Experimental study of tar and particles content of the produced gas in a double stage downdraft gasifier

Biomass gasification is not a new technology, but there is a renewed interest in its further development, mainly to produce power and heat as part of locally based combined heat and power plants. The produced gas mainly consists of H₂, CO, CO₂, CH₄, H₂O and some trace impurities such as H₂S, COS, NH₃, HCN, HCl, alkali, tar and particulate matter. These impurities are the responsible for clogging, corrosing, poisoning and carbon deposition on different elements of the power systems, like alternative internal combustion engines, gas turbines, also in fuel cells and auxiliary equipments, being necessary their removal or the adjusting of its concentration level, depending on the final application. This work presents the experimental evaluation of the tar and particle content in the produced gas from fixed bed downdraft gasifier with two stages of air supply. A very widely considered technology for the biomass gasification is the downdraft fixed-bed reactor, because had shown to produce a gas with lower tar level, compared with other gasifiers. The experiments were carried out varying the amount of air supplied to the reactor and the air ratio between the two stages (AR). The temperatures in different points along of gasifier and the gas compositions were also measured. The results show that there is an operational point where there is a coincidence of the highest conversion of the gasifier and the better quality of the gas, (higher calorific and lower tar content) and also that the use of a second stage can reduce the gas tar content up to 87%.