A pilot-scale investigation of ash and deposition formation during oil-palm empty-fruit-bunch (EFB) combustion

Abstract

In Thailand, oil-palm empty-fruit-bunch (EFB) a by-product of the crude-palm-oil milling process is currently one of the most promising potential energy resources. However, the ash-forming potassium, chlorine, silicon, and calcium constituents of EFB fuel can cause severe fouling, slagging, and ash meltdown, during combustion. This study is aimed to investigate EFB firing in a pilot-scale reciprocating grate-fired combustor with a 150kWth capacity. The study included chemical analyses of fuel and fuel ash, and samples of bottom ash, fly ash, and deposits derived from laboratory combustion tests. Experiments were conducted at temperatures of ≈800°C (low-temperature) and 900–950°C (high-temperature).Deposits mainly formed on the upstream side of the probe, and comprised two distinct layers, i.e., a thin white inner layer, and a gray outer layer. A swift growth of deposits on the cooled deposit probe, simulating superheater conditions, was evident, with significant retardation of heat transfer. Heat uptake by the probe appeared to reduce to 70% of the initial value within a 19-h period. The deposit mass flux was 167g/m2h, which corresponded with a fouling thermal resistance of 0.023m2·K/W. Following initial deposit formation of KCl condensate, particle impaction entailed deposit formation for incorporating Si- and Ca-rich fly ash particles into the deposits. XRF (X-ray fluorescence spectrometer) and SEM/EDX (scanning electron microscopy/energy dispersive X-ray) analyses revealed that not only did KCl mainly exist in the inner deposit layer, but also dominated the entire deposit mass (60–80wt.%), suggesting a crucial role for alkali condensation in deposit formation. If the bulk flue gases were sufficiently cooled, the KCl deposited on the probe by the transport of small solidified KCl particles. Corrosion attack was apparent near the metal surface and involved the deposition of KCl. SEM–EDX mapping exhibited that silica in combination with potassium led to the formation of low-melting-point compounds, which readily melted at high-temperature combustion. ICP-AES (inductively coupled plasma-atomic emission spectroscopy) analysis indicated that the potassium in the deposits had high mobility; the results were consistent with the XRF and SEM–EDX analyses.