Analysis of K~+ release kinetics of biomass ash with different pH

Abstract

Biomass ash is a kind of plant straw residuum produced by combustion of straw at 800 ℃. Biomass ash contains lots of nutrients, including phosphorus(P), calcium(Ca), magnesium(Mg) and especially potassium(K). In biomass ash, K mainly occurs as K2 SO4, KCl and K2 CO3. In recent years, biomass ash has been used to produce potassium-based fertilizers. However, less work has been done on the principles and mechanisms of biomass ash K+ release. In the new production system and method proposed to produce potassium fertilizers from biomass ash, pH is critical for K+ release. This paper used four types of biomass ash(cornstalk, rice straw, sawn wood and chaff) with different pH of 4, 7 and 9 adjusted by ammonium acetate(NH4 Ac) to extract available K by the method of penetration exchange. This study discussed the cumulative quantity and rate of release of K+, and analyzed K+ release dynamics of four types of biomass ash using three kinetic models. The results indicated that K+ release attained equilibrium in 20-80 min with a total capacity of K+ release significantly different for different pH. Total K+ release capacity was 14.77-255.17 cmol·kg-1at pH of 4, 4.83-106.71 cmol·kg-1 at pH of 7 and 12.11-224.33 cmol·kg-1 at pH of 9. Total K+ release capacity under each pH condition showed that rice straw released the most K+, followed by cornstalk and sawn wood, while chaff released the least K+. Correlation analysis suggested that total K content of biomass ash was critical for the time and total capacity of K+ release. The correlation coefficient of released K+ amount and total K content was 0.991-0.997. A significant difference was noted in K+ release rate among biomass ash types. These implied that the greater content of K contain the more amount of K+ release. This indicated that equilibrium time and quantity of released K+ varied with pH and total K content of exchange solution. Under the three pH conditions, released K+ amounts(Q) of rice straw and cornstalk were in the order of Q4 Q9 Q7, while those of sawn wood and chaff were Q9 Q4 Q7. The order of total K+ release capacity of biomass ash was Q Rice straw QSawn wood QChaff QCornstalks under the three pH conditions. Before equilibrium time, K+ release velocity(v) was logarithmically related with time(lnt) during each reaction stage. For the same biomass ash, the optimal kinetic mode of K+ release changed with pH condition. The best models at pH of 4, 7 and 9 were Elovich Equation(EE), Dual Constant Equation(DCE) and EE, respectively. Under the same pH condition, the model that best described K+ release process was the EE and DCE, while the Parabolic Diffusion(PD) was the most inappropriate model.