The effects of chemical impurities on the combustion of syngas were investigated, focusing on CH4, a common component of syngas, and trimethylsilanol (TMS), an unstudied impurity related to those common to landfill-based syngas. Ignition properties were systematically investigated at high-pressure low-temperature conditions relevant to gas turbine combustor operation using the University of Michigan Rapid Compression Facility. Pressure time history measurements and high-speed imaging of the ignition process in this facility were used to determine auto-ignition delay times and observe ignition behaviors. The four simulated syngas mixtures used were (1) pure syngas: 30% H-2, 70% CO fuel volume. (2) syngas with CH4: 27% H-2, 67% CO, 6% CH4, (3 & 4) pure syngas with 10 or 100 ppm TMS, all with fuel-to-O-2 equivalence ratios (phi) 01 0.1 at air dilution (i.e. molar O-2 to inert gas ratio of 1:3.76). and N-2 as the primary diluent gas. The pressures after compression were 5 & 15 atm with temperatures of similar to 1010-1110 K respectively. The results uniquely illustrated the occurrence of two-step ignition behavior at higher pressures, with two distinct regions of heat release and pressure rise. First and second auto-ignition delay times were therefore defined and interestingly the times were affected differently by the addition of impurities. The addition of CH4 consistently increased auto-ignition delay times up to 40% at 15 atm, while increasing delay times at 5 atm by up to a factor of three. Conversely, the addition of 10 ppm TMS impurity addition caused a consistent decrease of similar to 10-30% delay times at 15 atm with insignificant impact at 5 atm, and 100 ppm TMS impurity caused consistent decreases of 50-70% at 15 atm and 20-30% decreases at 5 atm. The marked pressure dependence of the auto-ignition delay time, typical for syngas at these conditions, was virtually eliminated for the 100 ppm TMS mixture. Kinetic modeling suggests that the promoting effects of TMS are related to enhanced consumption and/or reduced production of HO2. The impact of TMS is remarkably similar to that for SiH4 in pure H-2, suggesting a possible trend for poorly understood Si-based species to promote auto-ignition in syngas and hydrogen mixtures. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.