Conference Dates

May 22-27, 2016


The deployment and the exploitation of bioethanol as automotive fuel became more and more relevant to reduce the emissions of greenhouse gases and to limit the dependence on countries supplying fossil fuels. However, the production of second-generation bioethanol, i.e. using lignocellulosic biomass or scraps of agricultural crops as feedstock, generates a waste stream consisting of lignin-rich residues whose fate has to be found (1-2). This work aims at investigating the combustion of lignin-rich residues (in the following simply called lignin), coming from a second-generation bioethanol production plant, with coal in a pilot-scale bubbling fluidized bed combustor (FBC). The pilot-scale 200kWth FBC schematically shown in Fig. 1 basically consist of a AISI 310 stainless steel fluidization circular column (370 mm ID for 5.05 m and 700 mm ID for 1.85m in the upper part of freeboard), a continuous over-bed feeding system, two cyclones for flue gas de-dusting, a propane premixed burner for the start-up and different heat removal devices located along the fluidization column. On-line gas analyzers (ABB AO2020) measured flue gas composition sampled at the exhaust. Fuels were the lignin-rich residue, a bituminous coal and wood chips. Silica sand (0.8-1.2mm) was used as bed material. An experimental campaign was carried out to study gaseous and particulate emissions and thermal regimes during the co-combustion of different mixtures of coal-lignin varying the percentage of lignin fed with coal, the bed temperature, the excess air and the fluidization velocity. Figure 2 reports the main results in terms of normalized emissions of NO, SO2, particulate and carbon in particulate as a function of the O2 concentration measured at the exhaust obtained during the steady state operation of the pilot-scale FBC. A large part of the investigated experimental conditions regarded the operation using a mixture lignin-coal at 30%w in lignin. Experiments with coal, with a mixture at 40%w in lignin and with a mixture coal-wood chips at 20%w in wood chips were carried out for comparison. The analysis of the experimental results mainly highlights that: 1) the gaseous emissions do not significantly change with respect to coal or to reference biomass-coal mixture at least until the mixture content of lignin is 30-40%w; 2) the particulate emissions increase with the percentage of residues content, but, at the same, the carbon content is significantly reduced. Bottom bed particles were analyzed at the end of each experiments highlighting the absence of agglomerates but a significant enrichment of metals like Fe, Mg, Na, Ca and K coming from lignin ash when the FBC was operated for long time and at high temperature.

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