Effect of calcium phosphate monobasic on ash fusion and sintering characteristics of cornstalk

doi:10.11949/0438-1157.20191566

Abstract

Abstract:

Agricultural straw usually contains high alkali metals, which can easily cause high temperature melting and sintering of the boiler during the combustion process and affect the normal operation of the boiler. In this work, the effect of a phosphate additive, Ca(H₂PO₄)·H₂O (calcium phosphate monobasic, CPM), on ash sintering characteristic of a typical agricultural biomass cornstalks was investigated by using standard ash fusion measurement and ash sintering test at 700—1000℃. Meanwhile, the reaction behavior and the possible reaction products from the calcination of calcium phosphate monobasic and potassium salts at elevated temperature were investigated. The results showed that, during the heat treatment at elevated temperatures, temperature and mixing ratios of CPM and potassium salts had significant influence on reaction products. Meanwhile, when the mixture of CPM is excessive, in addition to the two reactions, there is high temperature decomposition reaction of CPM and secondary reaction between the reaction products. The reaction of CPM and potassium salts started at 300℃, with mainly Ca₂P₂O₇, Ca(PO₃)₂, KCa(PO₃)₃ at 300—600℃ and mainly K₂CaP₂O₇ at 700—900℃. Thermal decomposition of CPM and secondary reactions between the resultant products occurred when the P/K molar ratio was larger than 1, in addition to the reaction between CPM and K salts. In addition, it was found that K₂CO₃ has a higher reactivity with CPM compared to KCl. The addition of CPM in cornstalk significantly increased the ash fusion temperature and reduced the sintering degree by forming high melting phosphates mainly Ca₉MgK(PO₄)₇ and CaKPO₄. These results can provide theoretical guidance for the application of phosphorus-based additives in alleviating ash fusion and sintering issues during agricultural biomass combustion.

Key words: combustion, cornstalk, fusion and sintering, calcium phosphate monobasic

摘要：

农业秸秆通常含有较高的碱金属，在燃烧过程中易造成锅炉高温熔融烧结问题，影响锅炉正常运行。使用灰熔点测试仪并结合灰样烧结度测试研究磷酸二氢钙添加剂对玉米秆灰熔融烧结特性的影响规律。此外，使用固定床管式炉研究了磷酸二氢钙与两种不同钾盐之间的反应特性。结果表明燃烧温度和磷酸二氢钙与钾盐混合比例对反应产物影响显著。在碱金属盐过量时，磷酸二氢钙与碱金属盐在温度高于300℃时开始反应，300~600℃产物以Ca₂P₂O₇、Ca(PO₃）₂、KCa(PO₃）₃为主，700~900℃反应产物主要为K₂CaP₂O₇。当磷酸二氢钙过量时，除磷酸二氢钙与钾盐的反应外同时存在磷酸二氢钙的高温分解反应以及反应产物间的二次反应。此外，研究发现磷酸二氢钙与K₂CO₃的反应活性显著高于KCl。在玉米秆中添加磷酸二氢钙后可在灰分中形成高熔点的Ca₉MgK(PO₄）₇和CaKPO₄，显著提高生物质灰的熔融温度并在实验温度范围内有效缓解玉米秆的熔融烧结。研究成果可为磷基添加剂在缓解生物质锅炉灰熔融烧结中的应用提供理论指导。

关键词: 燃烧, 玉米秆, 灰熔融烧结, 磷酸二氢钙

CLC Number:

TQ 028.8

Youjian ZHU, Xianxian ZHANG, Yiming CHEN, Xuehong WU, Haiping YANG, Hanping CHEN. Effect of calcium phosphate monobasic on ash fusion and sintering characteristics of cornstalk[J]. CIESC Journal, 2020, 71(7): 3313-3321.

朱有健, 张显显, 陈奕名, 吴学红, 杨海平, 陈汉平. 磷酸二氢钙对玉米秆灰熔融烧结特性的影响研究[J]. 化工学报, 2020, 71(7): 3313-3321.

Figures/Tables 12

Table 1 Proximate, ultimate analysis and HHV of CS

Sample	Proximate analysis/%(mass, db)			Ultimate analysis/%(mass, db)					HHV/(MJ/kg)
Sample	A	V	FC	C	H	N	S	O	HHV/(MJ/kg)
CS	6.74	75.15	18.11	43.71	6.02	0.78	0.15	42.60	17.83

Table 2 Chemical composition of CS ash/%(mass)

Na₂O	MgO	Al₂O₃	SiO₂	P₂O₅	SO₃	Cl₂	K₂O	CaO	Fe₂O₃
3.96	11.39	3.89	29.05	3.41	3.01	11.25	23.58	9.68	0.78

Fig.1 Schematic diagram of fixed bed system

Fig.2 Mass loss rate of CPM and potassium salts at 200—900℃

Fig.3 XRD patterns of reaction product of CPM and KCla—KCl; b—Ca(H2PO4)2; c—CaHPO4; d—Ca2P2O7; e—Ca(PO3)2; f—KCa(PO3)3; g—Ca2KP3O10; h—Ca5(PO4)3Cl; i—K2CaP2O7; j—KH2PO4; k—K2H2P2O7·0.5H2O; l—(KPO3)n； m—Ca3(PO4)2

Fig.4 XRD patterns of reaction product of CPM and K2CO3a—K2CO3·1.5H2O; b—CaHPO4; c—Ca(OH)2; d—Ca2P2O7; e—Ca2KP3O10; f—KCa(PO3)3; g—Ca3(PO4)2; h—K2CaP2O7; i—KCaPO4; j—Ca(H2PO4)2; k—K2HPO4; l—Ca(PO3)2; m—(KPO3)n

Table 3 Ash fusion characteristic temperature of samples

Sample	Temperature/℃
Sample	DT	ST	HT	FT
CS	963	977	995	1036
CS+CPM1/2	1027	1181	1216	1259
CS+CPM1	1146	1219	1226	1272
CS+CPM2	1172	1359	1435	1446

Fig.5 Sintering degree of samples at elevated temperatures

Fig.6 XRD patterns of cornstalk asha—KCl; b—MgO; c—SiO2; d—K2CO3; e—CaMgSiO4; f—K2SO4; g—CaMgSi2O6; h—Ca2MgSi2O7; i—KAlSiO4; j—Mg2SiO4; k—K2MgSi3O8

Fig.7 XRD patterns of cornstalk with addition of CPMa—KCl; b—Ca9MgK(PO4)7; c—SiO2; d—KCaPO4; e—Ca2KP3O10; f—K2SO4; g—KMgPO4

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Table 4 EDS analysis of typical ash particle/%(mol)

Sample	Spot	Na	Mg	Al	Si	P	S	Cl	K	Ca
CS	C1	0	6.2	1.3	49.7	3.7	0	3.7	31.5	3.9
	C2	1.1	22.1	0.2	35.5	7.5	0.6	0.6	2.2	30.3
	C3	2.3	8.6	1.2	51.7	4.8	0	0.0	26.4	5.0
CS+CPM	P1	2.4	9.7	0.6	2.8	37.1	0	2.3	12.3	32.9
	P2	2.4	12.5	0	2.42	39.0	0.7	0	14.2	28.9
	P3	3.1	9.3	1.7	21.8	27.4	0	0	16.1	20.5

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