  Citation: Muri, H.I.D.I.; Hjelme, D.R. Sensor Technology Options for Municipal Solid Waste Characterization for Optimal Operation of Waste-to-Energy Plants. Energies 2022, 15, 1105. https: //doi.org/10.3390/en15031105 Academic Editor: Idiano D’Adamo Received: 6 December 2021 Accepted: 30 January 2022 Published: 2 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). energies Article Sensor Technology Options for Municipal Solid Waste Characterization for Optimal Operation of Waste-to-Energy Plants Harald Ian D. I. Muri * ,† and Dag Roar Hjelme † Department of Electronic Systems, Norwegian University of Science and Technology, 7491 Trondheim, Norway; dag.hjelme@ntnu.no * Correspondence: harald.muri@ntnu.no † These authors contributed equally to this work. Abstract: Reuse, refurbishing, and recycling are the most sustainable options for handling waste materials. However, for municipal solid waste (MSW) that is highly heterogenic, crude, contaminated, and decrepit, thermal conversion in waste-to-energy (WtE) plants is an option. In such plants, the fuel quality of MSW is difficult to predict and the substantial changes expected are challenging for incineration stability. Development of new online sensor technologies for monitoring waste properties prior to incineration is therefore needed. Sensors may contribute to increase WtE process stability, as well as reducing the probability of incineration stops or emissions exceeding legal limits. In this work, the operating principles of potential sensor systems for waste monitoring are categorized and assessed to be implemented for providing parameters for process control or indicators for process alarms in the waste incineration process. For transmissive settings, the use of inductance and hard X-ray sensors are most promising, whereas for reflective settings, utilization of photonic, inductive, soft and hard X-ray, as well as low-frequency radiowave sensors, are most promising. The analytic capacity of single-point measurements with inductance, radiowave, photonic, or X-ray sensors are limited to providing indicators for process alarms, whereas spectral imaging with X-ray or photonic techniques are feasible for providing parameters for both process control and indicators for process alarms. The results obtained in this sensor assessment will be important as a first step in guiding the evolution of monitoring waste properties in the WtE industry to increase repeatability, performance of energy production, and manual labor safety in controlling the waste incineration. Keywords: waste-to-energy; waste characterization; sensor technologies; online monitoring; feasibility studies 1. Introduction Municipal solid waste (MSW) that is highly heterogenic, crude, contaminated, and decrepit cannot be handled for reuse, refurbishing, or recycling. Such MSW materials are usually landfilled or incinerated at waste-to-energy (WtE) plants. In view of a sustainable future, landfills should only be used for pretreated waste. Thus, for waste streams for which material recovery is not applicable, energy recovery using WtE plants is the path to follow [1–3]. Due to variation in feedstock properties, WtE plants are today operated with significant process safety margins, resulting in limited energy efficiency. Thus, to increase the competitiveness of the WtE sector, optimization of the incineration process is needed. To this effect, significant efforts have been turned towards sensors that could provide supplementary information regarding the physical and chemical properties of the fuel before it enters the combustion process. Information from such sensors, if integrated with the process control system, could be used to automatically adjust the process to increase stability of the energy production and avoid emissions exceeding legal limits or excessive Energies 2022, 15, 1105. https://doi.org/10.3390/en15031105 https://www.mdpi.com/journal/energies