# Thermodynamic Equilibrium Study of Ash Transformation during Entrained Flow Conversion of Agricultural Biomass Focusing on the Potential Extraction of Valuable Si and K–P Compounds via Condensation from the Gas Phase

## TL;DR

This study explores how to extract valuable silicon and potassium-phosphorus compounds from agricultural biomass during combustion by controlling gas-phase condensation.

## Contribution

The novel approach is to extract valuable compounds via gas-phase condensation during thermochemical conversion, reducing the need for costly post-processing.

## Key findings

- High-purity Si compounds like SiC and Si2N2O can form under inert atmospheres and high Si/P ratios.
- K-bearing phosphates form alongside Ca-, Mg-, and Si-containing compounds during gas cooling.
- Results can guide practical extraction methods for agricultural biomass in entrained flow systems.

## Abstract

Agricultural biomass
is today largely underutilized in combustion
and gasification processes because of the abundant supply of other
easier-to-process biomass fuels. These biomass types generally have
a moderate to high ash content comprising valuable elements, such
as Si, K, and P, which can lead to ash-related operational problems.
The high share of Si, K, and P in agricultural biomass assortments
also has a significant economic value. These elements are usually
retained in the coarse or fly ash fractions. Extracting valuable Si-
and K–P-containing compounds with high purity from these ash
fractions often requires further postprocessing steps, which increases
operational costs. Therefore, a potential novel design concept could
be to control the combustion/gasification processes so that Si, K,
and P can be extracted by condensation from the flue/hot gases at
a quality that implies added value instead of extra costs. This work
aims to identify the possibilities of extracting valuable Si and K–P
compounds from the hot gases generated during entrained flow conditions
via stepwise controlled condensation in the close-flame regions or
heat exchanger zone. Thermodynamic equilibrium calculations were performed
by employing the databases (GTOX and SGPS) in FactSage 8.0 software.
The calculations were performed under varying conditions ,i.e., temperatures,
atmospheres, and fuel compositions. The selected fuels were rice husks
(Si-rich), brewer’s spent grains (P–Si-rich with moderate
to minor amounts of Ca, Mg, and K), and grass (K–Si-rich with
moderate amounts of Ca, Mg, and P). The results indicate that the
high-temperature formation of the valuable Si compounds, such as SiC
(s) and Si2N2O (s), would require an inert atmosphere
during both the release and cooling stages. Moreover, a high Si/P
molar ratio is needed to form valuable Si-containing compounds. The
predicted K-bearing phosphates during the gas cooling near the burner
zone were formed in the same temperature range as Ca-, Mg-, and Si-containing
compounds with all of the fuels. The results obtained by this study
can guide experimental research on the practical extraction of Si
and K–P compounds from different types of agricultural biomass
during thermochemical conversion in entrained flow conditions.