Experimental Investigation in Autothermal Chemical Looping Gasification

Abstract

In light of the current climate change efforts to reduce the emission of greenhous gases (GHGs) to the atmosphere are required. The sustainable utilization of biomass as carbon carrier and energy carrier opens up the possibility of carbon neutral or even carbon negative technologies. One technology where biomass can be utilized for the generation of a nitrogen free, high calorific syngas where no energy intensive air separation unit (ASU) is required is the chemical looping gasification (CLG) which has lately seen increased research interest. CLG has been demonstrated to work continuously within externally heated lab-scale reactors. In order to be economically viable commercial application of the process requires autothermal operation, which has not yet been demonstrated. Moreover, no process control concept applicable to large-scale reactors exists. In this work, the CLG technology is upscaled to the 1 MWₜₕ range and investigated under autothermal conditions. Equilibrium process simulations are used to develop a suitable process control concept for autothermal operation based on sub-stoichiometric air reactor (AR) operation. Based on heat and mass balance for an existing pilot plant modifications were designed and implemented retrofitting the pilot plant for CLG. Experiments carried out with industrial wood pellets (IWP), pine forest residue (PFR), and wheat straw pellets (WSP) as biomass feedstocks and ilmenite as oxygen carrier (OC) bed material are described in this work. A cold gas efficiency of around 50 % was achieved in the non-optimized pilot plant, indicating that higher values can be reached in a commercial unit when minimizing heat losses. The carbon conversion was above 90 %, and this value is expected to increase to almost 100 % when raising the temperature, residence time, and cyclone efficiency in a commercial unit. The syngas has a very high quality with low methane concentrations in the range of 7 vol.-% to 10 vol.-% and gravimetric tar content below 1 g/Nm³ measured via tar protocol. A new method for the determination of the solid circulation in a dual fluidized bed system utilizing solid samples from coupling elements to calculate the solids flux has been devised and was tested during the experiments showing a solid circulation of 1.2 kg s⁻¹ MW⁻¹ to 4.3 kg s⁻¹ MW⁻¹ with measurement uncertainty smaller than 20 %.