1. Introduction
The production of energy by burning solid fuels as the main item in energy mixes is prevalent across many countries in the world, including the largest energy producers, such as the USA, China, India and most of the European Union (EU) countries. It seems that coal fuels will remain an important element of their economies for at least several dozen years [
1]. The share of energy from renewable sources in final energy consumption in the EU-28 and in Poland in 2016 was 8.0% and 8.3%, respectively. In 2013–2016, there was an increase by 0.4% in the EU-28, but a decrease by 0.6% in Poland [
2,
3]. In 2016, the structure of primary energy production from renewable sources in the European Union included a share of 44.7% of solid biofuels and 4.7% of municipal waste in this stream but the results for Poland are different—70.7 and 0.9%, respectively. In 2016, in the EU, solid biomass had a 15.2% share in the structure of the consumption of household energy from individual energy carriers. A smaller index has been noted in Poland, 13.5% [
3], and for the whole world, 10–14% [
4]. The 27 EU member states have a high potential for waste biomass for energy applications, calculated at 8500 PJ·y
−1 [
5]. The use of biomass for energy production has a number of advantages, such as low costs and high availability, which lowers the costs of transport and reduces its environmental impact [
4,
6,
7]. It is also important that biomass can be used both in the boilers of the power industry to generate heat and electricity as small individual heating installations [
8]. The market for biomass pellets is growing systematically on a global scale. As a result, woody and herbaceous biomass will be more difficult to obtain [
9]. Difficulties are also noted in the access to wood and agricultural residues in many places around the world, i.e., African countries [
8].
Large amounts of waste with a significant potential for energy production are produced by the wood industry (logging and wood processing residues)—up to 27% of the wood mass. Up to 42% of this waste can be obtained for energy purposes [
8]. Poland is a country with a relative high forest residues theoretical potential in the EU [
5]. This waste contains 43–51% of carbon, and the heat generated by its combustion reaches 18.5–20 MJ·kg
−1, which makes it a valuable energy material [
10,
11]. An additional advantage is the low amount of ash produced during the combustion of the wood residues, ranging from 0.4 to 2.0%. The use of a mixture consisting of 80% of sewage sludge, 19% of wood dust and 1% of quicklime produced results that proved that a widespread use of this type of fuel was possible since the heat generated by its combustion was slightly over 13 MJ·kg
−1. Replacing wood dust with coal dust raised this value to 19 MJ·kg
−1. Attention is paid to the hygroscopy of this fuel and its susceptibility to crumbling under the influence of moisture [
11]. Due to the high price of wood obtained from forests by the wood industry using typical methods, slash and waste material obtained by pruning trees and shrubs are used to produce pellets. Wood from short cultivation cycles of
Populus, Salix, Eucalyptus and
Robinia is also used [
12]. This method of wood management is also used in agriculture in orchards [
13,
14]. Forest residues and straw are counted as the top two contributors of energy from the residual biomass in the EU—7000 PJ·y
−1 [
5]. Agriculture is the main source of fuel biomass in the world. In this context, apart from trees and shrubs, attention is paid to the cultivation of energy plants and the use of post-harvest waste for this purpose [
6]. Among agricultural crops, the typical biomass used for energy production are cereals, miscanthus, mallow, rapeseed, sunflower, and Jerusalem artichoke, but also agricultural residues such as vine, shrub and fruit tree shoots, corn stalks, peanut and hickory shells [
14,
15,
16]. According to Hamelin et al.’s elaboration, the straw theoretical potential in Poland is high compared with many other EU-27 countries [
5]. Pellets from vine shoot biomass have a standard calorific value for a majority of biomass types—18 MJ·kg
−1 [
14]. Due to the large diversity of agricultural crops and residues, the possibility of their recovery for energy purposes varies from 19 to 75% of the total mass [
8].
The third of the important biomass sources for modern energy production, after wood and agricultural products, is municipal waste. Solid municipal waste (residues from urban green areas, roadside vegetation, food, paper, textile) and sewage sludge are used for energy production, both as homogeneous fuels, as well as in mixtures with other biomass for co-combustion with coal. There is a lot of information about the difficulties in the management of municipal waste due to the significant differentiation of their properties [
17]. Chen et al. [
18] suggested using fuel in the form of granules from sewage sludge and wood dust in a proportion of 10:1, with a moisture content of 14.2–18.5%, in the form of granules with a diameter of 2 and 7 mm. The results (a calorific value of 21.8–23.4 MJ·kg
−1) were favorable and the fuel proved to meet the requirements of the Taiwanese company Taipower. Also, Jiang et al. [
19] mentioned sewage sludge as an interesting material that could be a biomass binder in the production of pellets. According to these authors, the addition of sewage sludge reduces the energy needed to compress and extrude materials during the production of pellets, increases the density and hardness of pellets (reducing dust during transport and operation) and improves combustion parameters. The downside is an increase in the weight of combustion residues in comparison to pure wood and herbaceous biomass. Stabilized sewage sludge contains 40–70% of carbon in its dry matter, which means that these materials could be used for energy production [
20,
21]. Nevertheless, the process of preparing sewage sludge for combustion and co-combustion is so expensive that the rolling costs of this type of use are not very favorable—in Poland, 375–438 € for 1 Mg d.m., compared to 75–150 in agriculture and reclamation [
20]. An additional problem is the amount of ash generated from this material, which is greater than in the case of most other fuels—an average of 36.4% for sewage sludge as against approx. 1% for wood, 6% for wheat straw and 19–22% for coal [
22,
23]. Therefore, the co-combustion of sewage sludge should be redefined to find an innovative method for the preparation of this material before its use for energy production. Apart from energy production, the new material has to give end users measurable financial benefits in the form of better furnace operating parameters. Yilmaz et al. [
24] showed that the best results could be obtained by burning pellets with a size of 35 mm made from waste from plant oil production and sewage sludge. These authors noticed the high susceptibility of pellets to mechanical degradation under the influence of moisture and, for this reason, they recommended short-term storage under conditions that would counteract the moistening of the material. Jiang et al. [
25] showed that better results could be obtained by burning pellets made from a mixture of sewage sludge and wood biomass than sewage sludge alone. As far as energy production is concerned, pellets are much more efficient than raw biomass. Increasing biomass density reduces transport costs and improves combustion parameters [
26]. When pellets are formed, it is possible to control their composition, and when they are finally used, it is possible to automatically feed them to the furnace.
In the literature, there is little mention of the process of granulation/pelletization of energy materials with the use of municipal waste. Relatively few tests of this type have been conducted on sewage sludge for only a couple of years. Li et al. [
27] presented optimum parameters for the production of pellets from biomass and sewage sludge (50 + 50%): pressure 55 MPa, temperature 90 °C and moisture in the material 10–15%. The energy needed to produce pellets using sewage sludge was 50% lower than the energy needed to produce pellets from pure biomass. Similar moisture parameters of material intended for pelletization were reported by Kliopova and Makariski [
28]. Kijo-Kleczkowska et al. [
23] obtained pellets with densities of (kg/m
3): 1089.2 (hard coal), 859.9 (sewage sludge), 802.6 (lignite), 363.1 (
Salix viminalis), 898.1 (50% sewage sludge + 50% hard coal), 803.5 (50% sewage sludge + 50% brown coal), and 515.9 (50% sludge + 50%
Salix viminalis). There were a number of changes in the process of combustion of particular solid biofuels after the addition of sewage sludge. Jiang et al. [
19] analyzed the possibilities of pelleting energy materials using sewage sludge and noticed an increase in the density of pellets obtained while increasing pressure to 28, 41, 55, 69 and 83 MPa. A further increase in pressure to 110 MPa no longer caused any significant differences. The increase in density and hardness of pellets was also the resultant of the share of sewage sludge (from 20 to 80%). The temperature during pelletization should not exceed 110 °C, and the content of moisture in the input material should not exceed 15%. Higher temperatures and humidity reduce the density of pellets. If granulates/pellets based on limed sludge and other selected solid waste are to be used for co-combustion with biomass in heating furnaces, it is necessary to prepare a production technology that will make them easy to produce repeatedly and make them profitable to producers, easy to transport, easy to use precisely, effective during biomass combustion and safe. They should also generate ash that is easy to remove from furnaces and environmentally safe. García-Maraver et al. [
29] noticed that the process of the preparation of heating pellets and their physical, physicochemical and chemical properties had a significant impact on the combustion process and emission parameters. Lehtikangas [
30] stated that it was necessary to use physically stable materials in the combustion process due to the possibility of disturbing the operation of automatic fuel feeders and advanced systems of automatic furnace control by dust. Moreover, this author noted that the temperature in the furnace could increase significantly (due to the combustion of dusts), which would lead to the melting of combustion residues. According to Sarenbo and Claesson [
31], the production of granulate has to be effective, the granulate binder must not adversely affect the properties of the whole aggregate, and the final chemical composition and stability of the aggregate has to be consistent with the recommendations for a particular type of use for the material. Thus, the problem arises of developing a technology that will make it possible to produce granulates/pellets with the desired characteristics and properties. During previous research projects dealing with the granulation/pelletization of materials, the problem of the durability of products and their homogeneity was noted.
Emissions from the combustion of carbonaceous materials may be an obstacle to a wider use of biomass for energy production [
7]. In this respect, attention is paid to the emission of CO
2 to the atmosphere as one of the main gases affecting the greenhouse effect. Nevertheless, in comparison to the combustion of solid fossil fuels, the combustion of biomass leads to a reduction in CO
2 emissions to the atmosphere when co-combustion technology is used [
6,
16]. As far as the so called carbon footprint is concerned, biomass is neutral to carbon circulation in the environment [
10]. Biomass combustion also results in the emission of gases other than CO
2 (including NOx, CO, SO
2, hydrocarbons) as well as dust polluting the atmosphere, especially when combustion does not take place in optimum conditions. It is often mentioned in the literature that the emission of these gases to the atmosphere could be reduced by mixing fuel with lime and lime and dolomite dust, which in terms of chemical properties are mainly calcium compounds: CaCO
3, CaO, Ca(OH)
2 [
7,
31,
32,
33].
The study was carried out in a region with the largest forest cover in Poland—49.3% of the total area and 51.7% of the land area of the region, compared to 30.5% for the whole country. The wood industry plays one of the key roles in its economy—wood acquisition amounts to 3.6 kdam
3 per year [
34]. Data on municipal waste management show that the amount of municipal waste that is combusted is still small, amounting to 19.4% of the total waste stream in the Lubusz region, compared to 24.4% in Poland [
35]. Counting the current energy production from biomass in Poland in relation to the theoretical potential of this source (20–30% of the final energy consumption [
5]), there is still a large reserve for activities intensifying this process. The use of waste from the wood industry as a fuel material allows to reduce the amount of this waste and reduce the energy demand from conventional sources. This approach also reduces the use of natural resources while meeting the energy needs of the population, so socio-environmental systems would be sustainable [
36]. In order to further develop the thermal management of waste biomass, it is necessary to solve problems related to the emission of pollutants into the atmosphere. The aim of this study was to show the possibility of using waste generated in this region in large quantities as a renewable energy source. In many wastewater treatment plants, sewage sludge is still hygienized with lime. Materials prepared in this way are usually used as a soil improver. Therefore, the question was asked whether it could also be used as an improver in the biomass combustion process. For this purpose, it was considered whether it would be possible to use waste from the wood industry and limed municipal sewage sludge to produce solid fuel in the form of pellets with good thermal properties and at the same time ecologically safe. It was hypothesized that the addition of limed sewage sludge to fuel made from biomass would reduce the emission of such gases as acidic anhydrides.
2. Materials and Methods
A number of tests were carried out to find out whether it would be possible to produce durable pellets with good thermal properties from waste biomass. They included:
Evaluation of input waste materials available in the region in large quantities;
Preparation of mixtures of input materials and checking their thermal properties;
Preparation of pellets from selected materials with the best properties in two groups—with lime and without lime;
Testing pellet durability under high humidity conditions;
Burning pellets under controlled conditions;
The input materials for the tests were: straw, wood shavings, wood dust and sewage sludge hygienized with lime;
Straw (S)—raw material obtained from a producer of straw pellets, homogeneous, crushed to a fraction of approx. 2 cm in length and packed in 10-kg bags; 100 kg of material was obtained for the needs of the experiments;
Wood shavings (WS)—raw material obtained from a carpenter’s workshop, material from debarked wood, non-homogeneous fraction from 1.5 to 3.5 cm; the material was not additionally sieved for the tests; 100 kg of material was obtained for the needs of the experiments;
Wood dust (WD)—raw material obtained from a carpenter’s workshop, material from debarked wood, homogeneous fraction, powder; the material was not additionally sieved for the tests; 100 kg of material was obtained for the needs of the experiments;
Sewage sludge (SS)—from the Krosno Water Utility Company Ltd. (Krośnieńskie Przedsiębiorstwo Wodociągowo-Komunalne Sp. z o.o.); municipal sludge treated with lime in an innovative installation for simultaneous hygienization and granulation of sewage sludge, using lime (CaO) in a sediment to a lime ratio of 1:0.9 by weight; the material was pre-screened through a 3-mm mesh sieve; the subscreen fraction was taken for testing; 100 kg of the material was obtained for the needs of the experiments.
For transport to the research place, the materials were packed in plastic bags, which were opened on the spot to avoid the phenomenon of organic material thermal degradation.
Each material was pre-homogenized to obtain a homogeneous mass with average properties. The homogenization process was carried out by hand, using a table for mixing substrates onto which a particular material was poured from the transport packaging. Each material was mixed for about 10 min. At the end of the homogenization process, 102 samples of materials (34 variants, 3 repetitions, 100 g each sample) were taken for laboratory analysis. In each case, the method of average pooled samples was used. The mixed samples consisted of 30 individual samples taken from the entire volume of the material.
The materials were analyzed in laboratory conditions in terms of:
bulk density—by weight in 5 repetitions, from which the mean and standard deviations were calculated;
the calorific value—using a calorimetric bomb Parr 6100, acc. to norm PN-C-04375-2:2013-07, in 3 repetitions, from which the mean and standard deviations were calculated;
the total carbon content—using the Pregla–Dumas method, the samples were combusted in a pure oxygen environment and the resulting exhaust gases were automatically measured using a CHNS/O 2400 Series II PerkinElmer elemental analyzer. The measurements were conducted for weights of 1.5–2.5 mg in three repetitions, from which the mean and standard deviations were calculated;
the content of heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) was measured using the ICP-OES method and a Perkin Elmer ICP-OES Optima 8000 spectrometer, after wet mineralization in concentrated nitric acid in a Perkin Elmer Titan microwave mineralizer, in three repetitions from which the mean and standard deviations were calculated;
the content of K, Na, Ca, Mg and Fe by ICP-OES was measured using a Perkin Elmer ICP-OES Optima 8000 spectrometer, after wet mineralization in concentrated nitric acid in a Perkin Elmer Titan microwave mineralizer, in three repetitions from which the mean and standard deviations were calculated;
pH—potentiometrically in a mixture of solid material (air dry) and water in a proportion of 1:5 using a pH-meter InoLab, with a WTW SenTix 41 glass electrode.
In order to obtain a homogeneous material, the samples were homogenized by grinding.
In laboratory conditions, mixtures of input materials were prepared and further components were weighed as shown in
Table 1. Each mixture was ground to obtain completely equal properties before further analysis.
Each mixture was analyzed in laboratory conditions in terms of:
the calorific value—calorimetrically in three repetitions, from which the mean and standard deviations were calculated;
pH—potentiometrically in a mixture of solid material (air dry) and water in a proportion of 1:5 using a pH-meter InoLab, with a WTW SenTix 41 glass electrode;
electrical conductivity (EC)—conductometrically in a mixture of air-dry solid material and water in a proportion of 1:5; using a Eutech Instruments Cyberscan PC300 and Elmetron CPC-411 devices with an EC-60 conductivity sensor;
the subtotal content of selected components—after the hot dissolution of substrate samples (in a Perkin Elmer MPS microwave oven) in a mixture of hydrochloric and nitric acid (aqua regia), using the ICP-OES method (Perkin Elmer Optima 8000)—ISO 11466 (1995);
the total carbon content—using the Pregla–Dumas method, the samples were combusted in a pure oxygen environment, and the resulting exhaust gases were automatically measured using a CHNS/O 2400 Series II PerkinElmer elemental analyzer. The measurements were conducted for weights of 1.5–2.5 mg in three repetitions, from which the mean and standard deviations were calculated.
Based on the results of these analyses, mixtures expected to bring the best results in terms of energy production were selected. A total of 10 kg of each selected mixture were prepared for pelleting. A pelletizer, ZLSP200, 7.5 kW made by Eko-Pal, with a capacity of approx. 80–100 kg h−1 was used for pelleting. The input materials were mixed in the right proportions and brought to a humidity of 12–13%. After wetting, the mixtures used for pelletization were rested for 24 h. A matrix with a diameter of 6 mm was used in the process of pelletization. The matrix was not lubricated with any substances. During this process, the rotation speed of the pelletizer was not regulated, and only the pressure of the roller on the matrix was corrected for better performance. Each of the mixtures intended for pelletization was passed through the device until pellets of the right quality had been obtained—in some cases, the process was repeated three times. After the pellets were obtained, the output material was rested for 24 h before packing. After each of the individual mixtures was pelletized, oat husks were passed through the pelletizer to clean the sieve before working with the next mixture. Each time, the first batch of pellets obtained from a new mixture was also discarded in order to get rid of the remnants of the oat husks in subsequent completed mixtures.
The pellets were combusted in fully controlled conditions to obtain information about the combustion process and exhaust emissions. The pellets were combusted in a prototype FORST boiler equipped with two burners: 24 and 48 kW (modified Forster Heizkessel, PE40 boiler), adapted for burning biomass, with an automatic pellet feeding and ash removal system. Combustion of individual pellets was carried out after preheating the furnace to its optimal temperature, using a standard willow tree pellet. Each of the test pellets was fed to the boiler for 4 h. The main combustion parameters: incineration temperature 700–800 °C, air flow 20 m
3 h
−1, mass fuel consumed 1,5 kg·h
−1, exhaust gas temperature 141–162 °C, exhaust gas mass flow 30 g·s
−1, O
2 in exhaust gas 11–14%, max. operating temperature 90 °C, max. operating pressure 2.5 bar, pellet calorific value 17.73 MJ·kg
−1, pellet residual moisture 8–9%, and ash production 3.84% of the pellet mass. The biomass boiler was included into the heat production and distribution system at the Laboratory of Thermal Technologies in the Renewable Energy Center (REC). The laboratory research system is equipped with an advanced measuring system based on measuring devices made by E&H. The exhaust duct of the boiler is equipped with a measuring connection for chemical analysis of exhaust gases. Measurement data are registered by the building management system, Building Management System (BMS), using the Wonderware System Platform. Exhaust gas was measured continuously using a TESTO 350 Xl exhaust gas analyzer. Emission measurements were conducted after the combustion conditions were stabilized, i.e., in the so called high-temperature combustion phase [
37].
The following analyses of the pellets were performed in laboratory conditions:
static and dynamic tests of moisture absorption by the pellets;
test of the mechanical strength of the pellets;
pH and EC analyses of water extracts after 48 h of incubation.
The static test of moisture absorption was carried out in 250-ml glass flasks. An amount of 10 g of pellets were poured into a flask and 90 cm3 of distilled water were added. After 48 h of incubation at room temperature, the samples were subjected to gravity dehydration on sterile gauze for 10 min. After that, a visual evaluation of the disintegration was carried out, the pellets were weighed and the leachate was collected to the volumetric flasks, then the mechanical strength of the pellets was examined.
The dynamic moisture absorption test was carried out in 250-cm3 plastic bottles. An amount of 10 g of pellets were poured into the bottles and 90 cm3 of distilled water was added. The samples were shaken on a mechanical stirrer at 60 rpm for 1 h and they were left for 24 h at room temperature. After 24 h, the samples were again subjected to mechanical shaking at 60 rpm for 1 h. The pellets were then subjected to gravity dehydration on sterile gauze for 10 min. Decomposition was assessed visually, the pellets were weighed, and the amount of leachate was measured using the method of quantitative gathering to the volumetric flasks. Then, the mechanical strength of the pellets was examined.
Due to the disintegration of some of the pellets during the static and dynamic tests, mechanical strength was tested only in the samples that did not disintegrate. The study consisted in dropping each pellet from a height of 1.5 m onto a concrete surface and a visual assessment of disintegration.
The pH and electrical conductivity test was carried out in water solutions obtained by gravitational drainage of the pellets, pH was measured potentiometrically (using a WTW Inolab ph level 1 pH meter, with a WTW SenTix 41 glass electrode) in the supernatant of a 1:2.5 dry solid material: water suspension and EC was measured conductometrically (using a Cyberscan PC300 Series conductor) in the water-saturated paste. In the samples that completely disintegrated, pH and electrical conductivity were measured in the suspension and in the leachate obtained by mechanical filtration of the suspension.