Performance and emissions characteristics of compressed spent coffee ground/wood chip logs in a residential stove

Spent coffee grounds (SCG), a processing by-product from the soluble coffee industry, was evaluated as a potential feedstock for preparing compressed logs for energy production. Hence, a blend of SCG/wood chips was transformed into densified logs using an industrial press. Chemical properties such as calorific value, ash content and elemental analysis were obtained. Combustion tests were carried out with a five-star-labeled stove in a French industrial research and development laboratory. Three different configurations were tested: a densified log containing 20 wt% of SCG blended with pine wood chips, another containing a mixture 50/50 wt% of SCG/pine wood chips and a classical beech log. This study is the first one that focuses on the potential of compressed logs as a new biocombustible for wood stoves. The obtained results show that the combustion of logs containing SCG leads to better yield of combustion. Nevertheless, CO and particle emissions increase when increasing the rate of SCG although the exhaust gas emissions and combustion yields always lead to the “five-star Flamme Verte” label achievement. Such attainment is essential to sell these densified biofuel logs in the French market. Results indicate that the blended densified logs are combusted rapidly leading to higher stove efficiency and therefore good heat recuperation from the stove walls. On the contrary, the addition of SCG leads to lower CO2 concentrations (7.7% and 9.05% for 20 wt% and 10 wt% of SCG, respectively). This behavior indicates that the combustion of compressed logs is not suitable for the stove design and may lead to an uncomfortable heating. One should be reminded that the evaluated stove was designed for the combustion of conventional wood logs (with respect to the NF Bois de Chauffage standard). This means that the results obtained during this study with the different densified fuel logs could be improved after an adjustment of the stove conception.