【作者】 李辉；

【导师】 路春美；

【作者基本信息】 山东大学 ， 动力工程及工程热物理， 2016， 博士

【摘要】 生物质直燃发电是生物质能规模化利用的主要技术之一,其中富钾生物质(如农作物秸秆)在我国生物质储量和直燃发电中占有重要地位。生物质炭可以显著提高生物质的可磨性和能量密度,提高其在燃烧中的利用效率,同时有效解决相对分散的生物质原料运输过程中物流成本过高的问题。但是富钾生物质及其炭在燃烧中析出的较高浓度的含钾化合物会造成电站锅炉受热面的积灰、结渣、腐蚀等问题,严重威胁着锅炉运行的安全性、经济性及其热效率。因此,为了实现富钾生物质及其炭的高效和清洁利用,需要对其燃烧中的钾析出和控制进行深入研究。本文采用实验和理论分析相结合的方法,对添加磷酸铵盐的富钾生物质及其炭燃烧中钾的析出特性和转化机理,以及预添加磷酸铵盐的富钾生物质炭混煤的钾析出特性和燃烧特性进行了研究。在管式炉燃烧系统上,研究了不同实验条件下添加磷酸铵盐(NH4H2PO4和(NH4)2HPO4)对富钾生物质燃烧中钾析出的影响规律。磷酸铵盐在700～1000℃内均有明显的钾析出控制效果,其中NH4H2PO4和(NH4)2HPO4均在1000℃时取得控制稻草中钾析出的最佳效果,分别将稻草的钾析出率降低了24.76和25.89个百分点。NH4H2PO4和(NH4)2HPO4对于不同富钾生物质均具有基本相同的钾析出控制效果及反应规律。富钾生物质中钾含量越高,其钾析出率越高,磷酸铵盐对其燃烧中钾析出的控制效果越好。磷酸铵盐中磷酸根p043-的添加量对磷酸铵盐控制富钾生物质燃烧中钾析出的效果影响较大,富钾生物质的钾析出率在各个反应温度下均随着P043-添加量的增加而降低,添加的磷酸铵盐与富钾生物质中钾摩尔比的适宜范围为1-2。采用热重分析法研究了添加磷酸铵盐的富钾生物质的燃烧特性。添加NH4H2PO4和(NH4)2HPO4对富钾生物质燃烧特性的影响基本相同,均提高了其燃尽温度,降低了其着火点,减小了其燃烧特性指数。磷酸铵盐与富钾生物质中钾的相互作用主要发生在300℃以上,此时磷酸铵盐与富钾生物质燃烧中的相互作用较强。采用X射线衍射(XRD)、电子显微镜(SEM)和灰熔点测试仪对添加磷酸铵盐的富钾生物质燃烧中钾的转化机理及其灰熔融性进行了研究。磷酸铵盐可以与富钾生物质中的钾通过不同的反应路径进行反应,生成具有较高熔点且不易析出的磷酸钾盐和K-Ca-P化合物,进而控制钾的析出,有效地缓解富钾生物质燃烧中的烧结和熔融现象,显著提高富钾生物质的灰熔点。选用KCl、CaO和NH4H2PO4分别为钾、钙和磷元素的来源化合物,在管式炉燃烧系统上结合XRD分析对K-Ca-P三元系统的反应特性进行了研究。在K-Ca-P反应系统中,磷更倾向于与钾首先发生化学反应,K-Ca-P反应系统中3种元素更倾向于经过相互反应生成KCaPO4和K2CaP2O7,与3种元素的摩尔比无关。在沉降炉实验系统上分别以NH4H2PO4和KCl为磷酸铵盐和气相含钾化合物的模型化合物,对磷酸铵盐与气相含钾化合物以及磷酸铵盐脱除NO的反应特性进行了研究。NH4H2PO4与KCl的反应产物主要为磷酸氢钾盐,延长反应时间和增大磷钾摩尔比均可以提高NH4H2PO4对KCl的转化率。NH4H2PO4不能明显降低模拟烟气中NO的浓度,烟气中的NO对NH4H2PO4与KCl的反应没有明显影响。分别以NH4H2PO4和稻草为代表磷酸铵盐和富钾生物质,在管式炉热解系统上对富钾生物质和预添加磷酸铵盐的富钾生物质热解中钾的析出特性进行了研究,并对所得稻草炭和预添加NH4H2p04的稻草炭的燃料特性进行了考察。NH4H2PO4在稻草热解中也有明显的钾析出控制效果。在各个热解温度下,预添加NH4H2PO4的稻草炭的质量产率和能源得率均高于稻草炭,但是其钾含量和高位发热量低于稻草炭,在热解温度为450℃下制备的预添加NH4H2PO4的稻草炭(RS+NH4H2PO4-45O)具有与晋中煤接近的燃料特性。采用热重分析法研究稻草及预添加NH4H2PO4的稻草热解特性,并计算其热解动力学参数。在相同升温速率下,预添加NH4H2PO4的稻草的最大热解速率均低于稻草。稻草和预添加NH4H2PO4的稻草热解中的表观活化能均随着转化率的提高呈现出先增大后减小,再增大的趋势,稻草热解中的平均表观活化能均小于预添加NH4H2PO4的稻草,但是其平均反应级数高于预添加NH4H2PO4的稻草。预添加NH4H2PO4的稻草炭的钾析出率在各个燃烧温度下均低于在同一热解温度下制备的稻草炭。RS+NH4H2PO4-45O在700～1000℃下燃烧产生灰样中的主要晶相化合物为具有较高熔点的磷酸钾盐和K-Ca-P化合物。随着热解温度的升高,稻草炭和预添加NH4H2PO4的稻草炭的着火点均升高,稻草炭的最大失重速率及燃烧特性指数均大于预添加NH4H2PO4的稻草炭,预添加NH4H2PO4的稻草炭的燃烧特性指数与长治、府谷和晋中煤相近。预添加磷酸铵盐的富钾生物质炭可以与煤以合适的比例掺混燃烧而不产生明显的负面影响。在各个反应温度下,随着RS+NH4H2PO4-450掺混比(RS+NH4H2PO4-450在混煤燃料中的质量百分数)的提高,混煤燃料的钾析出率降低,掺混比为10%和20%的RS+NH4H2PO4-450与晋中煤的混煤燃料的平均钾析出量小于晋中煤,并且在各个反应温度下,其钾析出率和析出量均小于具有相同掺混比的晋中煤混RS-450和晋中煤混稻草。掺混比为10%和20%的RS+NH4H2PO4-450与晋中煤的混煤燃料与晋中煤的燃烧特性相近。RS+NH4H2PO4-450与晋中煤混燃时存在对总体着火有利的正面协同效应,且该协同效应随着掺混比的提高而变得更加明显。更多还原

【Abstract】 Biomass direct combustion for power generation is one of the main technologies in large-scale us of biomass energy. Potassium-rich biomass such as agriculture straws plays an important role in biomass reserves and direct combustion for power generation in China. Biomass char can obviously improve the grindability and energy density of original biomass, which will increase the utilization efficiency of biomass during combustion and can be an effective solution to reduce the cost of logistics caused by transporting the geographically dispersed biomass. But the released higher concentrations of potassium compounds during combustion of potassium-rich biomass and its char will lead to ash deposition and slagging, corrosion and other problems of heating surface of boiler. In order to achieve the effective and pollution-free utlization of potassium-rich biomass and its char, the release and control of potassium during combustion of potassium-rich biomass and its char need to be studied. In this dissertation, by combining experimental study with theoretical analysis, the release characteristics and transformation mechanisms of potassium during combustion of potassium-rich biomass with ammonium phosphates and its char, and the release characteristics of potassium and combustion characteristics during co-combustion of coal with the pyrolyzed products of potassium-rich biomass with ammonium phosphates are studied.The effect of ammonium phosphates, i.e., ammonium dihydrogen phosphate (NH4H2PO4) and ammonium monohydric phosphate ((NH4)2HPO4) addition on release characteristics of potassium were studied in a tube furnace experimental system under different experimental conditions during combustion of potassium-rich biomass. These ammonium phosphates both have an obvious effect on controlling the release of potassium at 700～1000 ℃. NH4H2PO4 and (NH4)2HPO4 both control the release of potassium during combustion of rice straw (RS) most effectively at 1000 ℃, when the potassium release ratios of RS are decreased by 24.76 and 25.89 percentage points, respectively. The addition of NH4H2PO4 and (NH4)2HPO4 show almost the same effect on controlling the release of potassium and the same reaction behavior during combustion of different potassium-rich biomass. The potassium-rich biomass with higher initial potassium content has a higher potassium release ratio during combustion, and NH4H2PO4 and (NH4)2HPO4 also are more effective on controlling the potassium release of them. The addition amount of PO43- which comes from the ammonium phosphates has a great influence on the effect of controlling the release of potassium during combustion of potassium-rich biomass. The potassium release ratio of potassium-rich biomass decreases with the increasing addition amount of PO43- at every combustion temperature, and the appropriate molar ratios of added ammonium phosphates and potassium in the potassium-rich biomass are between 1-2.The combustion characteristics of potassium-rich biomass with ammonium phosphates addition were investigated by thermogravimetry. The addition of NH4H2PO4 and (NH4)2HPO4 show the similar effect on combustion characteristics of potassium-rich biomass, which both increase the burnout temperatures, increase the ignition temperatures and decrease the combustion characteristic indexes of potassium-rich biomass. Ammonium phosphates mainly react with potassium from the potassium-rich biomass at temperatures above 300 ℃, at which ammonium phosphates and potassium-rich biomass show a strong interaction during combustion.The transformation mechanisms of potassium during combustion of potassium-rich biomass with ammonium phosphates and their ash fusibility were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and an ash fusion point measuring instrument. Ammonium phosphates can react with potassium and form potassium phosphates and K-Ca-P compounds with high melting points which are difficult to volatilize through different paths during combustion of potassium-rich biomass with ammonium phosphates, which can contribute to controlling the release of potassium, mitigating the sintering and melting behavior during combustion of potassium-rich biomass, and increasing their ash fusion temperatures significantly.KCl, CaO and NH4H2PO4 were selected as source compounds of potassium, calcium and phosphorus, respectively, to investigate the reaction mechanisms of K-Ca-P ternary system in a tube furnace combustion system combined with XRD. For the K-Ca-P system, phosphorus is inclined to react with potassium firstly, and potassium, calcium and phosphorus are inclined to form KCaPO4 and K2CaP2O7 through interactions and have nothing to do with their molar ratios.NH4H2PO4 and KCl were selected as model compounds of ammonium phosphates and gaseous potassium compounds, respectively, to investigate the reaction characteristics of ammonium phosphates with gaseous potassium compounds and NO removal by ammonium phosphates in a drop tube furnace experimental system. The reaction products of NH4H2PO4 with KCl mainly are potassium hydrogen phosphates. Prolonging the reaction time and increasing the molar ratio of P/K can promote the conversion of KCl by NH4H2PO4. The NO concentration in the simulated flue gas cannot be decreased significantly by NH4H2PO4, and NO in the flue gas has no obvious influence on the reaction of NH4H2PO4 with KCl.NH4H2PO4 and RS were selected as representative ammonium phosphates and potassium-rich biomass, respectively, to study the release characteristics of potassium during pyrolysis of potassium-rich biomass and potassium-rich biomass with ammonium phosphates pre-addition in a tube furnace pyrolysis system. The fuel characteristics of the pyrolyzed products of RS (RS-char) and RS with NH4H2PO4 (RS+NH4H2PO4-char) were also investigated. NH4H2PO4 has an obvious effect on controlling the release of potassium during pyrolysis of RS. RS+NH4H2PO4-char show higher solid and energy yields, but lower higher heating values and potassium contents than RS-char at every pyrolysis temperature. The fuel characteristics of RS+NH4H2PO4-char pyrolyzed at 450 ℃ (RS+NH4H2PO4-450) are similar with Jinzhong coal (JZ).The pyrolysis characteristics of RS and RS with NH4H2PO4 were investigated by thermogravimetry under inert conditions. The pyrolysis kinetic parameters were also calculated. The maximum pyrolysis rate of RS with NH4H2PO4 is lower than that of RS under the same heating rate. The values of the apparent activation energy of RS and RS with NH4H2PO4 during pyrolysis first increase, then decrease and then increase with the increasing conversion fraction. The average value of apparent activation energy of RS is lower than RS with NH4H2PO4, but RS has a higher average reaction order than RS with NH4H2PO4 during pyrolysis.The potassium release ratios of RS+NH4PO4-char are all lower than RS-char pyrolyzed at the same temperature at every combustion temperature. The main crystalline compounds in the ash formed by RS+NH4H2PO4-450 at 700～1000 ℃ are potassium phosphates and K-Ca-P compounds with high melting points. The ignition temperatures of RS-char and RS+NH4H2PO4-char both increase with the pyrolysis temperature. The maximum weight loss rates and combustion characteristic indexes of RS-char are all higher than RS+NH4H2PO4-char. The combustion characteristic indexes of RS+NH4H2PO4-char are similar that of Changzhi coal, Fugu coal and Jinzhong coal.The pyrolyzed products of potassium-rich biomass with ammonium phosphates can combustion with coal with an appropriate blend ratio which will not produce obvious negative influences. The potassium release ratio of the blend of JZ and RS+NH4H2PO4-450 increases with the increasing blend ratio (the weight percentage of RS+NH4H2PO4-450 in the blend) of RS+NH4H2PO4-450 at every combustion temperature. The average potassium release amounts of JZ co-combustion with 10% and 20% RS+NH4H2PO4-450 are both lower than JZ, and their potassium release ratios and amounts are all lower than JZ co-combustion with RS-450 and JZ co-combustion with RS with the same blend ratios at every combustion temperature. The combustion characteristics of the blends of JZ with 10% and 20% RS+NH4H2PO4-450 are similar with JZ. There is a positive synergistic effect on improving the general ignition characteristics of the blends during JZ co-combustion with RS+NH4H2PO4-450, and the synergistic effect becomes more obvious with the increase of blend ratio.更多还原