A process control method for the on-line operation in real time of a low-rank-coal pyrolysis process producing a coal-char product, a pyrolysis gas, and a complex multi-component coal-tar-oil. The control method is based on measuring the concentration of selected compounds in the three products, a solid phase, a gas phase and a liquid phase condensed from the gas-phase, using a combination of spectrometric technology including scanning in the infrared, visible, ultraviolet and microwave spectral regions, and analyzing the data based on application of a modified Chi-Square data manipulation fitting technique developed for the specific products and process. This process control method provides a basis for accurate on-line control of the process operating parameters and allows optimization of the coal-char quality as well as the quality and yield of the extracted coal-tar-oil with unique chemical composition derived from low-rank coal in a pyrolysis process. The subject invention is based on the selection of 2-6 key compounds contained in each product to be measured and used as control point, calibration of the process operating conditions to the key compound composition and monitoring the changes in concentration on-line in real time.