Maximization of CO₂ storage for various solvent types in indirect carbonation using paper sludge ash

This study aimed to maximise the amount of CO₂ storage by indirect carbonation using paper sludge ash (PSA), which is an alkaline industrial waste. The properties of the following three types of solvents were considered: acids (acetic acid and hydrochloric acid), ammonium salts (ammonium chloride and ammonium acetate) and others (sodium citrate and water). When carbon dioxide was injected into calcium leachates prepared from PSA using acids, carbonation did not occur, and using ammonium salts as solvent, the carbonation efficiency was low. However, after pH swing of the leachates using sodium hydroxide and ammonia solution for acid and ammonium salt solvents, up to 324 kg CO₂/(ton PSA) and 175 kg CO₂/(ton PSA) were stored, respectively. When carbon dioxide was injected into calcium leachates prepared from PSA using sodium citrate or water, solid was initially generated which later dissolved. In order to solve this problem, CO₂ injection was stopped just before the pH of the solution reached 10.5 for sodium citrate and 8.3 for water, resulting in the storage of 136 kg CO₂/(ton PSA) and 29 kg CO₂/(ton PSA), respectively. Solids obtained through carbonation were pure calcium carbonate.