# Comparative Analysis of Partially Renewable Composite Fuels Based on Peat, Lignite and Plant Oil
## 1 Introduction  
Recent increases in the traditional energy resource prices have sparked a considerable interest in alternative fuels [1; 2] as well as in different ways of saving fossil fuels [3]. Many countries have their own deposits of low-grade fuels such as peat (P) and lignite (L). An evident increase in the lignite production and consumption occurred this year in Germany and Poland [4]. The producer price index for lignite shows jumped up to 128% of previous peak values this year [5]. These fuels are less efficient than the ones commonly used in the industry over the last decades, but they are cheap and highly available in different countries [6]. Germany, the Russian Federation, and some others countries have large enough deposits of peat and lignite that can be quickly taken in use. The main problem of low-grade fossil fuels is a smaller calorific value compared to traditional fuels. Variations in the chemical composition of fossil fuels depending on the deposit [7] cause additional difficulties for the industrial application of such fuels. The mineral matter content is up to 40-50 wt%, which leads to big amounts of ash residue. High moisture and volatile content result in a low heating value, slow ignition and an unstable combustion temperature [8]. All these drawbacks make these fuels unattractive for trivial combustion and most of the popular methods of thermochemical conversion. Various approaches to peat pyrolysis and lignite gasification, used over the last few years, show [9; 10; 11] that such techniques still need essential improvements. Another strong trend over the last few years, contributing to lower consumption of fossil fuels, is a growing utilization of renewable fuels [2; 12]. Different techniques of plant oil conversion into bio-diesel [12; 13] or bio-gas [14] are popular in the modern industry. However, these fuels are quite expensive because of their long growth cycle with largeplanting areas and expensive post-processing [15]. This factor has limited the spread of biofuels in industrial use. However, the perspective of the further decrease in oil availability stimulates the interest for bio-diesel and other plant-derived products, even taking into account the three times higher prices of these fuels [16] in comparison with oil. Bio-diesel production requires the thermochemical conversion of aliphatic acids with alcohol (esterification). Effective practical solutions here are compositions of plant aliphatic acids and methanol with a ratio of about 1:10 or even higher [17]. The production of bio-gas with a natural fermentation process looks much easier but it is a slow process. Efficient technologies utilizing this principle require significant investments in infrastructure located over large areas. Different processing techniques allow the production of biofuels with higher energetics or better ecological indicators [18]. Different pre-treatment approaches allow an essential enhancement of the availability of lignocellulosic for hydrogen production [19]. A special way of involving renewable components is the preparation of partially renewable fuel compositions [20; 21]. Plant oils can be easily mixed with low-grade fossil fuels to produce new compositions whose properties are quite appropriate for applications. Some co-firing approaches [22; 23] show a great potential of decreasing hazardous emissions as well as a rather high level of energy efficiency. The usage of the peat as a sorbent for spill oil shows the good potential of composing it with different aliphatic acids [24]. Mixing fossil fuels with renewables can extend the application area of low-grade fuels and make renewable fuels cheaper. The main task in this field is to determine the component ratio and thermal regime for the efficient use of a composite fuel that allows maximal production of combustible gases. Plant-derived aliphatic acids require rather high temperatures for efficient conversion [25].