Method for preparing biochar with high bioavailable phosphorus content, biochar and application thereof

Abstract

The present invention discloses a method for preparing biochar with high bioavailable phosphorus content, biochar and application. The method comprises: mixing biomass ash containing both potassium and calcium elements with dehydrated sludge, and performing co-pyrolysis to obtain biochar; wherein the potassium content in the biomass ash is greater than or equal to 30wt%, and the calcium content is greater than or equal to 10wt%. The content of bioavailable phosphorus in the biochar accounts for more than 90% of the total phosphorus content in the biochar. The method provided by the present invention can efficiently improve the bioavailability of phosphorus in sludge pyrolysis biochar and reduce the heavy metal content, without the need for additional addition of chemical reagents, and the process flow is simple, while being able to realize the resource utilization of biomass ash. The biochar provided by the present invention is rich in nutrients such as potassium and calcium, which is beneficial to plant growth.

Description

Technical Field

[0001] The present invention belongs to the field of sludge resource utilization, and more specifically, relates to a method for preparing biochar with high bioavailable phosphorus content, biochar and application. Background Art

[0002] In recent years, pyrolysis technology has been widely studied in the field of sludge phosphorus recovery because it can significantly reduce the volume of sludge, destroy organic pollutants and pathogens, and effectively increase the phosphorus content in the product. However, most of the phosphorus in sludge pyrolysis biochar exists in the form of insoluble orthophosphates, which is difficult to recycle directly. There are patents that recover phosphorus by transferring phosphorus to the liquid phase and then synthesizing products. For example, patent CN202110245451.1 discloses a method for efficiently recovering phosphorus from municipal sludge and simultaneously preparing porous biochar. The sludge is activated with an alkali metal activation agent and heated at 500-900 o C to obtain sludge pyrolysis biochar, which is then subjected to water immersion treatment. After solid-liquid separation, phosphorus-rich solution and porous biochar can be obtained, but there are problems such as the need to add additional chemical reagents, the required pyrolysis temperature is high, and the process is complicated. There are also studies that improve the bioavailability of phosphorus in the obtained pyrolysis biochar by adding inorganic substances containing calcium to the sludge for co-pyrolysis, so that the phosphorus in the biochar can be directly used by plants. For example, patent CN 108975649A adds calcium oxide to urban domestic sewage sludge for pyrolysis reaction to improve the availability of phosphorus in sludge biochar, but this method requires additional chemical reagents, which increases the operating cost.

[0003] In summary, the existing methods of recovering phosphorus by pyrolysis of sludge have the problems of requiring additional addition of chemical reagents, complicated processes, and long procedures. Summary of the invention

[0004] In view of the above defects or improvement needs of the prior art, the present invention provides a method for preparing biochar with high bioavailable phosphorus content, biochar and application, the purpose of which is to efficiently improve the bioavailability of phosphorus in sludge pyrolysis biochar, reduce the heavy metal content, and realize the resource utilization of biomass ash.

[0005] To achieve the above objectives, according to one aspect of the present invention, a method for preparing biochar with high bioavailable phosphorus content based on co-pyrolysis is provided, comprising: mixing biomass ash containing both potassium and calcium elements with dewatered sludge, and performing co-pyrolysis to obtain biochar; wherein the potassium content in the biomass ash is greater than or equal to 30 wt%, and the calcium content is greater than or equal to 10 wt%.

[0006] The principle of the method of the present invention is as follows: the phosphorus in the municipal sludge pyrolysis biochar is mainly present in the form of orthophosphates such as iron phosphate, calcium phosphate, and aluminum phosphate, and the phosphorus has low bioavailability. The present invention uses biomass ash containing both potassium and calcium elements (i.e., potassium-calcium-rich biomass ash) and dewatered sludge for co-pyrolysis. The potassium-calcium-rich biomass ash contains KCl, K 2 CO 3 , CaO and other phases can react with phosphate in the sludge during sludge pyrolysis to generate phosphorus-containing phases such as calcium hydroxyphosphate, calcium potassium phosphate, potassium iron phosphate, potassium hydrogen phosphate, etc., thereby improving the bioavailability of phosphorus in pyrolysis biochar. The biochar obtained by co-pyrolysis is also rich in nutrients such as potassium and calcium, which is beneficial to plant growth. 4 For example, the possible reactions are shown in equations (1) to (6). In addition, the chlorine in the ash can react with the heavy metals in the sludge at high temperatures to form heavy metal chlorides with lower boiling points and higher volatility, thereby promoting the removal of heavy metals in pyrolytic carbon and increasing the potential of sludge pyrolysis biochar to be used as fertilizer.

[0007] 4 FePO 4 +2 K 2 CO 3 =4 KFePO 4 +2 CO 2 (g)+O 2 (g) (1)

[0008] 12 FePO 4 +3 K 2 CO 3 =2 K 3 Fe 5 (PO 4 ) 6 +3 CO 2 (g)+Fe 2 O 3 (2)

[0009] 12 FePO 4 +6 KCl +3 H 2 O =2 K 3 Fe 5 (PO 4 ) 6 +6 HCl (g)+Fe 2 O 3 (3)

[0010] 2 FePO 4 +2 K 2 CO 3 +H 2 O =2 K 2HPO 4 +2 CO 2 (g)+Fe 2 O 3 (4)

[0011] 2 FePO 4 +4 KCl +3 H 2 O =2 K 2 HPO 4 +4 HCl (g)+Fe 2 O 3 (5)

[0012] 6FePO 4 +10 CaO+H 2 O=2 Ca 5 (PO 4 ) 3 (OH)+3 Fe 2 O 3 (6)

[0013] Preferably, the biomass ash contains 30-50 wt% potassium and 10-20 wt% calcium.

[0014] Preferably, the biomass ash includes at least one of sunflower straw ash, olive pomace ash and corn cob ash.

[0015] Preferably, the biomass ash is obtained by incinerating, drying, grinding and sieving the biomass in sequence. o C / min to raise the furnace temperature to 250-300 o C, and keep at this temperature for 60 minutes; continue to raise the temperature to 550-600 o C, and burn at this temperature for 3-4 hours; when sieving, for example, 80-mesh sieve can be used.

[0016] Preferably, the mass of the biomass ash accounts for 30-50% of the total mass of the mixture of biomass ash and dewatered sludge.

[0017] Preferably, the co-pyrolysis is carried out under the protection of an inert gas, and the pyrolysis temperature of the co-pyrolysis is 400-600 o C.

[0018] Preferably, the dewatered sludge is pre-treated before being mixed with the biomass ash, and the pre-treatment includes drying, grinding and sieving in sequence. The sieving process may be, for example, sieving through an 80-mesh sieve.

[0019] According to another aspect of the present invention, a biochar is provided, wherein the content of bioavailable phosphorus in the biochar accounts for more than 90% of the total phosphorus content in the biochar, and the bioavailable phosphorus refers to the phosphorus component that can be dissolved in plant roots and used by plants during the life cycle of the crop. The actual application value of the recovered phosphorus product or phosphate fertilizer can be evaluated, and the evaluation can be performed by testing the solubility of the recovered phosphorus product or phosphate fertilizer in 2% mass concentration of citric acid.

[0020] According to another aspect of the present invention, there is provided an application of biochar as a phosphorus-containing fertilizer.

[0021] In general, compared with the prior art, the above technical solution conceived by the present invention can at least achieve the following beneficial effects.

[0022] (1) The present invention co-pyrolyzes biomass ash containing both potassium and calcium elements with dewatered sludge, wherein the potassium content in the biomass ash is greater than or equal to 30 wt%, and the calcium content is greater than or equal to 10 wt%. This not only increases the proportion of bioavailable phosphorus in the obtained biochar to its total phosphorus content to more than 10 times the proportion of the corresponding substance in the biochar directly pyrolyzed from dewatered sludge without biomass ash, but also reduces the heavy metal content therein. At the same time, the biochar is rich in nutrients such as potassium and calcium, which is beneficial to plant growth. No additional chemical reagents are required, and the process flow is simple.

[0023] (2) The present invention performs low-temperature co-pyrolysis of potassium-calcium-rich biomass ash and dehydrated sludge instead of directly mixing biomass and dehydrated sludge. This is because biomass is mostly used as a raw material for biomass incineration power generation for resource utilization. However, as a by-product of biomass incineration power generation, most biomass ash is landfilled or randomly piled up, lacking proper disposal and utilization methods. At the same time, the potassium and calcium content in biomass ash is higher than that in biomass, which has a better effect on promoting the bioavailability of phosphorus in pyrolytic carbon. Therefore, this patent can achieve the synergistic resource utilization of biomass ash and sludge.

[0024] (3) The method of the present invention can simultaneously realize sludge pyrolysis resource utilization and phosphorus recovery, and the treatment process is short and the method is simple, which has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The XRD patterns of biochars obtained by direct pyrolysis of dewatered sludge and co-pyrolysis of sunflower straw ash and dewatered sludge in a mass ratio of 1:1, respectively. The pyrolysis temperature was 600 o C. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0027] The present invention provides a method for adding potassium-calcium-rich biomass ash to promote the bioavailability of phosphorus in municipal sludge pyrolysis biochar, comprising the steps of adding potassium-calcium-rich biomass ash to dehydrated sludge and mixing, and performing low-temperature co-pyrolysis. Specifically:

[0028] Step S1: adding biomass ash containing potassium and calcium obtained by sequentially burning, drying, grinding and sieving into dewatered sludge and mixing them evenly, the biomass ash includes at least one of sunflower straw ash, olive pomace ash and corn cob ash; wherein the potassium content in the biomass ash is greater than or equal to 30 wt%, and the calcium content is greater than or equal to 10 wt%. The added potassium-calcium-rich biomass ash accounts for 30-50% of the mass percentage of the mixture of biomass ash and dewatered sludge; the dewatered sludge is dried, ground and sieved through an 80-mesh sieve before mixing.

[0029] Step S2: The mixture of biomass ash and dewatered sludge is heated at 400-600°C under an inert atmosphere (nitrogen or argon). o The sludge pyrolysis biochar was obtained by pyrolysis at C temperature; and the bioavailable phosphorus content in the prepared pyrolysis biochar was determined using 2wt% citric acid.

[0030] The following are specific embodiments:

[0031] The municipal sludge in the embodiment of the present invention was taken from the concentration tank of Shahu Sewage Treatment Plant in Wuhan City, Hubei Province, with a total phosphorus content of 15.43 mg / g dry sludge. The sludge was conditioned and dehydrated using Fenton reagent to obtain dehydrated sludge. The preparation method of potassium-calcium-rich biomass ash in the embodiment is to heat the biomass at 105 o C for 24 h, crushed, ground, and sieved through an 80-mesh sieve, and the sieved material was collected and placed in a muffle furnace at a constant temperature of 10 o C / min and raise the furnace temperature to 250 o C and maintain this temperature for 60 min; then continue to raise the temperature to 550 o C, and burn at this temperature for 3 h; take it out after the heating program is completed and the temperature cools to room temperature.

[0032] Example 1

[0033] A method of adding potassium-calcium-rich biomass ash to promote the bioavailability of phosphorus in municipal sludge pyrolysis biochar in this embodiment is carried out according to the following steps:

[0034] (1) Sunflower straw ash (potassium content: 33 wt%, calcium content: 14 wt%) was mixed with dehydrated sludge. The dehydrated sludge was dried, ground and sieved through an 80-mesh sieve before mixing. Sunflower straw ash and dehydrated sludge were mixed in a mass ratio of 1:1.

[0035] (2) A horizontal tubular pyrolysis furnace was used to co-pyrolyze sunflower straw ash and dehydrated sludge under argon atmosphere at a pyrolysis temperature of 500 o C, pyrolysis time 2 h, to obtain sludge pyrolysis biochar; 2 wt% citric acid was used to determine the bioavailable phosphorus content in the prepared pyrolysis biochar, and it was found that the bioavailable phosphorus content in the biochar accounted for 90% of the total phosphorus content.

[0036] Example 2

[0037] A method of adding potassium-calcium-rich biomass ash to promote the bioavailability of phosphorus in municipal sludge pyrolysis biochar in this embodiment is carried out according to the following steps:

[0038] (1) Sunflower straw ash (potassium content: 33 wt%, calcium content: 14 wt%) was mixed with dehydrated sludge. The dehydrated sludge was dried, ground and sieved through an 80-mesh sieve before mixing. Sunflower straw ash and dehydrated sludge were mixed in a mass ratio of 1:1.

[0039] (2) A horizontal tubular pyrolysis furnace was used to co-pyrolyze sunflower straw ash and dehydrated sludge under argon atmosphere at a pyrolysis temperature of 600 o C, pyrolysis time 2 h, to obtain sludge pyrolysis biochar; 2 wt% citric acid was used to determine the bioavailable phosphorus content in the prepared pyrolysis biochar, and the measured bioavailable phosphorus content in the biochar accounted for 93% of the total phosphorus content.

[0040] Example 3

[0041] A method of adding potassium-calcium-rich biomass ash to promote the bioavailability of phosphorus in municipal sludge pyrolysis biochar in this embodiment is carried out according to the following steps:

[0042] (1) Olive pomace ash (potassium content: 44 wt%, calcium content: 10 wt%) was mixed with dehydrated sludge. The dehydrated sludge was dried, ground and sieved through an 80-mesh sieve before mixing. Olive pomace ash and dehydrated sludge were mixed in a mass ratio of 3:7.

[0043] (2) Olive pomace ash and dehydrated sludge were co-pyrolyzed in a horizontal tubular pyrolysis furnace under argon atmosphere at a pyrolysis temperature of 600 o C, pyrolysis time 2 h, to obtain sludge pyrolysis biochar; the bioavailable phosphorus content in the prepared pyrolysis biochar was determined using 2wt% citric acid, and the measured bioavailable phosphorus content in the biochar accounted for 90% of the total phosphorus content.

[0044] Example 4

[0045] A method of adding potassium-calcium-rich biomass ash to promote the bioavailability of phosphorus in municipal sludge pyrolysis biochar in this embodiment is carried out according to the following steps:

[0046] (1) Olive pomace ash (potassium content: 44 wt%, calcium content: 10 wt%) was mixed with dehydrated sludge. The dehydrated sludge was dried, ground and sieved through an 80-mesh sieve before mixing. Olive pomace ash and dehydrated sludge were mixed in a mass ratio of 1:1.

[0047] (2) Olive pomace ash and dehydrated sludge were co-pyrolyzed in a horizontal tubular pyrolysis furnace under argon atmosphere at a pyrolysis temperature of 400 o C, pyrolysis time 2 h, to obtain sludge pyrolysis biochar; 2 wt% citric acid was used to determine the bioavailable phosphorus content in the prepared pyrolysis biochar, and the measured bioavailable phosphorus content in the biochar accounted for 91% of the total phosphorus content.

[0048] Example 5

[0049] A method of adding potassium-calcium-rich biomass ash to promote the bioavailability of phosphorus in municipal sludge pyrolysis biochar in this embodiment is carried out according to the following steps:

[0050] (1) Olive pomace ash (potassium content: 44 wt%, calcium content: 10 wt%) was mixed with dehydrated sludge. The dehydrated sludge was dried, ground and sieved through an 80-mesh sieve before mixing. Olive pomace ash and dehydrated sludge were mixed in a mass ratio of 1:1.

[0051] (2) Olive pomace ash and dehydrated sludge were co-pyrolyzed in a horizontal tubular pyrolysis furnace under argon atmosphere at a pyrolysis temperature of 600 o C, pyrolysis time 2 h, to obtain sludge pyrolysis biochar; 2 wt% citric acid was used to determine the bioavailable phosphorus content in the prepared pyrolysis biochar, and the measured bioavailable phosphorus content in the biochar accounted for 99% of the total phosphorus content.

[0052] Comparative Example 1

[0053] (1) Dry, grind and sieve the dewatered sludge directly, and take the sieve residue for later use.

[0054] (2) A horizontal tubular pyrolysis furnace was used to directly pyrolyze the dehydrated sludge under an argon atmosphere at a pyrolysis temperature of 600 o C, pyrolysis time 2 h, to obtain sludge pyrolysis biochar; 2 wt% citric acid was used to determine the bioavailable phosphorus content in the prepared pyrolysis biochar, and the measured bioavailable phosphorus content in the biochar accounted for 9% of the total phosphorus content.

[0055] Comparative Example 2

[0056] (1) Corn straw ash (potassium content: 6 wt%, calcium content: 2 wt%) was mixed with dehydrated sludge. The dehydrated sludge was dried, ground and sieved through an 80-mesh sieve before mixing. The corn straw ash and dehydrated sludge were mixed in a mass ratio of 1:1.

[0057] (2) A horizontal tubular pyrolysis furnace was used to co-pyrolyze corn straw ash and dehydrated sludge under argon atmosphere at a pyrolysis temperature of 600 o C, pyrolysis time 2 h, to obtain sludge pyrolysis biochar; 2 wt% citric acid was used to determine the bioavailable phosphorus content in the prepared pyrolysis biochar, and the measured bioavailable phosphorus content in the biochar accounted for 54% of the total phosphorus content.

[0058] Comparative Example 3

[0059] This comparative example adopts the same steps as Example 1 to prepare biochar, except that in step (1), sunflower straw is used. o C for 24 h, and then crushed, ground, and sieved through an 80-mesh sieve to obtain sunflower straw powder, which was then mixed with dehydrated sludge in a mass ratio of 1:1.

[0060] The bioavailable phosphorus content in the prepared pyrolysis biochar was determined using 2 wt% citric acid, and the measured bioavailable phosphorus content in the biochar accounted for 32% of the total phosphorus content.

[0061] Table 1 shows the ratio of the bioavailable phosphorus content to the total phosphorus content of the pyrolysis biochar samples obtained in the above-mentioned embodiments and comparative examples. It can be seen that the bioavailable phosphorus content in the biochar directly pyrolyzed from dewatered sludge (Comparative Example 1) accounts for only 9% of the total phosphorus content. After adding sunflower straw and dewatered sludge for co-pyrolysis (Comparative Example 3), the bioavailable phosphorus content of the biochar accounts for 32% of the total phosphorus content, which is still relatively low. After adding corn straw ash with low potassium and calcium content and dewatered sludge for co-pyrolysis (Comparative Example 2), the bioavailable phosphorus content of the biochar accounts for 54% of the total phosphorus content, but the effect is still not ideal. After adding potassium-calcium-rich biomass ash and dewatered sludge for co-pyrolysis (Examples 1~5), the bioavailable phosphorus content of the biochar accounts for more than 90% of the total phosphorus content. XRD analysis of the pyrolysis biochar obtained in Example 2 and Comparative Example 1 is as follows Figure 1 As shown, it can be seen that the phosphorus in the biochar directly pyrolyzed by dewatered sludge mainly exists in the form of iron phosphate, and the bioavailability of phosphorus is low; after adding sunflower straw ash and dewatered sludge for co-pyrolysis, phosphorus-containing phases that are easily utilized by plants, such as hydroxycalcium phosphate, calcium chlorophosphate, potassium calcium magnesium phosphate, and potassium iron phosphate, appear in the biochar, which significantly improves the bioavailability of phosphorus in the pyrolysis biochar.

[0062] Table 1 The ratio of bioavailable phosphorus content to total phosphorus content of the pyrolysis biochar samples obtained in Examples 1 to 5 and Comparative Examples 1 to 3

[0063]

[0064] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing biochar with high bioavailable phosphorus content based on co-pyrolysis, characterized in that: include: Biomass ash containing both potassium and calcium elements is mixed with dehydrated sludge, and co-pyrolysis is performed to obtain biochar; wherein the potassium content of the biomass ash is 30-50wt%, and the calcium content is 10-20wt%; the biomass ash contains KCl, K2CO3, and CaO; the biochar contains calcium hydroxyphosphate, calcium potassium phosphate, potassium iron phosphate, and potassium hydrogen phosphate; the mass of the biomass ash accounts for 30-50% of the total mass of the mixture of biomass ash and dehydrated sludge; the content of bioavailable phosphorus in the biochar accounts for more than 90% of the total phosphorus content in the biochar; the dehydrated sludge is also pre-treated before mixing with the biomass ash, and the pre-treatment is drying, grinding and screening in sequence.

2. The method according to claim 1, characterized in that The biomass ash includes at least one of sunflower straw ash, olive pomace ash and corn cob ash.

3. The method according to any one of claims 1 to 2, characterized in that: The biomass ash is obtained by sequentially burning, drying, grinding and sieving the biomass; wherein the burning is to raise the furnace temperature to 250-300°C at a heating rate of 10°C / min and maintain this temperature for 60 minutes; then continue to raise the temperature to 550-600°C and burn at this temperature for 3-4 hours.

4. The method according to claim 1, characterized in that The co-pyrolysis is carried out under the protection of an inert gas, and the pyrolysis temperature of the co-pyrolysis is 400-600°C.

5. A biochar prepared according to the method according to any one of claims 1 to 4.

6. A use of biochar as claimed in claim 5, characterized in that: As phosphorus fertilizer.

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