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The aim of this article is to show the results of investigation how mixing producer gas (PG) (a product of a biomass gasification process) into the natural gas (NG) flow affects spectral characteristics of flame at specific wavelengths, representing formation reactions of OH*, CO* and C 2 * radical species, when the air equivalence ratio (ER) ranges from 1.0 to 1.3. In the current research, experiments were carried out for natural gas and mixtures of natural gas and producer gas. The producer gas was generated from wood pellets in a lab scale gasification reactor. Conventional flame emission spectroscopy methods were used for combustion process monitoring and control. This article presents the results of flame analysis by the emission spectroscopy method for registering chemiluminescent radical species OH*, CH* and C 2 *. After analysing the profile shape of chemiluminescent species intensity distribution along the burner vertical axis, it was determined that the quenching effect takes part in the combustion zone at some specific conditions. Additionally, the shift of chemiluminescense intensity towards the burner nozzle was registered when natural gas was mixed with producer gas. This effect was due to the presence of hydrogen in the producer gas.Key words: producer gas, combustion, chemiluminescence, renewable energy sources, spectroscopy INTRODUCTIONPartial or full substitution of natural gas (NG) with producer gas (PG) from waste or renewable sources is becoming a common practice nowadays; therefore, identification of trends in combustion of this type of fuel mixtures is important for developing both the burners, and monitoring and control systems. In the past decade, the usage of chemiluminesence in creation of active combustion process management systems made a great progress. During this time, the main meth ods of analysis were developed for processing the data acquired by non-intrusive optical monitoring. Currently, the most common tools for such research are the Cassegrain optical systems, reconstruction of 3D structure of flame by using inverse Abel transformation or 3D topography algorithms and use of Planar Laser Induced Fluo rescence (PLIF) [1]. Depending on a specific problem, each of these methods has some advantages and also some drawbacks. For example, PLIF is most successfully applied in small particle tracking, but it is also applicable for chemiluminescence research. The downside of this technique is that equipment is relatively expensive and applicable mostly for laboratory research. Also it is known that the ground state OH* radicals, which are abundant in the post-flame gas reaction zone, can be excited by PLIF, and this can generate errors in research. This problem does not apply when using Cassegrain (CS) optics because it detects only the natural chemiluminescence in the reaction zone [2]. The inverse Abel transform 211 Effect of producer gas addition on spectral characteristics of the natural gas flame and 3D topography methods may be the least expensive methods for analyzing data, sin...