3.2. Impact of Lignin on the Agglomeration Process
The substitution of coke in the agglomeration charge with biomass has a marked impact on the pelletizing process. There is little room for an extensive pre-treatment of biomass in the agglomeration process. The basic requirement for the use of biomass is to provide the desired grain size composition. It is not possible to carry out treatment (e.g., a reduction) of crude biomass in the same way as the treatment of coke powder because biomass has a soft fibrous texture. The study of biomass disintegration is related to the requirements for the properties of the agglomixture for the combustion process in the sintered layer. In this respect, it is necessary to address the possibilities of adjusting the granulometry of the biomass and the effect on properties such as mainly the burning rate, the change in the permeability of the agglomixture, and the overall thermal effect of sintering. Grain sizes for the substitution of coke with lignin were optimized based on experiments of burning coke and lignin with various grain sizes. It is apparent that the granulometry of mixed fuels (substitution of coke with lignin) was slightly shifted in terms of a drop in the finest fractions with the increasing substitution of coke with lignin, as shown in
Figure 8.
The pre-pelletization process may significantly differ from the standard charge with coke fuel, depending on the pelletizing properties of biomass and its wettability. The different densities of the fuels represent another factor affecting the pelletizing process. The increased substitution of coke with biomass has a more significant effect on the bulk density of a charge as well when there is a requirement for an energy substitution of fuel with different calorific volume, which is generally lower in biomass.
Figure 9 shows the impact of substitution on both the bulk density of the charge and the permeability.
The quality of a pre-pelletized charge and the share of biofuels in it were also reflected in the very course of charge sintering. An area of temperatures at which melt phases are formed can be analyzed from temperature curves. For this purpose, parameters of a so-called liquid phase formation area (LPFA) and duration of temperature for mineral melting (DTMM) were calculated (3) and (4).
where
te—final time for smelting;
ti—beginning time for smelting;
Ti—initial temperature of melting process;
Tt—temperature in enclosed area.
The temperature curves characterizing the course of temperature change in three horizons of the sintered layer showed various intensities and maximums reached for individual cases. The impact of substitution in terms of both the quantity and the type of biomass is evident and corresponds to the present knowledge from studies of biomass combustion in the conditions of the agglomeration layer.
Figure 10 illustrates the course of temperatures without the substitution of coke and with the 20% substitution of coke with lignin. Areas of temperatures over 1050 °C, as indicated in the graphs, represent the areas or temperature fields where the formation of an agglomeration melt is expected (LPFA—liquid phase formation area in [°C·s]) given the present components of the agglomeration charge. These areas were computed by the integration of a curve within the limits of a time interval for the temperatures over 1050 °C. The said duration of this time interval is expressed as a parameter of the DTMM (duration of temperature for mineral melting in [s]). The substitution of lignin for coke has manifested itself in the reduction of the DTMM and the drop in maximum temperatures, and thus in the decrease in the LPFA as well, as shown in
Figure 10b.
The comparison of temperature fields and sintering intensity for various types of biomass (that were analyzed in the previous research papers of the authors) with a constant substitution (20% of coke) is shown in
Figure 11. Sawdust from pine wood (SDPW) and sawdust from oak wood (SDOW) seem to be the least appropriate types of biomass here (among the tested substitute fuels) due to the observed effect of a non-homogeneous combustion with the degradation of temperature field toward the grate and the significant increase in the sintering time caused mainly by the decreased permeability and instability of the charge. The opposite extreme of an intensive course of the thermal change in the agglomeration layer was seen in case of sintering with hemp shives (HS), where the shortest sintering time was measured, while there was an overlapping of thermal waves.
Figure 12 shows the comparison of maximum temperatures in three horizons of the sintered layer and in the flue gas stream, as well as the total sintering time using various substitutions of coke with lignin. The maximum temperatures in the agglomeration layer decreased as the share of the substitute biomass increased, while the sintering time shortened as well. In general, biomass fuels burn more quickly than coke powder due to their high porosity and reaction surface, resulting in an increase in the vertical sintering rate. Lower temperatures in the sintered layer observed with the addition of lignin can also be attributed to the condensation of semi-volatile and volatile organic compounds. These compounds can reduce the heat transfer in the direction of burning, as the case may be.
Agglomerates with minor variations in the chemical and mineralogical composition were formed using lignin with up to 50% substitution of coke [
18]. The content of Fe
TOT in the agglomerates was 53–54 wt% at the basicity of 1.7–1.8. In terms of mineralogical composition, the agglomerates consisted of four major phases: iron oxides (40–60 wt%), ferrites (20–30 wt%, most of which were complex compounds of silicoferrites of calcium and aluminium (SFCA)), glass (<10 wt%), and calcium silicates (<10 wt%).
The performance parameters of sintering were determined based on the course of sintering, which is also confirmed by
Figure 13 below. The substitution of lignin for coke up to 50% was reflected in the increase in production parameters, primarily due to the higher vertical sintering rate. The calculation of the sintering rate takes into account different parameters for various research projects. It has also been determined that the heat wave moves through a bed with approximately constant velocity,
v [
19]. This constant velocity,
v, is expressed as:
where
h is the thickness of the sinter bed, and
ST is the sintering time.
The paper [
20] describes the sintering velocity using the following equation:
where
hg is the heat capacity of the gas;
hs is the heat capacity of the solid;
W is the normal volume of the fluid/unit cross-section of the bed/minutes; and f is the voids fraction/unit volume of the bed.
The sintering rate can be also expressed by the rate of thermal wave propagation (7), which depends on the volumetric flow rate of air sucked through the layer and the actual thermal capacity of gas and solid components. For this reason, the permeability of agglomeration charge is the key to the high sintering rate. A temperature profile, formed throughout the height of the sintered layer, has a substantial effect on both the type of produced agglomerate and its physical and reduction properties:
where
vs—sintering velocity;
wg—volumetric flow rate of gas;
ρg—density of gas;
ρf—density of mixture;
cg—thermal capacity of gas;
cf—thermal capacity of mixture; and
ε—permeability.
The vertical rate was calculated on the basis of (8):
where
Pr—specific production;
kA—conversion coefficient;
ρ—apparent density of mixture;
kPr—production coefficient;
V—sintered volume.
The impact of coke powder substitution with lignin on the granulometric and strength parameters of the agglomerate shows certain differences, as shown in
Figure 14. It is clear based on the analysis of the grain size distribution that better parameters were achieved with up to a 50% substitution of lignin for coke against the standard sintering without the substitution of coke powder. In a series of sinterings with modified lignin pellets, the agglomerate even showed an abrasion that was lower by ca. 1.5% at up to a 20% substitution of coke, which was manifested in the strength of the agglomerate as well. In the case of 50% substitution of coke with lignin, this positive effect persisted, but further increase in the substitution coefficient was already negatively reflected in the strength properties as well. Here, it is necessary to realize the effect of increased substitution on the bulk density and the yield of agglomerates in particular.
Agglomerates produced with the substitution of modified lignin pellets for coke showed a different nature as well, where a more fine-grained and heterogeneous porous structure was observed. The size of pores in the produced agglomerate (and the total agglomerate porosity at the substitution of up to 50%) increased as the substitution level was raised, as shown in
Figure 15.
The rise in porosity is probably associated with the rise in the volumetric proportion of biofuel in the mixture and the kinetics of its combustion, where the increased rate of lignin combustion and its quantity create the preconditions for the increased porosity. It may also positively affect the resulting reducibility of agglomerate with the substitution of modified lignin pellets for coke, since such agglomerates may have higher reducibility as well [
21]. The temperature profiles that became more downward as the substitution of coke increased constituted a relevant factor as well. At 86% substitution of coke, this effect was manifested more significantly along with the structure of original ore grains covered with a small proportion of the melt phase. The predominant part of agglomerate was characterized by the transition of sintered grains to the plastic state, i.e., to the area of softening. It is clear from the results that there was no marked change in the phase composition, i.e., no formation of new phases, at 20% and 50% substitution of agglomeration coke with lignin. The share of individual phases changed significantly. At 20% and 50% substitution, there is a decrease in the majority of oxide phases (magnetite and hematite) and increase in the bonding phases represented by silicoferrites (SFCA-1 and SFCA) and silicates (larnite and hedenbergite) [
18].
The emission profile of sintering with the addition of lignin varied depending on the amount of coke replacement, where a difference in the amount and course of gaseous component formation was observed during the sintering of agglomerate. The use of modified lignin pellets (LIG) for a partial substitution contributed to the increase in carbon oxides, where higher average concentrations were measured and higher specific amounts were calculated against the standard, as shown in
Figure 16a. Carbon dioxide is the main reaction product in the combustion of all carbonaceous fuels, and its emissions are directly proportional to the content of carbon in these fuels. It is released into the atmosphere not only in the reaction of carbon and oxygen during the combustion of different types of biomass but also in the combustion of carbon monoxide or organic matter, e.g., methane. In the sintering using lignin, a higher amount of H
2O (g) should be theoretically present in the gas phase, and due to the lower carbon content in the lignin, the amount of CO
2 (g) should be lower as well. The rise in CO
2 at the substitution of coke with lignin can be explained by the raised amount of carbon in the agglomeration process due to the increase in the total amount of fuel (coke + lignin) for individual substitutions, because lignin has a considerably lower calorific value. In sintering without the coke substitution, 4.35 kg of coke were used. At 20% substitution of coke with lignin, 5.52 kg of fuel (coke + lignin) were used, and 7.34 kg of fuel were used at 50% substitution of coke with lignin. However, the total amount of CO
2 is also affected by other factors—e.g., sintering time, amount of gas phase, temperature in the sintered layer, mechanism of carbon combustion, etc. In the future, the CO
2 emissions in the production using lignin can be reduced by optimizing the conditions of charge preparation and its sintering. However, its overall positive aspect is the reduction of carbon footprint in the agglomeration process as a result of the so-called zero CO
2 balance in the formation and processing of the lignin. The change in NO
X concentration during sintering with the substitution of lignin for coke copies the profile and the course of CO
2 curves, and it is also connected with the change in the concentration of oxygen during sintering. Overall, the substitution of coke with lignin reduces the content of NO
X, as shown in
Figure 16b. As a consequence of the significantly lower content of sulfur in the lignin, lower emissions of SO
x were found in the flue gas when coke had been substituted with lignin.