Study On Fluidized Bed Combustion Of Agricultural Biomass

The utilization of biomass can decrease the energy and environmental issues faced to the human today. Biomass direct combustion technology is one of the main ways that can utilize the biomass in large scale. As a large agricultural country, China has plenty of biomass of straw; therefore, it is necessary to develop the technology of straw combustion. However, the straw contains higher active metals-K,Cl,S than the other biomass and easily cause serious alkali metals problems such as slagging, agglomeration, deposition and high temperature corrosion, and so on. The alkali metals problems are the key issues that limit the development of straw biomass direct combustion. Considering the inherent advantages of fluidized bed in straw combustion, it was focused on the precept of fluidized bed combustion and related laws in fluidized bed combustion. Study the properties of alkali metals problems that limit the development of straw combustion, such as the properties of straw ash melts at low temperature, differentiate the slagging and the possible measures to remedy the slagging, agglomeration rules in straw combustion, as well as the transformation, migration of the active elements in biomass.At first, the study was carried out on the properties of biomass ash. It was found that in the biomass ash analysis, the ashing temperature should be under than 600℃in order to reduce the release of active elements, and the alkali mental compounds should be expressed as alkali chloride (ACl) and oxide (A2O). By this way, the deviation decreased largely. In order to meet the needs of description and comparison the different slagged ashes, by ashing directly and ash intensity determination, the sintering intensity index (SII) was created to quantify the sintering grade. On the basis of the SII; the risk grade of biomass fuel was set off using the TSmf of straw. The different additives were added to rice straw and wheat straw lignin, respectively, and the effect and mechanism of additives were also studied.Then, a series of experiments on a BFB combustor was carried out to study the rules of agglomeration with operation parameters, such as temperature, size and species of bed material, fluidized number and fuel feed forms. It was found the viscous ash is the direct cause of bed agglomeration, and it can be reduced largely by optimizing operation parameters, such as low temperature, small bed material particles and high fluidized velocity. A model of agglomeration was established coupling with the operation parameters and the surface free energy on particles, on the basis of agglomeration characteristics and sintering mechanism of silicates. The model adopted has been proved to good theoretical value on bed agglomeration by comparing the results of test and calculation on the BFB combustor. The results showed that the adjust of operation parameters was valid below 750℃but almost invalid above 850℃.Then, the active elements that cause the alkali metals problems in fluidized bed combustion (FBC) were researched. The transformation and release of the K in fixed bed was studied by changing the temperature and settle time, it was found that the rapid available potassium started to transform obviously at 700～800℃. The distribution and balance of the active elements during rice straw FBC (20kwth) for 21h were studied, and it showed that the ash and K in bed can reach a balance and most of the ash was in the forms of flyash,90% of potassium in flyash was water soluble. It was also found that the FBC of straw can reduce the SOx in some extent, by comparing the organic S and inorganic S in fuel, flayash and flue gas. The conclusions got from these experiments were proved by rice straw combustion on a bench scale CFB at low and high temperature.At last, the demonstration project of the first straw direct combustion was introduced. The CFB boiler combusted many biomass fuels with high efficiency and no agglomeration was found. The deposition and corrosion on the superheater in the boiler were light and do no harm to the operation. This fatherly proved the conclusions obtained from the agglomeration experiments, and it is a novel way to utilize agriculture straw is established by fluidized bed combustion.