Evaluation of Performance of Industrial-Scale Dual Fluidized Bed Gasifiers Using the Chalmers 2–4-MW_th Gasifier

Abstract

A general approach to evaluating the performance of industrial-scale dual fluidized bed (DFB) gasifiers was developed in this work. The approach is intended to simplify comprehensive evaluation of DFB gasifiers and to highlight important parameters, some of which are often missed or omitted in the literature. By applying this procedure, experimental results can be generalized, which is verified in this work using the Chalmers 2−4-MW th DFB gasifier. In a DFB gasifier, some of the fuel is converted to the desired calorific gas, while the remaining portion is combusted to meet the heat demands of the process. As shown here, the total heat demands limit the amount of chemical energy that can be restored from the fuel into the produced gas, whereby the main heat demands are from the drying and heating of the fuel, in addition to heating the combustion air and steam. By establishing a heat balance across the system, the chemical efficiency can be estimated. With lower heat demands, higher chemical efficiency is achievable, whereas with higher heat demands, more of the fuel must be burned and a lower chemical efficiency is achieved. It is experimentally complicated to quantify the level of fuel conversion and heat demands of a DFB gasification system. In this work, an experimental procedure is presented and implemented using the Chalmers gasifier to quantify the fuel conversion and heat demands. Furthermore, it was investigated how a variation in the amount of steam used for fluidization of the gasifier affects fuel conversion and other important parameters. To establish a reference case, silica sand was used as bed material and wood pellets was used as fuel to minimize the effects of ash and the bed material. By increasing the level of fluidization steam, the average residence time of the gas was decreased and the gas temperature, gas velocity, and steam-to-fuel ratio were increased, which resulted in increased conversion (up to 36%) of organic compounds (OC). However, limited char conversion was achieved (0%−4%), and the chemical efficiency remained unaffected by the amount of steam added to the process. The chemical efficiency of the Chalmers gasifier was determined to be 74% when using wood pellets as fuel. This is comparable to results from thermo-economic modeling of second-generation biofuels production processes, which, based on the heat demand, report the chemical efficiency of the DFB gasifier as being in the range of 74%−77% to maximize the overall efficiency. This shows that the required chemical efficiency is achieved, even with low char conversion, when using a fuel with a high content of volatiles, such as wood pellets.

Characteristics of olivine as a bed material in an indirect biomass gasifier

“…The gasifier is a 2-4-MW th bubbling fluidized bed coupled to the boiler via two loop seals. Detailed description of the system can be found elsewhere [6]. The letters A-D in Fig.…”

“…The heavy hydrocarbons are usually referred to as 'tar'; the usual definition is that the tar fraction includes hydrocarbons with a molecular mass greater than that of benzene. This fraction may represent 5-10% of the total energy content of the fed biomass [6]. These tar species start to condense already at temperatures of around 360°C, and they create operational problems, such as process shut-down and/or downstream catalyst deactivation [2,[7][8][9][10].…”

“…In the Chalmers 2-4-MW th dual fluidized bed gasification system, in which silica sand is usually applied as the bed material, the process has been thoroughly evaluated and reported upon [6]. Olivine has been tested on two occasions in consecutive years in the Chalmers unit under comparable operational conditions.…”

Magnetic tracer-particle tracking in a fluid dynamically down-scaled bubbling fluidized bed

“…To reduce this effect high excess air ratios are required to achieve full combustion in the furnace. The consequences of increased excess air are decreased plant efficiency due to the resulting lowered furnace temperature, increased investment cost for dimensioning of the flue gas cleaning and increased operational cost due to higher fan power.In indirect gasification (using interconnected fluidized beds), too fast lateral fuel mixing yields a fuel residence time too short to ensure complete gasification of the char [2][3][4]. In such a case, the non-gasified char is instead combusted in the combustor bed and thus decreases the rate of product gas obtained.…”

“…In indirect gasification (using interconnected fluidized beds), too fast lateral fuel mixing yields a fuel residence time too short to ensure complete gasification of the char [2][3][4]. In such a case, the non-gasified char is instead combusted in the combustor bed and thus decreases the rate of product gas obtained.…”