POTENTIAL AND PROPERTIES OF THE GRANULAR SEWAGE SLUDGE AS A RENEWABLE ENERGY SOURCE

The predominant method of the sewage sludge management in Poland is land disposal. However, since 01/01/2013, this method will be prohibited. Therefore, there is a strong need for the development of thermal methods of sludge disposal. In Polish legal system sewage sludge may be named as biomass or waste. For the purposes of determining the obligations of environmental regulations the definition of the Minister of Environment should be used. When disposing of sewage sludge in an amount up to 1% by weight of fuel, emission standards for fuel do not change. At the disposal of sewage in quantities of more than 1%, should be conducted continuous measurement of emissions, including HCl, HF, and continuous measurements of flue gas param eters (as for the installation of waste disposal). In order to meet the requirement to porduce energy from renewable sources we use the definition of Minister of Economy. In this case, in accordance with applicable law, sewage sludge shall be considered as pure biomass, thus it is CO 2 neutral. The use of sewage sludge as a fuel requires the determination of fundamental combustible properties. These properties should be in accordance with the requirements put fuels as an energy source. The paper presents the results of a detailed physico-chemical analysis of dried sewage sludge produced in the two Polish wastewater treatment plants. The results were compared with five representatives of biomass fuels: straw of wheat, straw of rape, willow, pine and oak sawdust. Ultimate and proximate analyses include a detailed analysis of fuel and ash. The results clearly indicate that sludge is a very valuable fuel similar to “traditional” biomass.


INTRODUCTION
Sewage sludge, originating from the treatment process of waste water, is the residue generated during primary (physical and/or chemical), secondary (biological) and the tertiary (additional to secondary, often nutrient removal) treatment [15][16][17]. Removal of sludges from Wastewater Treatment Plants (WWTP) represents a serious worldwide environmental problem [5]. Not long ago, it was thought that raw sludge was a valueless material that should be discarded, and then it was disposed of in landfills and/or thrown into the ocean. But huge amounts of the produced sludge make all these options environmentally unacceptable. High output of sewage sludge, which is increasing during recent years, and the limitations of the existing means of disposing sewage sludge highlight the need to find alternative routes to manage this organic material. The 6 th Environment Action Programme 2002-2012 of the European Commission has been described as a major factor in reducing sewage sludge disposal by 50% from 2000 by 2050. Moreover, European legislation prohibits the deposition of sewage sludge into landfill or water. Biomass and residues like sewage sludge are the only renewable energy sources that can provide C and H, thus it is interesting to process them by means of treatments that enable to obtain chemically valuable products like fuels. As a type of biomass fuel, sewage sludge is a renewable source and has an advantage of being CO 2 -neutral: no additional CO 2 is emitted into the atmosphere in the long term. The latest trends in the field of biomass and sludge management, (i.e., combustion, pyrolysis, gasification and co-combustion) have generated significant scientific interest [3,13,18]. Gasification is the process of converting a solid fuel into a gas by treating the solid fuel in a generator with oxygen, air, and steam or by other gasification methods [19]. As shown in Marrero et al., gasification of sewage sludge leads to a high-quality flammable gas that can be used for the generation of electricity or to support such processes as sewage sludge drying [6]. Gasification is one way of using sewage sludge and is an attractive alternative to other treatment methods. To determine the usefulness of sewage sludge as a biomass fuel for thermal transformation, it is necessary to know its basic physical and chemical characteristics. The elemental composition of sewage sludge and the contents of inorganic compounds depend on many factors, but it may be largely dependent on the country or region of origin.
The aim of the work is the comparison of physical-chemical properties of dried sewage sludge produced in the two Polish wastewater treatment plants with five representatives of "traditional" biomass fuels: straw of wheat, straw of rape, willow, pine and oak sawdust. Ultimate and proximate analyses include a detailed analysis of the fuel and ash.

RESULTS
Within this study straw of wheat and rape, oak, willow and pine sawdust and two sewage sludge samples were examined. The proximate and ultimate analysis are presented on Figure 1.
The moisture of the feedstock was obtained following the standard PN-EN 14774-3:2010 [9]. The infrared spectroscopy analyzer was used to carry out the ultimate analysis of the sewage sludge.
The volatile matter content was determined according to the standard PN-EN 15402:2011 [10]. The ash content was obtained using PN-EN 15403:2011 [11]. The calorific content was determined in accordance with the standards CEN/ TS15400:2006 [2].
As it can be seen from the ultimate analysis, there are no significant differences in the C, H, Cl and F content. Nevertheless, taking into consideration S, N and O contents, those differences between "traditional" biomass and sewage sludge are quite strong. Despite the fact that sewage sludge contains phosphorus, nitrogen and sulfur, the gasification of these components offers several advantages over a traditional combustion process. Gasification takes place in an environment with low levels of oxi- Fig. 1. Ultimate analysis of ash and water content in the analysed feedstock dizers (to prevent the formation of dioxins) and large quantities of sulfur and nitrogen oxides [1]. As mentioned above, sulfur is present in sewage sludge at low amounts; it is mainly converted to hydrogen sulfide (H 2 S) during gasification [7], whereas the nitrogen is transformed into ammonia [1].
It is worth noting that phosphorus in sewage sludge is partitioned into solid (not gaseous) residues [20] and that the volume of syngas produced from sewage sludge is low because gasification requires a fraction of the stoichiometric amount of oxygen necessary for combustion. For all of these reasons, gasification requires smaller and less expensive gas-cleaning facilities [8]. Analysing Figure 1 it can be also seen that the sewage sludge were characterized by higher ash content than "traditional" biomass feedstock.
It can be seen in Figure 2 that lower heating value is comparable to that of traditional biomass. Simultaneously, in Figure 3, it can be observed that volatile matter content in the sewage sludge is much lower in comparison to traditional biomass. The combination of low oxygen content and low volatile matter in sewage sludge indicates a low potential for creating large amounts of inorganic vapors during combustion and other thermal processes.
The results of the chemical analyses of the fly ashes are presented in Figure 4. The plasma spectrometer Thermo iCAP 6500 Duo ICP was used to carry out the ash analysis of the biomass feedstock.
Ash behavior and deposition tendencies were predicted through the use of empirical indices for biomass type ashes [4,12,14]. These indices, despite their shortcomings due to the complex conditions, which arise in boilers and their associated heat transfer equipment, are widely used and probably remain the most secure basis for decision making, if used in conjunction with pilot plant testing.
One simple index, the alkali index (AI) which is a frequently used parameter to describe the overall influence of catalytically active species within the ash and is defined as the ratio f the sum of the fraction of the basic compounds in the ash (CaO, MgO, K 2 O, Na 2 O and Fe 2 O 3 ) to the frac- (1) When the AI increases slagging tendency increases.
Another index, the base-to-acid ratio (R b/a )eq. (2). As Rb/a increases, the fouling tendency of a fuel ash increases. (2) A bed agglomeration index (BAI) -eq. (3) has been developed, relating ash composition to agglomerations in fluidized bed reactors. (3) Bed agglomeration occurs when BAI values become lower than 0.15.
Analysing data presented on Fig. 5 it can be concluded that sewage sludge is characterizing by higher slagging tendency in comparison to traditional biomass (especially oak and pine sawdust).
Simultaneously sewage sludge ash is characterised by lower fouling tendency than traditional biomass ash and higher tendency to create agglomerates.

CONCLUSIONS
The analysis of various biomass materials indented to be used as supplemental fuel in fossil fuel fired power plants has shown that there is always a range of the results sometimes with a big gap between minimum and maximum. Most noticeable for the sewage sludge was the highest share of ash, nearly 50% of the dry substance, compared to all the other fuels. Additionally, it should be emphasised that the combination of low oxygen content and low volatile matter in sewage sludge indicates a low potential for creating large amounts of inorganic vapors during combustion and other thermal processes. Moreover, sewage sludge is characterized by higher slagging tendency, lower fouling tendency and higher tendency to create agglomerates in comparison to traditional biomass.