Effects of ash forming temperature on water-soluble fraction of biomass ash and it's elements

Abstract: The implementation of biomass ash as a fertilizer is beneficial to the sustainable development of agriculture because it can return the elements taken by the plants from soil during the growth. Water solubility of biomass ash together with its elements is a vital property for the fertilizer's utilization. In this paper, the effects of ash forming temperature on the water solubility of biomass ash and its elements were investigated experimentally. Three typical types of agro-residues in China, i.e. wheat straw, corn stover and rice husk, were chosen for investigation. Materials were collected and the corresponding proximate and elemental analyses were performed. In preparation process, wheat straw and corn stover were milled into powder with the particle size of less than 1 mm. Ashes were prepared in a muffle furnace at different ash forming temperatures in the scope from 400 to 800℃ with the temperature interval of 50℃. About 4-5 g of each sample powder and rice husk were put in the crucibles, heated from ambient temperature to ash forming temperature, and then kept at the ash forming temperature for 4 hours in the muffle furnace. After the temperature of the furnace dropped to less than 200℃, the ashes were taken out to the cabinet dryer, cooled to the ambient temperature, and then weighted and sealed in the sample bag for further measurements. It showed that ash contents of wheat straw, corn stover and rice husk decreased significantly with the increase of ash forming temperature. They decreased from 9.8%, 10.9% and 18.4% at 400 ℃ to 6.9%, 8.7% and 16.8% at 800℃respectively. After the preparation of all the ash, two types of measurements were performed. The first was the water solubility measurement of the ashes according to the standard of GB8307-2002. Total ash of about 0.2-0.3 g was put in a beaker of 50 ml and about 25 ml distilled water was added in it. The mixture was heated and boiled for 2 min and then the solution was filtered through a quantitative filter paper. The filter residue was washed for several times using another 25 ml distilled water. After the burning of the filter residue, the insoluble ash was weighed and sealed in a sample bag. Results showed that the ash's water solubility of corn stover and wheat straw was much higher than that of rice husk. All of them were significantly affected by the ash forming temperature. They decreased from 48%, 35% and 12% to 12%, 8% and 0, respectively. The second was the water solubility measurement of the elements. This was based on the X-ray Fluorescence Spectrometer (XRF) tests of the original ash and water-insoluble ash using ZSX100e. The sample ash was pressed into a thin disc with the diameter of 30 mm. Then it was measured in vacuum with full scan mode by XRF. The data of measurement was processed using SQX (semi-quantitative nonstandard analysis program) and the mass concentrations of the elements were obtained. Result showed that there were respectively 11, 10 and 4 elements whose concentrations were more than 1% in the ash of wheat straw, corn stover and rice husk. And oxygen (O), silicon (Si), potassium (K), and calcium (Ca) were the major elements in all ashes, while chloride (Cl) was rich in stalks. The Cl concentration in the original ash decreased significantly with the increase of ash forming temperature. The water solubility of the elements in ash was calculated via two methods, i.e. mass conservation method and silicon trace method. Result showed that the major dissolved elements in ash were K, Cl and S (sulfur). The solubility of K and Cl decreased with the increases of ash forming temperature and the solubility of S was almost not affected by temperature. The experimental results can be used as the reference for environmental assessment and optimization of thermo-chemical process of biomass. And the method can be expanded to determine the recycling characteristics of other types of biomass ash.