The gasification and torrefaction of sewage sludge have the potential to make the thermal utilization of sewage sludge fully sustainable, thus limiting the use of expensive fossil fuels in the process. This includes sustainability in terms of electricity consumption. Although a great deal of work has been performed so far regarding the gasification of sewage sludge and some investigations have been performed in the area of its torrefaction, there is still a gap in terms of the influence of the torrefaction of the sewage sludge on its subsequent gasification. This study presents the results from the torrefaction tests, performed on a pilot scale reactor, as well as two consecutive steam gasification tests, performed in an allothermal fixed bed gasifier, in order to determine if torrefaction can be deemed as a primary method of the reduction of tar content for the producer gas, from the aforementioned gasification process. A comparative analysis is performed based on the results obtained during both tests, with special emphasis on the concentrations of condensable compounds (tars). The obtained results show that the torrefaction of sewage sludge, performed prior to gasification, can indeed have a positive influence on the gas quality. This is beneficial especially in terms of the content of heavy tars with melting points above 40 °C.

This study proposes an innovative installation concept for the sustainable utilization of sewage sludge. The aim of the study is to prove that existing devices and technologies allow construction of such an installation by integration of a dryer, torrefaction reactor and gasifier with engine, thus maximizing recovery of the waste heat by the installation. This study also presents the results of drying tests, performed at a commercial scale paddle dryer as well as detailed analysis of the torrefaction process of dried sewage sludge. Both tests aim to identify potential problems that could occur during the operation. The scarce literature studies published so far on the torrefaction of sewage sludge presents results from batch reactors, thus giving very limited data of the composition of the torgas. This study aims to cover that gap by presenting results from the torrefaction of sewage sludge in a continuously working, laboratory scale, isothermal rotary reactor. The study confirmed the feasibility of a self-sustaining installation of thermal utilization of sewage sludge using low quality heat. Performed study pointed out the most favorable way to use limited amounts of high temperature heat. Plasma gasification of the torrefied sewage sludge has been identified that requires further studies.

This research work presents the results of studies on the effect of modifiers comprising the salts CuSO 4 ·2H 2 O, NaCl, NH 4 Cl, MgSO 4 ·7H 2 O, CaCl 2 , and urea at various concentrations on the combustion of fine coal. The tests were carried out in a 12-kW laboratory boiler equipped with a rotary-grate retort furnace. The emission levels and concentrations of CO, CO 2 , SO 2 , NO x , and TOC in the flue gas were measured using analyzers. A modifier composed of 350 ppm Cu, Na, Mg, NH 4 + , Ca, and urea showed particularly high activity in the combustion of fine coal. The flue-gas levels of CO, NO x , and SO 2 were reduced by approximately 9%, 12%, and 10%, respectively, in comparison with the modifier-free sample. In this case, the boiler efficiency also increased from 65% in the tests with no modifier to 76% in the tests with the modifier. The proprietary application system, enabling the modifier to be added in exact amounts to the variable flow of fine coal, is also described. It was found that the use of the modifier in coal combustion tests results in lower emissions of harmful fuel components, a higher amount of heat obtained from the fuel unit mass, lower corrosive impact of the fuels, lower boiler maintenance costs, and extended service life.Energies 2019, 12, 4572 2 of 15In Poland, even as much as 80% of electric energy is still based on coal, including brown coal [10]. Looking at the latest investments in the power engineering industry in Poland, such as new power units in power plants, as well as the national energy policy, it seems that coal will continue to be the essential raw material in the generation of electric energy in Poland [10,11].The growing mechanization of coal mining leads to an increased efficiency of the production of fine coal (grain size 0-20 mm). Small coal has a high content of sulfur; therefore, its direct combustion is a serious source of environmental pollution. Reasonable utilization of fine coal is important for sustained development in power engineering [12].Direct coal combustion is a commonly used technology in the production of electric and thermal energy. There are essentially two coal combustion techniques: Bed combustion and suspension combustion. According to the former technique, coal is fed onto a fire grate and the combustion type depends on the directions of flow of the fuel and of the air. This type of combustion takes place in a stoker. The latter type, taking place in a fluidized bed, is a process in which suitably prepared solid particles are suspended in a uniformly distributed upward flow of the fluid. These are high efficiency devices, capable of attaining heating capacities as high as 135 tons of steam per hour [13].Stoker furnaces are the oldest and the most commonly used devices for industrial coal bed combustion. In these devices, crushed coal (typically, 95% particles below 32 mm and 20% to 60% below 6 mm) is fed onto the stoker where primary air flows upward through the coal bed. The crushed coal pieces are heated, dried, degassed, and combusted, lea...

Hard coal is widely used as a source of energy, and a number of catalysts have been developed to minimize the noxious impact of this fuel on combustion. This paper presents the cost-efficiency analysis of a system for improving the combustion of solid fuels, especially fine coal, in power boilers. The system is provided with a control and supervision device. It has been designed for better accuracy in controlling the boiler operating parameters, with a view to improving combustion efficiency due to the use of catalysts. The tests were carried out for system capacities ranging from 3 to 100 MW. It was found that, depending on the size of the system in the range of 3–100 MW, savings in the fuel consumption ranged from 2% to 8% due to the implementation of novel solutions in the boiler plant operation and from 2 to 6% due to the use of the combustion catalysts. Apart from boosting energy efficiency, the use of catalysts and the efficiency-boosting system resulted in the costs of overhauls being cut by about 20%. The payback time depends on system capacities, and it is between 6.75 and 1.74 years for capacities ranging from 3 to 75 MW and 2.0 years for a 100 MW plant.

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