Potential environmental benefits of integrating flue gas quench in biomass/waste‐fueled CHP plants

Due to stricter regulations, large biomass/waste incineration power plants are expected to reduce (i) pollutant emissions through water (such as organic compounds dissolved in the discharge water), (ii) the withdrawal of external freshwater, and (iii) the disturbance to the natural water by increasing the water recycle and internal reuse. To address such challenges, flue gas quench (FGQ) is playing a vital role that links flue gas (FG) cleaning and wastewater treatment. In this study, a detailed analysis based on the material and energy balance is performed regarding the pollutant distribution in the flue gas and the wastewater within a combined heat and power (CHP) plant. The real data from the reference CHP plant were used; and results show that the utilization of FGQ can result in less wastewater discharge (about 73 tonnes/d) together with less pollutant concentration to the municipal wastewater treatment plant, as compared to the system with only flue gas condenser but without FGQ. The integration of FGQ also results in less burden on the external freshwater use by increasing the amount of clean water for internal use (about 57 tonnes per day). In addition, the integration of FGQ can offer a potential annual energy saving of about 13.1 MWh in the municipal wastewater treatment plant due to the less wastewater coming from the CHP plant.


| INTRODUCTION
Biomass and waste usually have a high moisture content that can be up to 50% for biomass and even higher for municipal solid wastes (MSW). [1][2][3] To improve the energy efficiency of biomass/waste-fueled combined heat and power (CHP) plants, flue gas condensers (FGCs) are commonly employed, 4,5 resulting in a large amount of condensed water. For example, in Sweden, the annual amount of generated condensate from FGC is estimated to be nearly 400 Mton. 8 The condensed water gets contaminated with the organic and inorganic compounds in the flue gas (FG), such as acidic gases (SO 2 and HCl), NH 3 , and heavy metals. 6,7 Therefore, it needs to be treated before it can be discharged into the environment. In addition to the polluted water, a substantial amount of freshwater is also taken from rivers or lakes for cooling purpose and also as make-up water for steam cycles and the district heating. This adds more burden on the environment in terms of water and energy use. 9 Currently, stricter regulations require further reduction of negative impacts on the environment due to released pollutants like organic compounds, acids and heavy metals, polluted water, and solids. According to the European Commission, the large incineration power plants are expected to reduce emission through water, such as organic compounds dissolved in the discharge water. 10,11 Furthermore, the EU Water Framework Directive demands the reduction of freshwater withdrawal and increased water recycle with internal reuse in order to reduce the disturbance to the natural water. 12 For new biomass-based CHP plants, integrating a flue gas quench (FGQ) before FGC is gaining attention. In the FGQ, water is introduced to wash the FG and the water-soluble pollutants are removed from the FG. [13][14][15][16] As a result, the FG leaving the FGQ becomes cleaner that improves the water quality from the FGC. Since the FGQ employs wastewater from the FGC during the water scrubbing, a large amount of water can evaporate and leave with the FG. The FGC can then effectively reduce the amount of wastewater that needs be further treated and, consequently, save a considerable amount of energy in the wastewater treatment plant (WWTP).
Our previous study on the FGQ has briefly reviewed the research works available in the literature about the development and performance of the FGQ. 17 A mathematical model was developed to study the operation of FGQ. The current study is a continuation of our research work on the FGQ, and the primary objective is to establish knowledge about the potential environmental benefits when integrating the FGQ in biomass/waste-fueled CHP plants. The potential benefits are evaluated from the perspectives of (i) removal of pollutants from FG, (ii) reduction in energy demand of wastewater treatment, and (iii) reduction in external freshwater use. A detailed analysis based on mass balance is performed to investigate the distribution and transfer of pollutants from the FG to water. The combined role of the FGQ in the FG cleaning and wastewater treatment is also studied. The key knowledge gaps bridged by this study are: • What are the potential reductions in pollutants emission (removal of pollutants) through wastewater that will be discharged from CHP plants? • What is the potential reduction in external freshwater use from the integration of FGQ? • What are the potential energy savings from wastewater treatment plant due to reductions in the amount of wastewater? • Which factors can clearly affect the load reduction in the WWTP and coupled energy savings? 2 | SYSTEM DESCRIPTION AND

| Reference CHP plant and FGQ
In current study, a typical biomass-based CHP plant with the thermal capacity of 170 MW is taken as a reference. Figure 1 shows the schematic diagram of the flue gas cleaning process. 19 The NOx is removed when the FG passes through the primary de-NOx and the secondary de-NOx based on the selective noncatalytic reduction (SNCR). Most of the dust in the FG is filtered out by the bag house filter (BHF). Sulfur, halides, and alkali dissolved in water are removed using the wet scrubber. Typically, the wet scrubber is suitable for the cold downstream application. 23 In the FGC, moisture is condensed from the FG to recover heat. Since the FG still contains a large amount of pollutants, the condensate from the FGC also contains high levels of pollutants, such as SOx, HCl, NH 3 , HF, dioxin, furans, and heavy metals. 20 Such a stream requires treatment that consumes additional energy. 21,22 In order to reduce the amount of wastewater that needs treatment, the FGQ is proposed to be added before the FGC that can use the wastewater from the FGC. 4,24 In order to analyze the variations of pollutants in the flue gas and the process water under different operating conditions of the CHP plant, three cases have been developed and studied. A brief explanation of studied cases is presented: -Case 1: with FGC but without FGQ (Streams [1][2][3][4][5] This case corresponds to the conventional CHP plant. After BHF, the FG is introduced to the FGC to recover heat (Stream 1). The FG is released to the atmosphere through the stack (Stream 2). The FGC not only improves the boiler efficiency by recovering the sensible and latent heat of the FG, but also reduces the emissions of pollutants such as dissolved salts, aerosols pollutants that can be discharged into the atmosphere without the FGC. 25 The condensate from the FGC is discharged to the wastewater treatment unit (Stream 3) that produces clean water (Stream 5). The remaining water that contains high concentration pollutants is discharged into the WWTP (Stream 4).
-Case 2: with FGQ and FGC (Streams [6][7][8][9][10][11][12][13][14] In this case, the FGQ is integrated before the FGC as shown in Figure 1. The FG first passes through the FGQ before going into the FGC. It is important to note that the condensate can be distributed into two streams: one stream is distributed to the FGQ (Stream 9) and the other stream is distributed to the WWT (Stream 10). After the wastewater treatment, Stream 11 is also introduced to the FGQ. Streams 9 and 11 are used for FG scrubbing. Due to continuous operation of the FGQ, a substantial amount of pollutants from the WWT and the FG are accumulated in the quench water. In order to control the concentration of contaminants below the safe level, a part of the pollutant-rich wastewater is rejected to the boiler.
-Case 3: with FGQ but without FGC (Streams [15][16][17][18] In summer, due to the reduced heat demand, the FGC is not in operation. In such a scenario, there is no condensate produced from the FGC. To run the FGQ, external water is needed instead for the FG scrubbing.

| Mass balances of pollutants in flue gas and process water
A model based on mass balance is developed in order to analyze the pollutant distribution and the transfer from FG to the process water. Case 1: with FGC but without FGQ For FGC: For WWT: where m 1 and m 2 represent the mass flow rate of the FG at the inlet and outlet of the FGC (kg/s); m 3 , m 4 , and m 5 are the mass flow rate of condensate, wastewater discharged to the WWTP, and clean water (kg/s), respectively; C 1 and C 2 are contaminant concentrations in FG of inlet and outlet FGC (mg/Nm 3 ); C 3 , C 4 , and C 5 are contaminant concentrations in the condensate, wastewater, and clean water (mg/L); ∆m FGC and ∆m WWTP are the mass flow rate of contaminant in the condensate and wastewater (mg/s); and i represents the different contaminants.

Case 2: with FGQ and FGC
For FGC: For FGQ: where m represents the mass flow rate of the FG (stream 6, 7) and water (Stream 8-12) (kg/s); C is contaminant concentration in FG and water (mg/Nm 3 or mg/L); similarly, dm is water evaporation in FGQ (kg/s); and ∆m FGQ,i is the amount of contaminants captured by quench water from FG (mg/s). For the model of mass balance of FGC and WWT, it is identical with the Case 1. The contaminants removed from FG can be calculated by Equation (16): where ∆m i is the amount of contaminants captured by water (mg/s); C j,i represents the different contaminant concentrations in different streams (mg/L); m j,i represents the mass flow rate of FG or water in different steams (kg/s); and λ i is the coefficient of pollutant removal from FG or water.

| Inputs and assumptions
In current study, three main contaminants, i.e. NH 3 , SO 2 , and HCl, are considered. It is assumed that 90% of NH 3 , SO 2 , and HCl can be removed from the FG by water scrubbing. 18,[26][27][28] In order to reduce the pollutant concentrations, it is assumed that the removal rate of NH 4 -N, HCl, and SO 2 are 99.7%, 80%, and 96%. [29][30][31] The contents of such contaminants in external water and FG (Stream 1 and Stream 6) are listed in Table 1, 20 and Table 2. 32 According to our previous work, 17 the profiles in various streams are shown in Table 3.

| Results on mass balance
Based on the mass balance model and the measurement data in Tables 1-3 Figure 3 shows that the concentrations of NH 4 -N, Cl, and S in the process water are up to 976, 857, and 5266 mg/L, respectively, and the process water is then discharged to the WWTP.   In the FGQ, a large amount of water is evaporated due to the high temperature of the FG. In consequence, more condensate is produced in the FGC. Although a part of condensate water is sent to FGQ, the flow rate of Stream 10 in this case is still higher than Stream 3 in Case 1. Therefore, more clean water is produced in Case 2, with concentrations of NH 4 -N, Cl, and S at 0.03, 1.9, and 2 mg/L.

Case 3: with FGQ but without FGC
Typically, the FGC is not in operation during the summer and the water needed by the FGQ is taken externally due to the nonavailability of the condensate. The concentrations of NH 3 , HCl, and SO 2 of FG after the FGQ (Stream 16) are estimated to be 0.76, 0.83, and 4.25 mg/Nm 3 , and the concentrations of NH 4 -N, Cl, and S of rejection water (Stream 18) are 4624, 5250, and 26 074 mg/L (see Figure 6). The released FG shows higher pollutant concentrations in Case 1 and Case 3 than in Case 2. For water, the pollutant concentrations of rejection water to boiler in Case 3 are lower than in Case 2, reducing by 9.05%, 4.06%, and 8.2%, respectively, for NH 4 -N, Cl, and S. This will lead to more pollutants that are sent into the atmosphere.

| Potential savings in WWTP
The comparison between Case 1 and Case 2 shows a significant reduction in the amount of wastewater that needs to be treated at the WWTP in Case 2. For the reference plant, CHP without FGQ discharges about 73 tonnes per day of wastewater (with 976 mg/L of NH 4 -N, 857 mg/L of Cl, and 5266 mg/L of S) to the WWTP, whereas, when FGQ is integrated, nearly an identical amount of wastewater from internal WWT unit is used as injection water to FGQ and the rejection water from FGQ is further introduced to the boiler. Moreover, the concentrations of contaminants are also substantially lower when FGQ is integrated, that is, the injection water to FGQ contains about 79 mg/L of NH 4 -N, 69 mg/L of Cl, and 425 mg/L of S.
In addition, the stream data from the FGC highlight lower contents of contaminants in the wastewater that need to be treated by the internal WWT unit due to the FGQ integration, i.e. 8 mg/L of NH 4 -N, 9 mg/L of Cl, and 45 mg/L of S in comparison with 99 mg/L of NH 4 -N, 108 mg/L of Cl, and 554 mg/L of S in the case without the FGQ integration.

| Potential reduction in freshwater use
The condensate after treatment can be used internally. Without the FGQ, the amount of wastewater for the internal WWT is about 727 tonnes per day and 654 tonnes per day of clean water is produced. In comparison, with the FGQ, the amount of wastewater is about 791 tonnes per day that produces 711 tonnes per day of clean water. This implies that nearly 57 tonnes per day of more clean water are available for internal use, resulting in less burden on the external freshwater use.

| Energy savings in the WWTP
The potential reduction in the amount of wastewater to be treated by the WWTP results in considerable energy savings. The average energy consumption at Scandinavian wastewater treatment plants is quantified as an average of 0.49 kWh/m 3 with a standard deviation of 0.197, 33 and the density of wastewater is between 998 and 1001 kg/m 3 . 34 The comparative analysis between systems with and without FGQ shows a potential reduction of 73 tons of wastewater per day to the WWTP that may results in potential annual energy savings of about 13.1 MWh based on the reference CHP plant capacity.

| Sensitivity analysis
In this section, a brief sensitivity analysis is performed for Case 1 and Case 2 to analyze the variation of potential load reduction on WWTP, potential reduction in the freshwater use, and energy savings in WWTP due to the fluctuation of pollutants concentrations in the FG. Case 3 is not included in the sensitivity analysis as no water is discharged.
T A B L E 4 Sensitivity analysis for the concentrations of NH 3 , HCl, and SO 2 in different streams in Case 1   The pollutant concentrations in Streams 7/8/9/10, and 11-14 are changed after varying the pollutant concentrations in FG before FGQ, as shown in Table 5 Figure 8 shows the impacts of variation of pollutant concentrations on potential load reduction of the WWTP. The flow rate of different streams is changed after the pollutant concentrations increased or decreased. Thisresults in increasing or decreasing in potential load reduction of the WWTP. Figure 8 shows that the potential load reduction increases with the increase of pollutant concentrations. With variations in the pollutant concentrations (∓ 20%), there is a significant impact on the potential load reduction of the WWTP, that is, load reduction varies between 66 tonnes/d and 81 tonnes/d and decreased or increased by 11%. This variation will further have direct implications on the energy consumption and energy savings in the WWTP. Figure 9 shows the impacts of variation of pollutant concentrations on potential reduction in freshwater use. It shows that the potential reduction in freshwater use increases with the increase of pollutant concentrations. This is due to that the flow rate of condensate water increases with the pollutant concentrations. The maximum and minimum of potential reductions in freshwater use are 73 and 41 tonnes per day, respectively, decreased or increased by 28%.  Figure 10 shows the impacts of variation of pollutant concentrations on energy saving in WWTP. It also shows that the energy saving in WWTP increases with the increase of pollutant concentrations. It indicates that the produced injection water increases with increase of pollutant concentrations. When FGQ is integrated into the CHP system, the injection water is introduced into FGQ that can reduce the energy consumption of the WWTP. The maximum and minimum of energy savings in WWTP are 11.8 and 14.5 MWh per year, respectively.

| CONCLUSIONS
In this study, a detailed analysis is presented to understand the role of flue gas quench (FGQ) in the wastewater treatment and the FG cleaning for a biomass fueled CHP plant. The impacts on the distributions of pollutants, the energy demand of the WWTP, and the withdrawal of external freshwater have been investigated. Based on the reference CHP plant, the answers to the identified key research questions are given: (i) The system with FGQ has less amount of wastewater to be treated (about 73 tonnes/d) together with less pollutant contents in the wastewater (in terms of NH 4 -N, Cl, and S) compared to the system without FGQ. (ii) There is relatively more clean water (about 57 tonnes/d) available for internal use within FGQ system resulting in less burden on the external freshwater use. (iii) The CHP plant with FGQ may result in a potential annual energy saving of about 13.1 MWh in WWTP due to less wastewater and pollutant load. (iv) The sensitivity analysis shows that the variations of pollutant concentrations have a significant impact on load reduction in MWWTP and coupled energy savings.