. However, an excessively high pH increases the scale concentration in the circulating water, resulting in the possibility of occlusion of piping, etc. In this plant, the unit is operated with a pH of around 6.5 and can fully satisfy the flue gas control value under this condition (Table 7).

Salt concentration monitoring is the key in the stable operation of the wet scrubber. As the salt concentration in the circulating water increases, it is likely that scale will adhere to the circulation piping, the pump, etc. and have a detrimental effect on the stable operation of the units.

The optimum salt concentration depends on the operating conditions. We found that although the flue gas treatment system can operate stably and without any problems with a salt concentration of a little less than 5 %, it causes the local formation of crystals when the salt concentration exceeds 8 %.

When waste incineration facilities adopt a wet type flue gas treatment unit, a steam heater is installed in a later stage of the wet scrubber in many cases to reheat the flue gas cooled in the wet scrubber to prevent lowtemperature corrosion or reduce white smoke. Consequently, many incineration facilities adopting a wet type flue gas treatment unit consume a large amount of steam in processes, produce less electric power, and experience economic difficulties.

This plant achieves both advanced flue gas treatment and high energy efficiency and ensures higher economy by adopting a highly corrosion-resistant GGH made of resin and through the process of indirectly reheating the flue gas at the outlet of the wet type flue gas treatment unit using the flue gas at its inlet without using steam to reheat the flue gas.

In China, it is rarely that white smoke exhausted from the stacks of Waste-to-Energy plants become a problem. However, this plant was to be constructed in Shanghai, which is one of the largest cities in China, and is required to reduce white smoke.

The temperature of the flue gas at the outlet of the wet scrubber is approximately 65 ℃ . To reduce white smoke from the outlet of the stack and prevent lowtemperature corrosion of the duct and the stack, this plant adopts the process of heating the flue gas to approximately 125 ℃ , which is higher than the sulfuric acid dew point, by the GGH and discharging it into the air through the stack.

Examples of the design values and actual operation values of the GGH operated in Fengxian plant are shown in Table 8.

We confirmed that the heat exchange performance of the GGH is almost as designed.

This means that approximately 21200 tons of steam can be saved per year, assuming that this plant is operated for 8000 hours yearly and that the conditions of the steam used for the flue gas reheater are 1.3 MPa and 300 ℃ (superheated steam). This can help increase the annual electric power output by approximately 3200 MW per year.

In addition, the temperature of the flue gas at the inlet of the wet scrubber is generally set to approximately 150 to 160 ℃, but in this plant, consumption of water is successfully reduced compared to regular wet scrubbers because the temperature of the flue gas at the inlet is reduced to approximately 110 ℃ thanks to the GGH.

The temperature of the flue gas at the outlet of the wet scrubber is low (65 ℃), and it may corrode the steel pipe heat exchanger. However, the GGH adopted is made of PTFE resin, and it was confirmed that 11 months after the commencement of operation, it was sound and free of corrosion, wear, etc.

In a performance test, all items met the guaranteed values, except for those that could not be verified for reasons on the client’s side. The performance test results of the incineration facility in Fengxian plant are shown in Table 9.

The incineration facility in Songjiang plant also met the guaranteed values of all items.

In many cases, incineration facilities operated in large cities in China are required to achieve the highest level of environmental performance from a global perspective. Therefore, it is necessary to propose a process that can comply with the different design conditions and customer requirements of each facility and ensure both environmental performance and economy.

Following up on the report before last, this report describes China’s incineration facilities, which are outfitted with state-of-the-art flue gas treatment equipment capable of achieving a hydrogen chloride concentration of 10 mg/m³ (NTP).

The report before last covered a different flue gas treatment process (combining a semi-dry calcium hydroxide slurry injection and dry sodium bicarbonate injection) from the flue gas treatment system described herein. We hope you will read that report as well.

We are determined to further continue to improve technology and contribute to waste treatment and environmental conservation in China.

In conclusion, we would like to thank all those who helped in this project.

1) Kazushige Kurosawa, Zhengbing Wang, et al.: Report on the Establishment and Stable Operation of Incineration Treatment Technology for Stoker-Type Incinerators in China (3rd Report), Technical Papers for the 38th National Urban Cleaning Research and Case Study Workshops (January 2017).
2) Kazushige Kurosawa, Zhibao Zhang, Shengbing Wang: Report on Delivery and Operational Condition of Gratetype (Stoker-type) Incinerator with Advanced Flue Gas Treatment System in China -Nanjing City, Jiangsu Province-, Ebara Engineering Review, No. 252, pp. 64-68 (October 2016).