Transfer Behavior And Influence Of Phosphorus In Biomass And Coal Co-gasification

Entrained flow gasifier is not only an important equipment for clean coal technology, but also represents an attractive alternative to the thermal treatment system for biomass and pollutants. Base on the studies of using gasifier for biomass utilization, this paper mainly focus on the transfer behavior of phosphorus in gasifier and its influence on gasification process. The detailed contents and conclusions of this study are summarized as follows:(1) An laboratory-scale high frequency heat gasifer is employed to investigate the volatilization behavior of phosphorus during sewage sludge/coal co-pyrolysis and gasification. The temperature range investigated is 900℃-1300℃. Volatilization ratio of phosphorus monotonically increases with the gasification temperature. The volatilization of phosphorus mainly takes place during fuel pyrolysis process. Organic phosphorus compounds can evaporate under low temperature, but most of them are immobilized through transforming to calcium iron phosphate and aluminum phosphate. The evaporation of inorganic phosphorus within the given temperature range because calcium phosphate can react with quartz and reduced by carbon or reducing gas. Aluminum phosphate firstly transforms to calcium phosphate and then vaporized. Phosphorus is speculated mainly in the form of elementary substance in syngas. There are not more than 33.51% of total phosphorus vaporizes into gaseous products, and most of phosphorus in sludge are fixed in slag in the form of phosphorus glass.(2) The influence of sewage sludge and blue-green algae on the AFT of six kinds of coal is investigated. Melting mechanism of mixtures is discussed base on X-ray diffraction and Raman spectrum investigation. Phosphorus has significant influence on mixture melting characteristics. And the influence of phosphorus on mixture AFT depends greatly on the Al2O3/(CaO+Na2O+K2O) (A/CNK) molar ratio of bulk ash. When alkali metal content is low, phosphorus tends to combine with calcium and form refractory minerals in subaluminous system (A/CNK<1). While phosphorus in peraluminous system (A/CNK>1) is mainly amorphous and beneficial for decreasing blended ash fusion temperature. While alkali metal content is high, it can not only combine with phosphorus directly and forms low temperature molten glass, but also can reduce molten temperature of refractory calcium phosphate. (3) The co-gasification characteristics of sewage sludge-coal and algae-coal mixture are investigated by using thermo gravimetric balance. Volatile matters of biomass and coal evaporate independently with each other, but solid pyrolysis products of biomass covers on the surface of coal. It has great influence on coal pyrolysis gaseous products and the property of coke. Adding biomass inhibits graphitization process and increases the specific surface area of coke. These are benefit for making high reactive chars. On the other hand, the ash and phosphorus content of sewage sludge and algae are higher than coal. Ash of biomass can cover on the surface of coke and inhibits the gasification reaction. Phosphorus can lead to the inactivation of alkali and alkaline earth metals. It is proved by experiment that sewage sludge will reduces the activity of high calcium content coal, like Shenfu coal, and increases the activity of high aluminum content coal. High content of alkali metal exits in the form of glass, which will lead to low reaction activity of algae coke. Adding in low ratio, algae can improves the gasification reactivity of Shenfu coke, because phosphorus can react with calcium from Shenfu coal and release alkali metals.(4) Volatilization behavior of sulfur, potassium and sodium in sewage sludge and Shenfu coal is studied and the influence of phosphorus on it is discussed. The investigated condition includes pyrolysis process in 600℃～1400℃and gasification process in 900℃～1400℃Sulfur in Shenfu coal is inclined to escape at lower temperature when mixed with sewage sludge. This is because aluminum phosphate in sewage sludge can react with anhydrite and occupying calcium. Most alkali metals in Shenfu coal volatilize during gasification at high temperature. It is found alkali metals can be controlled by addition of municipal sewage sludge. This is because phosphorus can react with alkali metal and forms low temperature molten glass, and this effect is strengthened by increasing sludge adding ratio and react temperature.