Study On Release And Control Of Potassium During Combustion Of Potassium-rich Biomass And Its Char

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.