Fertiliser-catalysed biochar production: an analytical and techno-economic feasibility study

Abstract

The current agricultural method of burning leftover plant material in farmlands before planting a new crop is outdated. While this technique may be fast and cost-effective, it is very unsustainable. This practice produces large amounts of carbon black, resulting in health issues, and reduces soil fertility [Pang, 2019]. It is also well known that the process used to produce nitrogen-containing fertilisers, the Haber-Bosch process, is thermodynamically unfavourable at high temperatures. To overcome this limitation, the process is operated at extremely high pressures, making the process very energy intensive [Razon, 2014]. Like fossil fuels, the world’s phosphorus reserves are limited and can’t sustain current agricultural practices indefinitely, driving the search for sustainable alternatives [Desmidt et al., 2015]. With 71 billion metric tons of phosphate ore available and a production rate of 0.22 billion metric tons in 2021 [Jasinski, 2022], the use of specialty fertilisers is expected to rise due to the irreplaceable role of phosphorus in farming.

The objective of this research was to identify fertilising agents that reduced the pyrolytic onset temperature of biomass, producing a fertilising-biochar. By lowering the pyrolytic ignition temperature, energy reductions were observed, resulting in a more economically viable product. The enriched biochar, containing nitrogen (N), phosphorus (P), and potassium (K), aims to provide essential nutrients to the soil for crop cultivation. 79 prospective fertilising agents, each representing one of the three macronutrients (NPK) required by biomass, were analysed. K2CO3, K3PO4, and Ca(NO3)2·4H2O were chosen to represent potassium, phosphorus, and nitrogen, respectively. As for the biomass of choice, Eucalyptus grandis was selected, due to its widespread abundance, not only in South Africa, but in the world.

The fertilising agents were each added in concentrations of 1 wt.%, 2 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, and 50 wt.% to the biomass with deionised water and then dried at 105 °C. Thermogravimetric analysis was conducted on the dried samples with a Hitachi STA7300 horizontal-beam TGA-DTA system and compared to a neat sample of E. grandis. It was established that both K2CO3 and K3PO4 suppressed the pyrolytic onset temperature of the biomass. K2CO3 exhibited an ignition temperature suppression of -11 °C, -25 °C, -48 °C, -71 °C, -83 °C, and -97 °C for 1 wt.%, 2 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, and 50 wt.%, respectively, with a maximum possible suppression of -98 °C. This resulted in an energy saving of 18.7% for the 50 wt.% loading. K3PO4 also showed significant suppression of the pyrolytic onset temperature, with 1 wt.%, 2 wt.%, 5 wt.%, 10 wt.%, 20 wt.%, and 50 wt.% loadings resulting in suppressions of -2 °C, -20 °C, -33 °C, -49 °C, -55 °C, and -75 °C, respectively. The highest fertiliser loading (50 wt.%) indicated an energy saving of 16.7%. Conversely, Ca(NO3)2·4H2O demonstrated no catalytic effect on E. grandis.

After establishing that the fertilising agents had a catalytic effect on E. grandis and resulted in an energy reduction, a techno-economic feasibility analysis was conducted to investigate the economic viability of an enriched biochar. To quantify the feasibility, four financial indicators were utilised: the net present value (NPV), internal rate of return (IRR), return on investment (ROI), and payback period (PP). In addition to the sale of the nutrient-enriched biochar, the by-products (synthesis gas and pyrolysis oil) were also offset to increase economic feasibility. Four scenarios for the sale of by-products were considered, with Scenario 2 and Scenario 4 proving to be the most viable. In Scenario 2, electricity was sold to the grid by combusting the synthesis gas and wood vinegar fraction, while the phenolic fraction was distilled and sold separately. In Scenario 4, the phenolic fraction was extracted through distillation, and both the wood vinegar and phenolic fractions were sold. Process parameters including the daily tonnage, fertiliser loading, biochar yield, fertiliser agents ratio, and the final selling price were varied to optimise the final selling price.

The first price comparison was between neat biochar to that of coking coal, used in the steel manufacturing industry. It was discovered that at a selling price of 20 R·kg⁻¹, neat biochar attained an IRR of 3.4% and a payback period of 10.4 years (with the plant lifespan set at 20 years). The selling price of neat biochar was calculated as almost four times the current price of coking coal (5.04 R·kg⁻¹). The other price comparison that was investigated was between NPK fertilising products, currently used in agriculture, to the enriched biochar. Due to the difficulty of obtaining bulk prices for K2CO3, K3PO4, and Ca(NO3)2·4H2O, wholesale prices for muriate of potash, diammonium phosphate, and urea were used, respectively. Although the selling price was kept as low as possible, an IRR level of 16% was set to attract investors, given that the plant would require a major fixed capital investment. The analysis indicated that employing a biochar with 51.7% loaded fertiliser resulted in a 32-fold price increase per hectare if the required amount of fertiliser is applied. Using a 5.3% loading, the cost increase would soar to 127 times higher per hectare. The price increase is primarily attributed to the large amount of biochar needed—if an enriched biochar of 5.3% loading is used, 19 times more product is required per hectare compared to using neat fertilisers.