Carbon-relative molar mass for the modeling of the lignocellulosic biomass torrefaction and slow pyrolysis performance and biochar fuel properties

• The influence of the HTT pyrolysis and CRMM of the biomass on biochar yield and properties has a polynomial character. • The biochar yield parameters can be predicted based on the HTT of pyrolysis and the CRMM of biomass. • The biochar fuel properties can be predicted based on the HTT of pyrolysis and the CRMM of biomass. • The validation confirmed the high accuracy of the biochar yield and fuel properties prediction. One of the identified issues related to the development of biochar production is the accurate prediction of biochar yield and fuel properties. Numerous regression and machine learning models were developed. These models were based on numerous (13–16) independent variables describing the feedstock parameters and thermal treatment conditions. In this paper, we propose a new parameter, the carbon-relative molar mass (CRMM) of the biomass, as a parameter unifying all biomass parameters like moisture content, volatile matter, ash content, content of C, H, N, S, and O. Based on this approach, the experiment with 12 mixtures of biomass structural components (lignin, cellulose, and hemicellulose) was designed. The torrefaction and slow pyrolysis occurred for 60 min under the highest treatment temperature (HTT), ranging from 200 to 600 °C with an interval of 25 °C. 204 samples of biochar were produced. The ultimate and proximate analyses of the used mixtures and produced biochar were done. Using CRMM of the biomass and HTT as independent variables, the non-linear response surface regression function was applied to model two groups of output variables process performance indicators such as biochar yield, energy densification ratio, and energy yield, and fuel related properties of biochar including carbon, hydrogen, nitrogen, sulfur, oxygen, ash content, higher heating value (HHV), and CRMM of the biochar. For models’ validation, real samples of biomass: hardwood (apple tree), softwood (pine tree), and herbaceous biomass (hemp straw) were thermally treated for 60 min under HTT from 200 to 400 °C. Predicted biochar yields and biochar fuel properties were compared with the experimental data from real biomass. Executed experiments showed that CRMM of the biomass and HTT may describe the biochar yield and fuel properties of biochar with the coefficient of determination R 2 > 0.74, except for nitrogen and sulfur. The validation of determined models with the use of real biomass revealed that the proposed models have a high matching degree of the predicted data with measured values in the case of the HHV, CRMM of the biochar, and C, O, and H content in the biochar. These findings open a new niche for the experimentation and optimization of the torrefaction and slow pyrolysis processes and biochar production.