Fluidized-Bed Combustion of Mixtures of Rapeseed Cake and Bark: The Resulting Bed Agglomeration Characteristics

Abstract

The bed agglomeration characteristics resulting from the combustion of 11 mixtures of rapeseed cake and spruce bark were studied in a bench-scale bubbling fluidized-bed reactor (5 kW). The objective was to determine the defluidization temperatures and the prevailing bed agglomeration mechanism as functions of the fuel mixture. Controlled fluidized-bed agglomeration tests were performed for each mixture with quartz sand as the bed material. The total defluidization temperatures and the initial defluidization temperatures were determined based on the measured pressure and temperature profiles in the bed. After combustion, bottom ash samples, agglomerates, and fly ash samples were analyzed by means of scanning electron microscope combined with energy dispersive X-ray detector (SEM-EDX). The composition of the ash-forming matter produced by the combustion of rapeseed cake is significantly different from that produced by the combustion of bark, resulting in different bed agglomeration tendencies. Bark contains ash-forming matter dominated by calcium, with some silicon and potassium, whereas rapeseed cake is rich in phosphorus, potassium, and sodium. The total defluidization temperature for pure bark was above 1045 °C, whereas, for rapeseed cake, defluidization occurred during combustion (800 °C). During the combustion of bark, the formation of a potassium-rich layer on the silica-bed grains was found to be a crucial for the formation of agglomerates. The low defluidization temperature for the rapeseed cake can be attributed to the formation of sticky ash, which is dominated by phosphates. Two main phosphate forms were observed in the neck between the silica grains: calcium–potassium/sodium phosphates, and magnesium–potassium phosphates. As the proportion of bark increased, the Ca/P ratio increased in the fuel mixture, and the formation of high-temperature melting phosphates in the ash was favored. However, the addition of bark also favored the formation of a potassium-rich layer on the silica bed material, leading to the coexistence of both bed agglomeration mechanisms. In the present work, mixtures with a minimum of 60 wt % bark resulted in significantly increased defluidization temperatures and reduced bed agglomeration tendencies, compared to what occurs in rapeseed cake monocombustion.

Investigation on Ash Slagging Characteristics During Combustion of Biomass Pellets and Effect of Additives

“…The abundance of phosphorous in the seed-originating biomass materials like wheat bran, cereal grain and meals was reported previously. 8,[13][14][15] Phosphorus is a vital nutrient for cell division and development of new tissues, which is essential for seeds formation and growth consequently. 14 Therefore, a large amount of phosphorous are found in seeds, cereals, grains, husks and grans.…”

“…12 A general trend is that large amount of K and/or P can be found in agricultural and food processing wastes, together with other main ash-forming elements like Si and Ca. [13][14][15] Different than woody biomasses, relative concentrations of key ash forming elements (i.e., K, Si, P and Ca) vary significantly for agricultural and food processing wastes. [16][17][18] It was reported that the content of phosphorus in some agricultural wastes is even higher than that of silicon.…”

Effects of air staging on emission characteristics in a conical fluidized-bed combustor firing with sunflower shells

“…However, this fuel ash also contained the significant amounts of calcium and magnesium, which was reported to increase the ash and ash/bed melting temperatures and to reduce the risk of bed agglomeration problem [18,19]. Silica sand (SiO 2 ¼ 99.5 wt%) with solid density z2500 kg/m 3 and particle size 0.3e0.5 mm was used as bed material at the static bed height of 30 cm and to avoid the bed agglomeration, it was replaced every 18 h of use.…”

Composition of agglomerates in fluidized bed reactors for thermochemical conversion of biomass and waste fuels

“…The formation of ash coatings and agglomeration has been subject to numerous studies and there are mechanisms suggested in the literature [17,18,23,26,27,[31][32][33]. An attempt to describe the agglomeration phenomena with a simplified mechanistic model can be found in [34,35] and is used in [36] to explain the agglomeration during co-combustion of bark and rapeseed cake in a fluidized bed, Fig. 1B.…”