John Mbũrũ Ngũgĩ

The utilization of biomass-coal briquettes offers a promising approach for improving the sustainability and performance of solid-fuel combustion systems. This study experimentally investigated the combustion characteristics of sawdust-coal briquettes in a fixed-bed reactor to evaluate the influence of fuel composition on combustion behavior. Briquettes containing 0–60 wt.% coal at 10 wt.% intervals were produced and combusted under a constant air mass flux of 0.10 kg/m²s. Combustion performance was assessed using temperature distribution, mass-loss history, ignition time, flame propagation speed, and burning rate. Temperature profiles exhibited a single dominant peak, indicating substantial overlap of fuel heating, devolatilization, volatile combustion, and char oxidation processes. Peak bed temperatures decreased from 813 °C for briquettes containing 40 wt.% sawdust to 657 °C for pure sawdust briquettes, while combustion duration decreased from 26 to 21 min with increasing sawdust content. Mass-loss rate, flame propagation speed, and burning rate increased with increasing sawdust content, while ignition time decreased progressively. Overall, higher sawdust contents promoted earlier ignition, faster fuel conversion, and greater combustion reactivity, whereas higher coal contents resulted in elevated combustion temperatures and longer combustion durations. These findings provide insights for optimizing biomass-coal briquette formulations for fixed-bed combustion applications.

Trimethyl orthoformate (TMOF: HC(OCH3)3) has recently been examined as a viable biofuel. TMOF is a branched isomer of oxymethylene ether-2 (OME2) that, due to its high oxygen content and lack of direct carbon-carbon bonds, considerably reduces the formation of soot particles. To meet the challenges of a more flexible and sustainable power generation, a detailed understanding of its combustion properties is essential for its safe and efficient utilization, neat or in blends. In this work, two fundamental combustion properties of TMOF were studied: (i) Auto-ignition of TMOF/synthetic air mixtures (φ = 1.0; diluted 1:5 with N2) using the shock tube method at pressures of 1, 4, and 16 bar, and (ii) Laminar burning velocities of TMOF/air mixtures using the cone angle method at ambient and elevated pressures of 3 and 6 bar. Furthermore, the impact of TMOF addition to a gasoline surrogate (PRF90) on ignition delay times was studied using the shock tube method at φ = 1.0, 1:5 dilution with N2, T = 900–2000 K, and at 4 bar. The experimental data sets have been compared with predictions of the in-house chemical kinetic reaction mechanism (DLR concise mechanism) developed for interpreting the high-temperature combustion of a broad spectrum of different hydrocarbon fuels as well as oxygenated fuels, including TMOF. The results demonstrate that the ignition delay times of TMOF and OME2 are nearly identical for all pressures studied in the moderate-to high-temperature region. The results obtained for the blend indicate that ignition delay times of the TMOF/PRF90 blend are shorter than those of the primary reference fuel 90 (PRF90) at 4 bar. In the lean-to stoichiometric region, the results obtained for laminar burning velocities of TMOF and OME2 are similar. However, in the fuel-rich domain (φ > 1.0), laminar burning velocities for TMOF are noticeably lower, indicating a decreased reactivity. The model predictions based on the in-house model reveal a good agreement compared to the measured data within the experimental uncertainty ranges. In addition, sensitivity analyses regarding ignition delay times and laminar flame speeds were performed to better understand TMOF oxidation.

Oxymethylene ethers (OMEn, n = 1–5) are a promising class of synthetic fuels that have the potential to be used as additives or substitutes to diesel in compression ignition engines. A comprehensive understanding of their combustion properties is required for their safe and efficient utilization. In this study, the results of a combined experimental and modeling work on oxidation of OME2 are reported: (i) Ignition delay time measurements of stoichiometric OME2/synthetic air mixtures diluted 1:5 with nitrogen using the shock tube method at pressures of 1, 4, and 16 bar, and (ii) laminar flame speeds of OME2/air mixtures using the cone angle method at atmospheric and elevated pressures of 3 and 6 bar. The experimental data sets obtained have been used for validation of three detailed reaction mechanisms of OME2 obtained from literature. The results of ignition delay time measurements showed that ignition of OME2 is characterized by pre-ignition activity at the low temperature side of the measurements regardless of the pressure. Regarding the performance of the different reaction mechanisms, the model from Cai et al. (2020, “Auto-Ignition of Oxymethylene Ethers (OMEn, n = 2–4) as Promising Synthetic e-Fuels From Renewable Electricity: Shock Tube Experiments and Automatic Mechanism Generation,” Fuel, 264, p. 116711) best predicted the temperature and pressure dependence of ignition delay times. For laminar flame speeds, the experimental data were well matched by the mechanism from Ren et al. (2019) at p = 1, 3, and 6 bar and for all equivalence ratios considered. From sensitivity analyses calculations, it was observed that chain reactions involving small radicals, i.e., H, O, OH, HO2, and CH3 control the oxidation of OME2. The comparison of the results of this work and our previous work (Ngugi et al. (2021)) on OME1 show that these two fuels have similar oxidation pathways. The results obtained in this work will contribute to a better understanding of the combustion of oxymethylene ethers, and thus, to the design and optimization of burners and engines as well.

*Demands of energy will increase worldwide. The use of alternative and renewable energy resources is an attractive option to counteract climate change connected with the burning of fossil fuels. Moreover, improvements in fuel flexibility are a pre-requisite to meet the challenge of a sustainable production of energy in the near future.* *Within this context, oxygenated molecules,