Experimental and economic study of small-scale CHP installation equipped with downdraft gasifier and internal combustion engine Witold Elsner a , Marian Wysocki a , Pawel Niegodajew a,⇑ , Roman Borecki b a Czestochowa University of Technology, Institute of Thermal Machinery, al. Armii Krajowej 21, 42-200 Czestochowa, Poland b Institute of Research & Machinery Implementation Ltd., ul. Sokola 4, 42-256 Olsztyn, Poland highlights  Innovative design of downdraft gasifier with internal heat exchanger.  Stable working conditions regardless lower calorific value of a fuel.  Self-adjustment of gasifier operation under rapid changes in gasifying agent load.  The most profitable scenario of CHP plant operation is proposed. article info Article history: Received 20 February 2017 Received in revised form 17 May 2017 Accepted 22 May 2017 Keywords: Downdraft gasifier Piston engine Cogeneration plant Sewage sludge Biomass waste CHP profitability abstract This paper concerns the experimental and numerical analysis of a combined heat and power (CHP) instal- lation equipped with a biomass downdraft gasifier, gas purification system and gas piston engine. The numerical studies were aimed at identifying waste heat in the CHP installation for further utilisation. In turn, a new innovative method of heat recovery inside the gasifier was proposed and subsequently applied in the experimental CHP installation. The newly developed facility is able to produce up to 75 kWe of electrical power. An extensive parametric study was performed under the steady and unsteady facility operation and was supplemented by economic analysis of the process. The main objective of the study was to examine the possibility of sewage sludge gasification and the impact of produced syngas quality on gas engine performance. During the investigation a number of different types of biomass were investigated including wood pellets, sewage sludge and their blends. The results showed a stable opera- tion of the CHP facility in terms of produced syngas load, calorific value and content – even when dried sewage sludge was used alone. Results indicate that a 40/60% blend of wood pellets with sewage sludge is recommended, which allowed the lower heating value (LHV) of 4.45 MJ/Nm 3 . Additionally, it was con- firmed that the fixed bed gasifier is able to self-adjust to temporary changes in gasifier agent load. The economic analysis was performed taking into account policies and regulations in the Polish energy market sector. The study showed that it is more profitable to use the generated electricity and heat for its self-consumption rather than selling it on the market. Even with the supporting policies the payback time is not less than about eight years. Taking into account calorific value and limited local biomass availabil- ity, it seems justified that the described CHP installation is suitable for decentralised power generation such as small farms or horticultural businesses, where the produced electricity and heat can be suffi- ciently utilised. This technology also contributes to reduction of CO 2 emissions into the atmosphere, which is one of the most problematic issues for energy sector today. Ó 2017 Elsevier Ltd. All rights reserved. 1. Introduction Current trends in energy and environmental sciences show an on-going global effort to replace fossil fuels with renewable energy sources. Particularly interesting are biomass conversion methods (including biomass wastes) which represent a valid alternative to fossil fuels [1]. Wide biomass availability and its largely uniform distribution put bio-energy production ahead of many other renewable energy sources. Diverse biomass types are commonly available as agricultural waste, food waste, industrial wastewaters and sewage sludge. All are generated in large volumes and have already found their usefulness as non-conventional energy source http://dx.doi.org/10.1016/j.apenergy.2017.05.148 0306-2619/Ó 2017 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: niegodajew@imc.pcz.czest.pl (P. Niegodajew). URL: http://www.imc.pcz.pl (P. Niegodajew). Applied Energy 202 (2017) 213–227 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy