**1. Introduction**

Preheating furnaces use substantial amounts of energy in industrialized countries. A great number of furnaces and substantial energy consumption accompanying the heat treatment processes lead to the significant carbon footprint in Europe and beyond. In Germany, industrial processes are responsible for nearly 40% of natural gas consumption [1]. In Poland, over 45% of natural gas is used by industry and construction sectors [2]. High energy consumption of furnaces results from the need of heating the processed load to a specific temperature over a certain period of time. On an industrial scale, where the volume of the treated material is large, it means high demand for thermal power, associated with proportional operating cost and environmental impact.

On the other hand, it is deemed necessary to reduce the anthropogenic pressure on climate change [3] and the diversity of the biosphere, correlated with the CO2 emissions from combusting fossil fuels. This is followed by concrete actions at the level of the European Union: 2020 climate and energy package, 2030 climate and energy framework, 2050 long-term strategy, and the resulting initiatives for public-private partnerships like Sustainable Process Industry through Resource and Energy Efficiency aiming to make the European process industry more efficient and sustainable, and less resource consuming [4].

Natural gas is a commonly used energy source in the industry due to its wide applicability and availability at a low price. Although its CO2 emission factor is the lowest of all hydrocarbons and it is considered as a much cleaner alternative to oil or coal, combustion of natural gas is still associated with polluting the atmosphere with greenhouse gases and the climatic consequences of that. Thus, measures limiting both fossil fuel consumption and gaseous emissions from the industry are sought. In the steel industry, the process by-products such as coke oven gas (COG), blast furnace gas (BFG) [5], and basic oxygen furnace gas (BOFG) can be used as a feedstock for reheating furnaces [6]. Usage of those gases in reheating furnaces and annealing lines with radiant tube burners was briefly analyzed [7]. It was found that there are no significant constraints for application of those gases, although cleaning the gas has to be done prior usage to avoid damaging the equipment. Researchers [8] investigated the influence of the syngas impurities on scale formation on steel slabs in the reheating furnaces, and found that they can cause corrosion and slagging.

Usage of other alternative gases (such as biogenic syngas fuels) has not been extensively explored, especially in the preheating furnaces where the temperatures are significantly lower and the conditions for complete combustion are not favorable. Moreover, the biogenic syngas may contain a significant amount of tars, which can either require gas cleaning (inherently connected with exergy losses) or cause difficulties in process equipment selection and operation in temperatures above the tars dew point.

Other contaminants which could be present in the biogenic syngas are halogen species (Cl, Br) [9] and, to a smaller extent, the alkali metals (Na, K) [10] which can cause operational problems [11]. Taking into account the above mentioned, there are no studies of the application of the syngas coming from the biomass gasification in the preheating furnaces.

One of the possible methods for low-emission combustion is partial substitution of natural gas with gaseous biofuels, whose CO2 emission factor, according to the European Commission regulations, is equal to 0 [12]. An example of such gas is syngas obtained from the solid biomass gasification process.

There is a noticeable interest in the issue of replacing natural gas as a source of heat in the industry with renewable gas fuels. The possibility of reducing CO2 emissions by co-firing syngas in gas turbines [13] and coal-fired boilers [14] has been studied. Research has been done on emission levels of nitrogen oxides (NOx), carbon monoxide (CO), polycyclic aromatic hydrocarbons (PAH), and volatile organic compounds (VOC) accompanying the combustion of syngas [15]. Partial replacement of natural gas by biogas has been proven to be a sensible approach for implementation in the glass processing industry as no negative effects in product quality have been observed [16]. There have been no studies investigating the impact of hot biogenic syngas co-firing on heat treatment process parameters in steel preheating furnaces. The preliminary study preceding this work has shown that the use of low-calorific alternative fuels (specifically: basic oxygen furnace gas and biogenic synthesis gas) does not excessively change the course of the heating process, and that the introduction of dedicated syngas burners is a preferred option of organizing co-firing in the considered furnace [17].

The goal of this study is to analyze the effects of partial substitution of natural gas with biomass-derived synthesis gas on temperature of a typical load treated in an exemplary steel sector preheating furnace. The potential of introducing carbon-neutral low-calorific syngas to the process sector has not yet been studied. The results are to be compared with the ones gathered from the furnace fired in standard operation mode (leaning on natural gas only) in order to assess the possibility of applying this method of carbon dioxide emission reduction to other industrial units without worsening thermal parameters of the process.
