An investigation of lime addition to fuel as a countermeasure to bed agglomeration for the combustion of non-woody biomass fuels in a 20kWth bubbling fluidised bed combustor

Hetian Chi,Miguel A Pans,Chenggong Sun,Hao Liu

doi:10.1016/j.fuel.2018.11.122

Abstract

The unique fuel flexibility of fluidised bed system facilitates the combustion of low quality solid fuels such as agricultural residues, but may suffer from serious bed agglomeration problems especially when operating at the high end of the typical fluidised bed combustion temperature range (800–900 °C). In this study, a 1.5 wt% lime addition to biomass fuels was investigated as a countermeasure to agglomeration for the combustion of pelletised wheat straw and miscanthus in a 20 kW bubbling fluidised bed (BFB) combustor using quartz sand as the bed material. Comparing to the widely studied agglomeration countermeasures such as bed additives and alternatives, the lime addition to fuel can provide a continuous on-site agglomeration counter action. Without lime addition, defluidisation as a result of bed agglomeration occurred once the bed temperature reached 860 °C and 877 °C for miscanthus and wheat straw respectively. However, with the lime addition, stable combustion could be achieved at 900 °C for both fuels with a substantially prolonged operation time. The obtained agglomerates were characterised using CAMSIZER, XRD and SEM/EDX and the results showed that the presence of lime in the fuel could significantly reduce the agglomerates size by forming high melting point calcium silicates/phosphates which hinder the K-rich molten substance attachment and result in heterogeneous coating layers in the agglomerates. The results of this study clearly indicate that using lime as fuel additive can realise the combustion of the problematic non-woody biomass fuels by considerably reducing the bed agglomeration propensity in practical fluidised bed combustion plants.

Highlights

Biomass as a CO2 neutral fuel has been considered to be a viable alternative to coal used in power plants and a secure energy resource with various environmental benefits [1]
Comparing to the raw agricultural residues, pelletised biomass fuels are friendlier to their feeding system and their combustion process tends to be more stable, which further leads to an improved energy conversion efficiency
When the bed temperature was controlled at around 820 °C, defluidisation did not occur within one-shift operation and the test had to be interrupted after one shift operation, and restarted and continued on the second day until defluidisation occurred

Summary

Introduction

Biomass as a CO2 neutral fuel has been considered to be a viable alternative to coal used in power plants and a secure energy resource with various environmental benefits [1]. Given that agricultural crops have a much shorter harvest period comparing to woody biomass, agricultural residues may have significantly different physical and chemical properties in comparison to woody biomass materials. They usually have lower bulk density, fibrous structures and higher ash content [4]. As a consequence, their feeding system may be malfunctioning and their combustion process may be more difficult to control, which may further result in various ash-related issues, for example, fluidised bed agglomeration, corrosion and particulate matter aggregation [5]. Other factors including ash fusion temperature, excess air ratio, bed height etc. have been found to have minor relations to the agglomeration tendency [16,18,19]

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Results

Conclusion