TASK 1: Indirect gasification and gas filtration

In smaller scale production, less than 200 MW fuel input, indirect steam-blown gasification combined with an externally heated reformer offers a competitive alternative for the concept based on oxygen-blown gasification and autothermal reforming, especially in applications where methane reforming is not required or methane is the desired end product.

Compared to oxygen-blown gasification indirect gasification offers the following benefits:

• No need for oxygen production
• The amount of carbon dioxide to be removed in the gas cleanup phase is considerably smaller.

In the UCG project in 2004-05 a preliminary evaluation of the indirect gasification process concept was carried out based on bench-scale experiments. Indirect gasification was found to have the following limitations:

• Lower gasification temperature → high concentrations of tar and lighter hydrocarbons in the gasification gas 
• High requirements for the fuel particle size and prehandling especially in bubbling fluidised-bed gasifiers
• Ash-related issues, such as ash sintering, especially in the oxidation zone operating at higher temperatures than the gasifier when the target gasification temperature is above 850 °C. 

In this task the research is focused on indirectly heated steam-blown gasification. Gasification experiments will be carried out with the following bench-scale atmospheric pressure test units:

• AFB60 – fluidised-bed reactor coupled with a hot gas filter and a reformer
• IDFB – indirectly heated fluidised-bed gasifier for air-blown and steam/O2 gasification coupled with a hot gas filter.

The objective is to study the effect of various process parameters on carbon conversion, gas yield, gas composition as well as on tar and nitrogen compound concentrations in the gasification gas. The main variables to be studied are as follows:

• Steam/fuel input ratio
• Gasification temperature
• Bed material, such as sand, olivine and calcium/magnesium containing bed materials
• Introduction of air or oxygen.

Test runs will be carried out with different biomasses and variable fuel particle sizes.

A laboratory-scale thermobalance unit will also be used to study the reactivity and the ash sintering behavior of the chosen biomass samples in steam gasification conditions. In addition, oxidation of char produced in steam gasification will be studied in laboratory and bench-scale units with emphasis on ash sintering behavior.

The results obtained from laboratory and bench-scale experiments in this subtask will be used in modifying an existing Aspen gasification model to be applicable for indirect steam-blown gasification. The model will be applied in process evaluation studies.