Energy and High Surface Area Siliceous Ash From the Combustion of Rice Hulls

Biomass combustion is an attractive energy production method since it is a zero net emitter of carbon dioxide. Rice hulls are a significant source of biomass fuel: approximately 100Mt of rice hulls are produced yearly from the processing and milling of paddy rice. Previous studies have shown that the ash produced from the combustion of rice hulls possesses pozzolanic properties. When used as a concrete additive, the ash can behave similar to silica fume and improve the strength and porosity of the finished concrete. In such cases, the surface area attributed to the pozzolan is related to the residual carbon content of the ash, which is often well in excess of 5%. The excessive carbon level stems from the awkward geometry of the hulls combined with the low melting point and heat sensitive nature of the ash minerals: these create problems for commercial combustion devices. As a result, the ash produced at these facilities contains either high residual carbon or a high proportion of crystalline silica, both of which renders the ash unattractive for use as a concrete additive. However, if the ash is of sufficient quality to be sold as a concrete additive, the economics of energy generation from the rice hulls can be improved. Test work conducted in Mississauga, Canada has produced an ash with less than 3% residual carbon, and greater than 90% silica, nearly all of which is amorphous. This work was done at temperatures higher than those in conventional operations without significant conversion of the siliceous component from the amorphous state to crystalline forms such as crystobalite or quartz. The key to this unexpected result is an enhanced combustion environment in which the solids residence time at elevated temperatures, and therefore the opportunity for silica phase changes, is minimized. In this instance, the total measured surface area of this ash is lower than that of the other ashes due to the reduced residual carbon content, but the siliceous surface area is equal or greater due to the high amorphous content. Consistent with this fact, the rice hull ash performed well in a number of standard concrete tests, in which the ash replaced 7.5% to 12.5% of the Portland cement for similar water to cement mixture ratios. With proper application of a TORBED process reactor as a combustor, a high value and high surface area siliceous ash can be produced as energy is simultaneously recovered from the system.