Spent brewing grains into engineered hydrochar-based adsorbent for direct air carbon capture

Spent brewing grains are considered one of the major waste streams in the UK, with around 7,500 trillion metric tonnes produced per annum. Generally, spent grains are either landfilled or used for complementary animal feed. Hydrothermal carbonisation (HTC) is an environmentally friendly process for the conversion of wet biomass into carbonaceous material (hydrochar) that can be used as an adsorbent. Recently, amino acid salts have been recognised as effective green solvents for direct air capture (DAC) systems. In addition, amino acid, along with metal oxide, can be utilised as additives to adsorbents to enhance DAC performance. This study utilised a novel method for integrating both HTC of spent grains with the application of amino acid salts for an efficient DAC system. In a one-pot reaction, spent grains were valorised into efficient CO₂ adsorbents. This was achieved by mixing spent grains with amino acid in an HTC reactor at 240 °C for 1 h. The resulting engineered hydrochar was filtered, dried, and tested for DAC application. The DAC experiment was conducted by exposing the engineered hydrochar to an air stream containing a CO₂ concentration of 555 ppm (±20 ppm) at 293.15 K (±1.0 K) and 1008 hPa (±10 hPa), with relative humidity held at 42 % (±6 %). Response surface methodology (RSM) via Box–Behnken design (BBD) was used to investigate the effect of process parameters, including reaction temperature, time, and amino acid/metal oxide ratio, and to predict the optimal process conditions. The experimental design work revealed a maximum CO₂ adsorption capacity of 4.58 mmol of CO2 per gram of engineered hydrochar, highlighting a potential material for direct air capture applications. These findings demonstrate the technical viability of using hydrochar derived from spent brewing grains as a carbon capture medium, offering a promising route for enhancing the sustainability of industrial processes while mitigating climate change.