energies Article Model-Free Control of UCG Based on Continual Optimization of Operating Variables: An Experimental Study Ján Kaˇ cur * ,† , Marek Laciak † , Milan Durdán † and Patrik Flegner †   Citation: Kaˇ cur, J.; Laciak, M.; Durdán, M.; Flegner, P. Model-Free Control of UCG Based on Continual Optimization of Operating Variables: An Experimental Study. Energies 2021, 14, 4323. https://doi.org/10.3390/ en14144323 Academic Editor: Adam Smoli ´ nski Received: 2 June 2021 Accepted: 15 July 2021 Published: 18 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Institute of Control and Informatization of Production Processes, Faculty BERG, Technical University of Košice, Nˇ emcovej 3, 042 00 Košice, Slovakia; marek.laciak@tuke.sk (M.L.); milan.durdan@tuke.sk (M.D.); patrik.flegner@tuke.sk (P.F.) * Correspondence: jan.kacur@tuke.sk; Tel.: +421-55-602-5176 † These authors contributed equally to this work. Abstract: The underground coal gasification (UCG) represents an effective coal mining technology, where coal is transformed into syngas underground. Extracted syngas is cleaned and processed for energy production. Various gasification agents can be injected into an underground georeactor, e.g., air, technical oxygen, or water steam, to ensure necessary temperature and produce syngas with the highest possible calorific value. This paper presents an experimental study where dynamic optimiza- tion of operating variables maximizes syngas calorific value during gasification. Several experiments performed on an ex situ reactor show that the optimization algorithm increased syngas calorific value. Three operation variables, i.e., airflow, oxygen flow, and syngas exhaust, were continually optimized by an algorithm of gradient method. By optimizing the manipulation variables, the calorific value of the syngas was increased by 5 MJ/m 3 , both in gasification with air and additional oxygen. Further- more, a higher average calorific value of 4.8–5.1 MJ/m 3 was achieved using supplementary oxygen. The paper describes the proposed ex situ reactor, the mathematical background of the optimization task, and results obtained during optimal control of coal gasification. Keywords: underground coal gasification (UCG); optimization; syngas; calorific value; optimal control; operating variables; control algorithm 1. Introduction The technology of underground coal gasification (UCG) enables the extraction of coal energy by thermic decomposition. The coal is transformed into syngas by the utilization of gasification agents injected into an in situ georeactor. The produced syngas is exhausted on the surface, where it is transformed into the desired form of energy, or various chemicals are produced. Compared with classical coal mining, the UCG is a less expensive technology, also attractive in terms of environmental protection. In the case of UCG, at least two boreholes must be drilled into a coal seam, i.e., inlet or injection hole and outlet production hole [1]. Before gasification can begin, a highly permeable path (i.e., channel) within the coal seam is established between the wells. This link is required as the in situ properties of the coal seam do not permit the gas flows required for economical gasification. Many of the known coal resources are currently uneconomic to mine using conventional techniques. The potential of UCG is also in the case of deposits with tectonic faults and in deposits that are unavailable for traditional mining. As coal reserves are much larger than those of natural gas, it seems likely that coal gasification will be used more frequently for generating synthesis gas to make chemicals and liquid fuels. The essential performance parameter of coal gasification is the calorific value of the syngas. The raw dry gas from UCG consists of hydrogen, carbon monoxide and carbon dioxide, methane, higher hydrocarbons, and traces of tars and pollutants. The valuable gases in Energies 2021, 14, 4323. https://doi.org/10.3390/en14144323 https://www.mdpi.com/journal/energies