Effects of attapulgite clay on anti-slagging behavior of densified biofuel

Abstract: Biomass has been the fourth largest energy source after coal, oil, and natural gas in the world. Densified biofuel has also drawn much attention, due to the simple preparation, high thermal efficiency, convenient storage and transportation. However, the slagging can induce the uneven heating of the grate during combustion, leading to the low heat-transfer efficiency, even some serious accidents in some cases. The additives can be an effective way to deal with slagging. But, the currently developed additives remain unclear on the anti-slagging behavior. Therefore, it is necessary to find more efficient and cheap additives for the improved anti-slagging behavior in the densified biofuel. In this study, the attapulgite clay was proposed to serve as the composite anti-slagging agent. Firstly, a series of densified biofuels were fabricated under the various doses of attapulgite clay, together with the blank control group. A proximate analysis was carried out for the high calorific values during combustion of densified biofuel. The results showed that the calorific value decreased less when the attapulgite clay addition was less than 3%. However, the ash increased rapidly with the increase of attapulgite clay addition. Secondly, the macro morphology analysis and micro morphology analysis were carried out to determine the slagging tendency of biomass ash, and the macro morphology of ash samples was collected by a high-speed camera. The results showed that the volume shrinkage of densified biofuel ash was less with the attapulgite clay addition than that without, while the SEM spectrum of biomass ash for micro morphology showed that the small ash particles were evenly distributed without agglomeration when the amount of attapulgite clay addition was more than 3%. Thirdly, the composition of biomass ash was identified by XRF and XRD, in order to clarify the anti-slagging mechanism of attapulgite clay. The organic potassium in the biomass was volatilized to the gas phase in the form of K at the temperature of 200-500 ℃ during heating. It is found that during the heating process, with a temperature of 200-500 ℃, the organic potassium in biomass was volatilized to the gas phase in the form of K, which was adsorbed by attapulgite clay. when the temperature reached 900 ℃，the release of K reached the maximum, and the XRD patterns of all ash samples at 2θ=8.57°, 12.56°, 19.59°, and 32.7° were showed no characteristic peak of attapulgite clay, indicating that the crystal structure of attapulgite clay has completely collapsed，as a result，the released K element mainly from biomass and Mg, Al and Si elements mainly from attapulgite clay formed more high melting point double salts such as KAlSiO4, KAlSi2O6 , Ca2Mg(Si2O7). Therefore, the attapulgite clay changed the migration path of the K element to reconstruct the slagging mechanism of densified biofuel ash, thereby improving the anti-slagging performence of densified biofuel. In addition, there was a relatively low cost of attapulgite clay as an anti-slagging agent for the densified biofuel. The optimal amount of attapulgite clay was 3% addition.