Projeto e análise de viabilidade econômica do pré-tratamento de casca de café para produção de briquetes e aplicação em bioenergia

Abstract

Brazil, the world's largest coffee producer and exporter, generates substantial amounts of coffee husks, the main solid residue from wet processing. Recent research has demonstrated its energy potential. However, the high moisture, low density and irregular particle size require pre-treatment for efficient thermochemical conversion. This study aimed to design and perform the economic analysis of a coffee husk pre-treatment plant, including drying, comminution and densification steps, for the production of briquettes. The economic feasibility of the bioenergy plant, i.e., the pre-treatment associated with the combustion of briquettes for electricity generation, was also analyzed. Sensitivity analyses of the design and economic variables were performed, focusing on the influence of the drying operating conditions. The Net Present Value (𝑁𝑃𝑉), Internal Rate of Return (𝐼𝑅𝑅), simple payback (𝑃) and discounted payback (𝑃𝑑) were evaluated. The Levelized Cost of Production per Briquette Mass (𝐿𝐶𝑀) for the pre-treatment plant and the Levelized Cost of Electricity (𝐿𝐶𝑂𝐸) for the bioenergy plant were calculated. The results demonstrated that the commercialization of the briquettes was viable, with 𝑁𝑃𝑉 in the range of US$ 38,635 to US$ 192,046, 𝐼𝑅𝑅 of 11.62% to 11.95%, simple payback of 5.66 to 5.74 years and discounted payback of 9.27 to 9.48 years. The lowest 𝐿𝐶𝑀 of R$ 602 t−1 (US$ 0.115 kg−1) occurred from higher values of drying air temperature (120 ºC), bed height (4 cm) and biomass flow rate (1500 kg h−1). Under these conditions, the minimum price (𝑃𝑏𝑟𝑖𝑞) and sales value (𝑉𝑏𝑟𝑖𝑞) of the briquette were R$ 609 t−1 (US$ 0.116 kg−1) and R$ 811 t−1 (US$ 0.155 kg−1), respectively, within the ranges reported in the literature and some advertisements in the Brazilian market. However, 𝑉𝑏𝑟𝑖𝑞 was not attractive when compared to firewood, which may make it difficult to accept coffee husk briquettes in practice, if there are no tax incentives. The bioenergy plant was unfeasible, with an average 𝑁𝑃𝑉 of − US$ 7.92 million, mainly due to the high investment in combustion technology (63.7% of the total investment) and high drying costs (56.8% of the operating costs). The lowest 𝐿𝐶𝑂𝐸 (US$ 198.58 MWh−1) was achieved at the highest air temperature in the optimistic scenario (𝐶𝑢𝑛 = US$ 2,000 kW−1), but above the base tariff (US$ 67.32 MWh−1). Air temperature, followed by bed height, were the drying variables with the greatest influence on the bioenergy plant economics, while air velocity was inelastic. The unitary cost of conversion technology (𝐶𝑢𝑛) was the economic variable with the greatest impact. Therefore, in addition to considering economic improvements in drying, it is also necessary to reduce the capital costs of combustion or investigate more affordable conversion technologies. The pre-treatment plant thus emerges as the most promising alternative for the utilization of wet-processed coffee husks.