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• Lean combustion in SI engines fueled with both pure CO and hydrogen-rich gas. • Low efficiency of the engine when using pure CO. • Emissions were reduced for syngas H 2 /CO ∼ 1. The possibility of using renewable energy generated in the industrial sector for the combined generation of electricity and heat is becoming one of the most efficient energy generation methods, without the need for sophisticated systems of biogas compression for its use. Different fuels are able to extend the lean threshold of spark ignition engines (SI). Although synthesis gas (syngas) lean combustion raises combustion efficiency, its use can result in high carbon monoxide (CO) emissions, making it urgent to investigate the CO combustion process in SI engines. Currently, only data referring to the combustion of pure hydrogen are found. Also, studies on the emissions from the use of syngas generated from peach kernels and rice husks are scarce. Therefore, the innovation and goal of this paper were to investigate the emissions resulting from lean combustion in SI engines fueled with both pure CO and hydrogen-rich gas with close characteristics of syngas generated from rice husk and peach kernel, two highly produced biomasses in Brazil. As the main results, regarding nitrogen oxide (NO x) emissions, the combustion of pure CO was quite high when compared to syngas. The syngas H2/CO ∼ 1 was also responsible for the largest emission of hydrocarbons (HC), a little less than twice that of H2/CO ∼ 0.38. Furthermore, by allowing lean combustion, the temperatures of the flared gas can be kept below the NO x formation limit. Moreover, the combustion of pure CO resulted in reduced efficiency and increased COV of IMEP. Besides emissions reduction, the use of waste for biofuel production is essential for the overall strategy for a transition to a circular economy. [ABSTRACT FROM AUTHOR]