A method for recovering nitrogen and phosphorus in sludge hydrothermal solution and improving the combustion performance of hydrothermal carbon

Abstract

The present invention discloses a method for recovering nitrogen and phosphorus in sludge hydrothermal liquor and improving the combustion performance of hydrothermal carbon. The method utilizes the struvite crystallization method to recover magnesium ammonium phosphate (commonly known as struvite) during the aqueous phase recycling process of sludge hydrothermal carbonization. The sludge is subjected to hydrothermal carbonization treatment at multiple reaction temperatures and numbers of aqueous phase recycling. The influence of the aqueous phase after struvite crystallization treatment on the combustion performance of hydrothermal carbon during the aqueous phase recycling process of sludge is systematically studied, and hydrothermal carbon with better combustion performance is obtained. Moreover, struvite is usually used as a fertilizer in agriculture. The method described in the present invention realizes the resource utilization of sludge hydrothermal carbonization liquid, improves the combustion performance of sludge-based hydrothermal carbon and reduces its nitrogen content, reduces environmental pollution, is easy to operate, and provides an environmentally friendly method for sludge treatment.

Description

Technical Field

[0001] The present invention belongs to the field of physical chemistry and green energy, and particularly relates to a method for recovering nutrients in sludge hydrothermal carbonization liquid and improving the combustion performance of sludge hydrothermal carbon. Background Art

[0002] Due to the relatively low temperature and universality for sludge with different water contents, hydrothermal carbonization is a common technology in the sludge treatment process. The main product of sludge hydrothermal carbonization is sludge-based hydrothermal carbon, which is a potential fuel source. However, during the sludge hydrothermal carbonization process, much more attention has been paid to hydrothermal carbon than to hydrothermal carbonization liquid. The solvent for sludge hydrothermal carbonization is usually water, and the hydrothermal carbonization liquid after the reaction contains a large amount of substances such as ammonia nitrogen and phosphate. Direct discharge will cause environmental pollution. Therefore, developing a method for recovering nutrients in hydrothermal carbonization liquid and improving the performance of hydrothermal carbon is of certain significance for the environmentally friendly treatment and resource utilization of sludge.

[0003] Some researchers have studied the recovery of nutrients in the aqueous phase and found that the struvite crystallization method has the advantages of simple operation and spontaneous reaction without additional energy input for recovering nitrogen and phosphorus resources from hydrothermal carbonization liquid. Therefore, the struvite crystallization method provides a feasible way for recovering nutrients in sludge hydrothermal carbonization liquid. Other researchers have shown that sludge aqueous phase recycling is a feasible way for the treatment of sludge hydrothermal carbonization liquid.

[0004] In recent years, the aqueous phase problem in the sludge treatment process has attracted people's attention. However, current research mainly focuses on the recovery of substances in the pure aqueous phase or the treatment of the pure aqueous phase, and there is almost no discussion on recovering nutrients in the aqueous phase and simultaneously realizing the treatment of the aqueous phase. On this basis, combining hydrothermal carbonization aqueous phase recycling with the struvite crystallization method can improve the performance of hydrothermal carbon while recovering nutrients in hydrothermal carbonization liquid, providing a reliable solution for the recovery and treatment of the aqueous phase in sludge hydrothermal carbonization. Summary of the Invention

[0005] Aiming at the deficiencies of the existing hydrothermal carbonization aqueous phase treatment, the present invention provides a method for recovering nutrients in sludge hydrothermal carbonization liquid and improving the combustion performance of sludge hydrothermal carbon.

[0006] To achieve the above objectives, the specific technical solutions adopted by the present invention are as follows:

[0007] A method for recovering nitrogen and phosphorus in sludge hydrothermal liquid and improving the combustion performance of hydrothermal carbon, the method comprising the following specific steps:

[0008] Step 1: Using sludge as raw material, adding water as solvent, conducting hydrothermal carbonization reaction alone under the reaction conditions of 200 - 260 °C for 60 min to obtain hydrothermal carbon and hydrothermal carbonization liquid under the said reaction conditions; wherein, the mass ratio of the sludge to water is 1:1;

[0009] Step 2: Detect the content of NH4 + -N and PO4 3- -P in the hydrothermal carbonization liquid obtained in Step 1, and use the struvite crystallization method to recover NH4 + -N and PO4 3- -P in the hydrothermal carbonization liquid in an equimolar ratio. The reaction equation is:

[0010] Mg 2+ +NH4 + +PO4 3- +6H2O → MgNH4PO4·6H2O

[0011] The struvite crystallization is carried out in a beaker and stirred using a magnetic stirrer; using magnesium chloride, ammonium chloride, and dipotassium hydrogen phosphate as the magnesium source, nitrogen source, and phosphorus source, adding magnesium chloride, ammonium chloride, and dipotassium hydrogen phosphate to the hydrothermal carbonization liquid, so that the molar ratio of the three ions is n(Mg 2+ )∶n(NH4 + )∶n(PO4 3- ) = 1.0∶1.0∶1.0, adjusting the pH to alkaline, stirring for 30 min, with a stirring intensity of 150 r·min -1 , at a temperature of 25 °C. After stirring, let it stand for 30 min and then filter to obtain magnesium ammonium phosphate, i.e., struvite, and the aqueous phase after recovering struvite. Detect the concentration of NH4 + -N and PO4 3- -P in the aqueous phase after recovering struvite to calculate the recovery rate;

[0012] Step 3: Replace the solvent water in Step 1 with the aqueous phase obtained in Step 2, and conduct the operation in Step 1 to obtain hydrothermal carbon with a 29.70% improvement in combustion performance and hydrothermal carbonization liquid;

[0013] Step 4: Conduct the operation in Step 2 on the hydrothermal carbonization liquid in Step 3 to obtain magnesium ammonium phosphate, i.e., struvite, and the aqueous phase after recovering struvite;

[0014] Step 5: Replace the solvent water in Step 1 with the aqueous phase obtained in Step 4, and conduct the operation in Step 1 to obtain hydrothermal carbon with a 130.83% improvement in combustion performance and hydrothermal carbonization liquid;

[0015] Step 6: Conduct the operation in Step 2 on the hydrothermal carbonization liquid in Step 5 to obtain magnesium ammonium phosphate, i.e., struvite, and the aqueous phase after recovering struvite;

[0016] Step 7: Repeat Step 5 and Step 6; repeat at least 2 times, and the combustion performance of the hydrothermal carbon obtained is improved to 223.03%.

[0017] The present invention combines the struvite crystallization method with the aqueous phase recycling process of sludge hydrothermal carbonization. Experiments have found that it can significantly improve the combustion performance of sludge hydrothermal carbon, and can efficiently recover ammonia nitrogen and phosphate in water bodies; the method described in the present invention realizes the resource utilization of sludge hydrothermal carbonization liquid, improves the combustion performance of sludge-based hydrothermal carbon and reduces its nitrogen content, reduces environmental pollution, is easy to operate, and provides an environmentally friendly method for sludge treatment. Description of the Drawings

[0018] Figure 1 It is a Van diagram of hydrothermal carbon and sludge produced by two aqueous phases at different cycle times and temperatures. Detailed Embodiments

[0019] The following describes the present invention in detail with reference to the drawings and embodiments.

[0020] Example 1

[0021] The struvite crystallization method is coupled with the aqueous phase recycling of sludge hydrothermal carbonization to achieve the recovery of nutrients in the aqueous phase and the improvement of the combustion performance of hydrothermal carbon.

[0022] In this embodiment, in order to better prove the improvement of the combustion performance of hydrothermal carbon by the aqueous phase recycling of the struvite crystallization method combined with sludge hydrothermal carbonization, the aqueous phase that has not been recycled by the struvite crystallization method is also recycled, and the difference in the performance of hydrothermal carbon under the same reaction conditions is compared.

[0023] For convenience of description, the aqueous phase recovered by the struvite crystallization method is represented by 1#, and the aqueous phase not recovered by the struvite crystallization method is represented by 2#; 200, 230, 260 represent the reaction temperature; +0, +1, +2, +3, +4 represent the number of cycles, and +0 represents the aqueous phase or hydrothermal carbon that has not been recycled after the first reaction.

[0024] 1.1 Mix municipal sludge (wet basis) and water in a reaction kettle at a mass ratio of 1:1, seal the reaction kettle, and heat it for reaction according to the set reaction conditions (reaction temperature is 200 °C, 230 °C, and 260 °C, and reaction time is 60 min). After the reaction is completed, move the reactor to an ice bath and cool it to room temperature. The product is a mixture of solid and liquid, and the solid product (hydrothermal carbon) and aqueous phase are separated by vacuum filtration. The solid product is dried to constant weight in an oven at 105 °C, and the aqueous phase is used for the recycling experiment.

[0025] 1.2 The struvite crystallization method experiment was carried out in a 100 - milliliter beaker and stirred using a magnetic heating stirrer. After a typical hydrothermal carbonization experiment, the contents of ammonia nitrogen and phosphate in the water phase to be recycled were tested. MgCl₂·6H₂O and K₂HPO₄·3H₂O were used as the magnesium source and phosphorus source, and sodium hydroxide solution was used as the additional alkali solution to adjust the pH value of the water phase to 10. The reaction was carried out at 30 °C for 30 minutes. After the reaction, it was left standing for 30 minutes, and then the struvite and the water phase after recovering the struvite were separated by vacuum filtration. The ammonia nitrogen and phosphate concentrations of the separated water phase were tested, and the recovery rates of ammonia nitrogen and phosphate were calculated (Table 1).

[0026] 1.3 The separated water phase was used for the next reaction. In the recycling experiment of the water phase after struvite crystallization method recovery, each hydrothermal carbonization reaction needed to treat the water phase according to step 1.2, and the water phase was recycled four times during the experiment process.

[0027] Elemental and thermogravimetric tests were carried out on the obtained hydrothermal carbon (Table 2, Table 3).

[0028] It can be seen from the data obtained in the experiment that for the water phase recovered by the struvite crystallization method, the N content of the hydrothermal carbon after recycling is less than that of the untreated water phase. And after four - cycle recycling, it can be observed that at the same temperature, for the water phase recovered by the struvite crystallization method, the ignition temperatures of the hydrothermal carbon 1#200 + 4, 1#230 + 4 and 1#260 + 4 obtained by recycling (269.99 °C, 268.95 °C and 273.55 °C) are all higher than those of 2#200 + 4, 2#230 + 4 and 2#260 + 4 (241.39 °C, 230.23 °C and 251.30 °C). The higher ignition temperature makes the fuel have better performance in terms of safe handling, storage and transportation. In addition, the burnout temperatures of 1#200 + 4, 1#230 + 4 and 1#260 + 4 (336.04 °C, 359.89 °C and 365.02 °C) are lower than those of 2#200 + 4, 2#230 + 4 and 2#260 + 4 (429.24 °C, 459.66 °C and 410.97 °C). This shows that the recycling of the water phase after struvite crystallization method recovery can make the hydrothermal carbon burn faster and the flame more concentrated. The combustion indices of the hydrothermal carbon obtained from the recycling experiment of the water phase after struvite crystallization method recovery are 115.13 (1#200 + 4), 74.50 (1#230 + 4) and 51.49 (1#260 + 4) respectively, and their combustion performance is better than that of the hydrothermal carbon produced by recycling the untreated water phase (40.67 (2#200 + 4), 41.23 (2#230 + 4) and 34.07 (2#260 + 4)). According to the Van Krevelen diagram ( Figure 1 ), the hydrothermal carbon obtained by recycling the water phase after struvite crystallization method recovery is closer to the lignite region and has a higher degree of coalification. This is also confirmed.

[0029] Table 1 Recovery rates of ammonia nitrogen and phosphate in the aqueous phase by struvite crystallization method at different temperatures (aqueous phase after recovery by 1# struvite crystallization method; aqueous phase not recovered by struvite crystallization method)

[0030]

[0031] Table 2 Elemental analysis of hydrothermal carbon produced from two aqueous phases at different cycling times and temperatures (aqueous phase after recovery by 1# struvite crystallization method; aqueous phase not recovered by struvite crystallization method)

[0032]

[0033] Table 3 Combustion characteristics of sludge and hydrothermal carbon at 20 °C / min. (aqueous phase after recovery by 1# struvite crystallization method; aqueous phase not recovered by struvite crystallization method)

[0034]

[0035]

[0036] The preferred embodiments of the present invention have been specifically described above. However, the present invention is not limited to the described embodiments. Those skilled in the art can also make various equivalent variations or substitutions without departing from the spirit of the present invention. These equivalent variations or substitutions are all included within the scope defined by the claims of this application.

Claims

1. A method for recovering nitrogen and phosphorus from sludge hydrothermal fluid and improving the combustion performance of hydrothermal carbon, characterized in that: The method comprises the following specific steps: Step 1: using sludge as raw material, adding water as solvent, and performing hydrothermal carbonization reaction alone under the reaction conditions of 200-260° C. and 60 min to obtain hydrothermal carbon and hydrothermal carbonized liquid under the reaction conditions; wherein the mass ratio of sludge to water is 1:1; Step 2: Detecting NH4 in the hydrothermal carbonization liquid in step 1 + -N and PO4 3- -P content, using struvite crystallization method, equimolar recovery of NH4 in hydrothermal carbonization liquid + -N and PO4 3- -P, the reaction equation is: Mg 2+ + NH4 + + PO4 3- + 6H2O → MgNH4PO4·6H2O The struvite crystallization was carried out in a beaker and stirred with a magnetic stirrer; magnesium chloride, ammonium chloride and potassium dihydrogen phosphate were used as magnesium source, nitrogen source and phosphorus source, and magnesium chloride, ammonium chloride and potassium dihydrogen phosphate were added to the hydrothermal carbonization liquid to make the ratio of the three ion substances be n (Mg 2+ )∶n (NH4 + )∶n (PO4 3- ) = 1.0∶1.0∶1.0, adjust pH to alkaline, stir for 30 min, stirring intensity 150 r·min -1 , the temperature was 25℃, after stirring, it was allowed to stand for 30min and filtered to obtain magnesium ammonium phosphate, i.e., struvite, and the aqueous phase after the struvite was recovered. The NH4 + -N and PO4 3- -P concentration in order to calculate the recovery rate; Step 3: replacing the solvent water in step 1 with the water phase obtained in step 2, and performing the operation in step 1 to obtain hydrothermal carbon and hydrothermal carbonized liquid with a combustion performance improved by 29.70%; Step 4: performing the operation of step 2 on the hydrothermal carbonized liquid of step 3 to obtain magnesium ammonium phosphate, i.e., struvite, and a water phase after recovering the struvite; Step 5: replacing the solvent water in step 1 with the water phase obtained in step 4, and performing the operation in step 1 to obtain hydrothermal carbon and hydrothermal carbonized liquid with a combustion performance improved by 130.83%; Step 6: Perform the operation of step 2 with the hydrothermal carbonization liquid of step 5 to obtain magnesium ammonium phosphate, i.e., struvite, and a water phase after recovering the struvite; Step 7: Repeat steps 5 and 6 at least twice, and the combustion performance of the hydrothermal carbon is increased to 223.03%.

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