Combustion of biomass based pellets with pyrolysis bio-oils

Abstract

Due to increased energy demands and environmental concerns, biomass has become an increasingly attractive primary source in the energy mix. It can be used as heat supply to both domestic and industrial sectors, but also as fuel in dedicated or co-generation-based power plants and it is also considered CO2 neutral. Amongst the most widely used formats, wood pellets are very attractive because of their standardized properties which makes them efficient over a wide range of applications. In Portugal alone, over 1 million tons a year are produced. However, due to their attractiveness, the pressure on traditional raw materials for their fabrication (saw dust) has driven the cost upwards and induced bottlenecks upstream in the supply chain. One way to expand the range of raw materials is to combine a biomass based matrix with alien materials into the same pellet format. In this way one can also find alternative uses for other materials which, some of them wastes, are often dumped into landfills. Such an approach has been pursued into combining industrial wastes with biomass in pellets. This project concerns the evaluation of the combustion behavior of saw dust based pellets fortified with pyrolysis bio oils produced from waste vegetable oils. The bio-oil is an organic liquid obtained by pyrolysis of used vegetable sunflower oil, at 420 ºC and 10 min. The pellets were manufactured with pine saw dust with bio-oil incorporation up to 5% in mass. The resulting pellets were fully characterized for their properties, including the heating value, which increased when compared with standard saw dust pellets. These pellets were afterward tested in a fully instrumented 20 kW fixed grate combustion facility. The flue gases were analyzed and the resulting ashes were tested for their chemical composition in order to assess their ability to melt during combustion. The results show that this route can be beneficial in terms of exhaust emissions, including promoting the reduction of NOx during combustion. However, the physical characteristics of the pellets have some restrictions, since they do not meet minimum requirements for its certification ENplus level of density, mechanical durability and fines content.