A comparison between Miscanthus and bioethanol waste pellets and their performance in a downdraft gasifier

Abstract

Pelletised biomass has been found to have excellent potential for their utilisation in small to medium sized energy systems because of its advantages over loose feedstock. The energy density is increased and so the space occupied in transportation is decreased and the amount of problematic dust or fines is also decreased. Furthermore, pellets provide a more uniform fuel, allowing easier feeding and improved performance in thermal conversion processes. The pellet manufacturing process, or pelletisation process, plays a major role on the quality of pellets produced. Changes to pelletisation parameters such as feedstock moisture content, die diameter, particle size (or screen size), addition of lubricants or binders can significantly alter the quality of the pellets and therefore the ease with which the pellets can be gasified in a downdraft gasification process. One important quality parameter that greatly affects the downdraft gasification process is the strength or durability of pellets. Durability can be defined as the ability of pellets to resist mechanical breakdown during transport or during feeding into an energy plant. Other important parameters that affect downdraft gasification are the ash content and composition of the pellets. The ash is derived from the minerals in the feedstock, the addition of binders or lubricants and also the pellet production method. Furthermore, gasification efficiency can be also affected by the process parameters such as air-to-fuel ratio, air or biomass feed rate and operating temperature. The current article compares the properties of three different types of pellets and their gasification performance. Two types of Miscanthusand a bioethanol production reside (distiller’s dried grains with solubles (DDGS)) were used to make the pellets. The pellets made were of similar size (6–8mm) and ultimate analysis, so the paper focuseson the most important differences; these were durability, ash content and gasification parameters, expressed through the equivalence ratio which relates the actual air-to-fuel ratio with the calculated stoichiometric value. A series of experiments were conducted in a 50kWth pilot scale downdraft gasifier with the equivalence ratio varied in the range 0.2–0.3. The quality of the gas produced and the gasifier performance were assessed in terms of the gas composition, yield, heating value, cold gas efficiency and carbon conversion efficiency.