Feedstock and Recovery Ash Composition: Values and Variability
Chemical Composition
Physicochemical properties of the feed biomass consumed during the 15 months are summarized in Table 3 and the chemical composition of biomass ash is shown in Fig. 3. Unfortunately, when the samples taken in December (1st year) and August (2nd year) were inspected, they were rotted, so there are no results from these samples. Values obtained show that C, H, N, S, volatile matter, and calorific value obtained in all the tests are quite constant over time (their respectively coefficients of variation (Cv) are less than 10%), while Cl and ash content are more variable, particularly chlorine content which has an extremely high coefficient of variation, 41 %. The average ash content at 550°C (11.11 % wt.) is close to the maximum of the typical range shown in the literature for exhausted olive cake [12], the main component of biomass samples in this plant (≥ 95 %). Mean contents of carbon, hydrogen, nitrogen, sulfur, chlorine and oxygen are within the typical range for orujillo. However, the sample from November (1st year), has an extremely high concentration of chlorine (Table 3) and sodium (Fig. 3), more than expected for orujillo and the other biomass used in the feed mixture [12]. This extremely high Cl and Na content also occurs during December (1st year) and was reflected on the fly ash composition (Fig. 4). The unusual high chloride and sodium content should be related to an orujillo contamination, which could derive from different causes. For example, the infrequent employment of table olives dressed with sodium chloride and, in some cases, with lye, in the olive oil production process. The process to obtain table olives from fresh olives increases the Cl and Na contents. For instance, Na content of olive pulp and peel varies from around 10–40 ppm of fresh olive [27] to 5700–16600 ppm for table olives (value variable depending on origin and treatment) [20]. Spanish standard [28] prevents from the thermal use of fuels derived from table olives. Other eventual source of Na and Cl contamination are the exposition to sea water or road salting [12, 18] or other non-controlled contaminations.
This period of unusual high Cl and Na content (November (1st year) and December (1st year)) caused a huge trouble at the boiler, leading to an uncontrolled superheater deposit growth and to a complete flue gas blockage and plant shutdown in December 1st year. Therefore, it is necessary to include the control of Cl and Na content in the plant feedstock´s quality control. Traditionally, power plants control fuel moisture [26] and, some of them, heating value. However, these measurements cannot alert from high Cl and Na contents.
Table 3
Physicochemical properties of biomass samples (dry basis)
Month
|
Year
|
Proximate analysis
|
Ultimate analysis
|
Heating value
|
Volatile matter
(VM)
|
Ash
(A)
|
C
|
H
|
N
|
S
|
Cl
|
High Heating Value
(HHV)
|
Low Heating Value
(LHV)
|
Weight %
|
MJ/kg
|
November
|
1st
|
70.66
|
8.59
|
49.40
|
6.27
|
1.50
|
0.12
|
0.56
|
20.90
|
19.54
|
December a
|
1st
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
January
|
1st
|
70.64
|
9.10
|
50.39
|
5.81
|
1.56
|
0.11
|
0.24
|
20.17
|
18.91
|
February
|
1st
|
70.49
|
10.10
|
49.55
|
5.51
|
1.64
|
0.10
|
0.26
|
19.50
|
18.31
|
March
|
2nd
|
69.80
|
12.83
|
48.29
|
5.71
|
1.55
|
0.11
|
0.18
|
19.28
|
18.11
|
April
|
2nd
|
69.89
|
11.37
|
48.69
|
5.69
|
1.90
|
0.12
|
0.33
|
19.41
|
18.25
|
May
|
2nd
|
69.70
|
12.44
|
48.03
|
5.68
|
1.59
|
0.13
|
0.25
|
19.35
|
18.20
|
June
|
2nd
|
69.46
|
12.16
|
48.18
|
5.63
|
1.48
|
0.12
|
0.23
|
19.36
|
18.20
|
July
|
2nd
|
70.08
|
11.27
|
48.94
|
5.73
|
1.61
|
0.13
|
0.23
|
19.70
|
18.52
|
August a
|
2nd
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
September
|
2nd
|
71.22
|
9.93
|
49.00
|
5.95
|
1.62
|
0.13
|
0.20
|
20.28
|
18.99
|
October
|
2nd
|
68.73
|
12.23
|
48.58
|
5.83
|
1.67
|
0.14
|
0.23
|
20.11
|
18.85
|
November
|
2nd
|
68.99
|
12.29
|
48.15
|
5.84
|
1.61
|
0.14
|
0.21
|
19.93
|
18.67
|
December
|
2nd
|
66.37
|
12.15
|
48.99
|
5.55
|
1.67
|
0.13
|
0.17
|
19.90
|
18.70
|
January
|
2nd
|
69.73
|
9.91
|
48.39
|
5.82
|
1.48
|
0.13
|
0.16
|
19.62
|
18.36
|
Minimum
|
|
66.37
|
8.59
|
48.03
|
5.51
|
1.48
|
0.10
|
0.16
|
19.28
|
18.11
|
Mean
|
|
69.67
|
11.11
|
48.81
|
5.77
|
1.61
|
0.12
|
0.25
|
19.81
|
18.58
|
Maximum
|
|
71.22
|
12.83
|
50.39
|
6.27
|
1.90
|
0.14
|
0.56
|
20.90
|
19.54
|
Average deviation
|
|
0.79
|
1.22
|
0.52
|
0.14
|
0.07
|
0.01
|
0.06
|
0.37
|
0.33
|
Sample standard deviation
|
1.21
|
1.41
|
0.67
|
0.19
|
0.11
|
0.01
|
0.10
|
0.47
|
0.41
|
Coefficient of variation Cv(%)
|
1.7
|
12.7
|
1.4
|
3.4
|
6.8
|
8.8
|
41.4
|
2.4
|
2.2
|
Typical values for exhausted olive cake (EN ISO 17225-1, 2014 [12])
|
-
|
3.4–11.3
|
48–52
|
4.6–6.3
|
1.4–2.7
|
0–0.5
|
0.1–0.4
|
18.1–21.6
|
13.9–19.2
|
a Corrupted and not analyzed |
For fly ash the coefficient of variation of the different compounds is around 20–35 % except for potassium (K2O), that remains almost constant (coefficient of variation 5.8 %), sulfur and phosphorous (8.8 and 9.7 % respectively), and sodium, with an extremely high variation (coefficient of variation 118 %). The high sodium’s coefficient of variation is mainly due to the extremely high content in November (1st year) and December (1st year), that constitute an unusual composition with respect to the results from all other months (as shown for feedstock samples). Excluding these months, all coefficients of variation decrease, but this decrease is considerably higher for sodium (which drops to 57 %, half of the initial one) and chlorine (which drops to 6.2 %, being 22.6 % the initial one). Differentiation from November and December (1st year) with respect other months is confirmed by the statistical analysis carried out using clustering techniques. As can be seen in Fig. 6, the smallest and most homogeneous group, when analyzing simultaneously all chemical compounds, was observed for November (1st year) and December (1st year) samples. Furthermore, the separation between groups shows a high linkage distance between the November-December 1st group from the group formed by the other samples. This linkage distance is similar to that observed by Zajᶏc et al. [29] between different kinds of biomass. Therefore, separation between groups seems to confirm the unusual Cl and Na contents at these months. This high Na content, together with the high chlorine content, led to harmful deposit formation, due to the major role that high alkaline and chlorine contents play in the deposition process [1, 5, 30].
For bottom ash the coefficient of variation of the different compounds is around 10–30 %. The least variable oxide content is that of Fe2O3 (coefficient of variation 9.8 %) whereas the most variable one is the Na2O content (coefficient of variation 111.8 %), accordingly with fly ash results. Likewise, the variability decreases when excluding the months with an unusual composition (November (1st year) and December (1st year)).
Table 4 shows the average composition and standard deviation of laboratory, fly and bottom ash considering all samples except November (1st year) and December (1st year) due to their unusual composition. The main component, as commented before, in fly ash is K2O (39.96 % wt.), four times higher than the second one (CaO (10.23 % wt.). Contents in SiO2 (7.31 % wt.), SO3 (5.91 % wt.) and Cl (4.47 % wt.) are also important. For bottom ash, K2O (29.66 % wt.) is also the main component, but SiO2 (19.75 % wt.) and CaO (17.93 % wt.) contents are also high, whereas SO3 (1.41 % wt.) and chlorine (1.28 % wt.) are less abundant.
Table 4
Average composition of laboratory, fly and bottom ash (discarding November and December (1st year) samples due to their unusual composition) (wt. %)
Oxides
|
Laboratory ash
|
Fly ash
|
Bottom ash
|
SiO2
|
12.08 ± 2.69
|
7.31 ± 1.72
|
19.75 ± 2.43
|
Al2O3
|
2.93 ± 0.65
|
1.68 ± 0.41
|
3.67 ± 0.62
|
Fe2O3
|
1.60 ± 0.29
|
1.46 ± 0.27
|
2.34 ± 0.25
|
MgO
|
4.40 ± 0.88
|
2.05 ± 0.35
|
3.92 ± 0.43
|
CaO
|
12.78 ± 2.31
|
10.23 ± 1.75
|
17.93 ± 2.84
|
Na2O
|
0.34 ± 0.22
|
0.33 ± 0.23
|
0.43 ± 0.32
|
K2O
|
32.32 ± 1.68
|
39.96 ± 1.70
|
29.66 ± 3.83
|
P2O5
|
4.30 ± 0.40
|
2.63 ± 0.22
|
4.36 ± 0.45
|
SO3
|
2.66 ± 0.28
|
5.91 ± 0.43
|
1.41 ± 0.45
|
Cl
|
2.29 ± 0.23
|
4.47 ± 0.30
|
1.28 ± 0.60
|
According to their composition, all ash samples are classified as low acid “K” type following the chemical classification system of inorganic matter in biomass and biomass ash proposed by Vassilev et al. [18] (Fig. 7). This classification is also use by Eliche-Quesada and Leite-Costa [25] for the same kind of ash, although in their article they misclassify their sample as “C” type. Categorization of ash type is useful when looking for possible applications. These applications are discussed in the last section (Ash Applications).
Results from Table 4 are graphically presented in Fig. 8 to show more clearly how the orujillo ash forming matter (denoted as laboratory ash) is distributed among the ash fractions (fly and bottom). As shown, the enrichment in volatile elements (K, S and Cl, expressed as oxides in the figure) in fly ash and the enrichment in Si, Al, Fe, Ca and P in bottom ash with respect to the laboratory composition can be clearly observed. The higher K, S and Cl contents in fly ash are due to the vaporization of these elements, their reactions, and the subsequent condensation of the formed species containing these elements, on already existing ash particles or nucleation of new particles. These results are consistent with other combustion biomass studies [1–7]. Nevertheless, the fly ash from orujillo shows higher potassium content than fly ash from straw (biomass also with high K and Cl contents) as shown in other studies, despite different analysis techniques [1]. This could be explained not only by the initial high potassium content of orujillo but also by the better release of potassium, at pulverized combustion conditions, for olive residue than for straw, as shown by Shah et al. [31].
Phase Composition
The study of phase minerals identified by XRD shows some general conclusions. A broad halo can be observed from the XRD patterns, which is associated with presence of a large amount of amorphous materials. There are some phases which have been identified in all months, while other phases could only be identified in certain months. Table 5 summarizes the identified crystalline phases for each kind of ash, and distinguishes those which have been identified for all samples from those which have been identified in most samples but not all. As a representative example, diffractograms showing XRD results are shown in Figs. 9 to 11, and the peaks of the identified crystalline phases are marked with the acronyms shown in Table 5. As an example and for a better visual comparison, the diffractograms of all kinds of ash for a particular month and, their water insoluble fraction, are shown in a single figure (Fig. 12). In these figures (from 9 to 12), the halo associated with amorphous material can be observed, and it is remarkable at the moving range of 25–35 (2Θ for Cu Kα1 radiation). The relative importance of the different identified phases clearly changes along the different months. For example, at laboratory ash (Fig. 9), Fairchildite is the peak of maximum intensity at June 2nd whereas Quartz is the most intense peak at November 2nd. As shown in Table 5, most of the phases identified are common in the different kinds of ash, although with different relative intensity. Sulphates and chlorides are more intense in laboratory ash and, especially, in fly ash, while silicates and phosphates are intense in bottom ash (Figs. 9 to 11). Carbonates are also identified in all kinds of ash. These results are consistent with their classification as “K type” ash, and with the results from XRF (for example, higher S and Cl content in fly ash).
Laboratory ash is useful as a reference to compare with industrial ash. However, the high temperature and short residence time of biomass in a suspension-fired boiler affect the amount of volatile matter present in the bottom and fly ash (some of this volatile matter, such as alkaline salts, governing fouling) and the crystalline phase transformations. Although the analysis of laboratory ash is useful to prevent from possible problems, it does not provide information about possible deposit locations (grate, superheaters, etc.), nor about the components causing these problems.
The main carbonates found are potassium and calcium carbonates (Table 5). Calcite has been identified in all laboratory samples, although it is not always marked in Fig. 9 for clarity. The same applies to most of the bottom and fly ash samples. Fairchildite is specific of laboratory ash and huntite appears in both fly and bottom ash, especially in the latter. In bottom ash, the carbonates of calcium and magnesium are more significant than the potassium ones.
Quartz (SiO2) has been identified in all ash samples (Table 5). Other silicates, such as potassium aluminosilicates (KAlSiO4) are found in all bottom and fly ash. However, in fly ash, this phase is masked by a high water-soluble peak. Although under more aggressive conditions it could be solubilized, in this case it is better identified in the insoluble fraction (Fig. 12 peak at 2 θ = 28.50º). A specific silicate, talc, is only identified in the insoluble fraction of the laboratory ash (Fig. 12 peak at 2 θ = 28.60º and 2 θ = 9.47º, the last not shown), although it cannot be observed in the original ash. Talc is not a usual phase in biomass ash, but it is a common additive used to improve oil extraction which finally ends up at the olive cake [32]. It is usually added in a range of 0.5 % to 2 % of the olive weight.
Phosphates in the form of apatites (chloro- and hydroxyl-) are the most repeatedly found. They are present in fly and bottom ash (Figs. 10–12, where the range of most intense peaks is 2 θ = 31º – 34º), but their importance is higher in bottom ash, result which agrees with a higher content of phosphorous and calcium (Table 4). On the contrary, apatites do not appear in the laboratory ash (only soluble compounds appear within the range of most intense peaks for apatites), which suggest that they are not in the original biomass. According to these results apatites are supposed to be secondary phases, formed during the combustion process. This agrees with Vassilev et al. [33] who stated that the dominant origin of phosphates is secondary, as a result of the interaction of oxides, such as those of Ca, Fe, K, Mg, with phosphorous from the organic matter.
The presence of sylvite (KCl) in the orujillo ash is clear. It is always present on laboratory and fly ash (Table 5) and is the most intense phase in fly ash. XRD profile of November 1st laboratory and fly ash show a perfect pattern of sylvite (Fig. 9 and Fig. 10). Furthermore, its reduction (or disappearance) in the diffraction intensities in bottom ash can be explained with the volatilization and silication of KCl, [34, 35] which for some samples is not complete and still appears in the bottom ash. On the other hand, KCl present in fly ash is mostly attributed to condensation of gaseous KCl (derived from K and Cl volatilization and recombination) over existing ash. KCl plays an essential role on deposit formation because it promotes the formation of the first deposit layer and acts as glue [36] for the impacting ash particles, facilitating deposit growth. The role of sylvite on deposits formed at this plant was widely studied in a previous work [21] and the presence of this phase was evident (it was the most intense phase in all deposits, and specially at the inner layers).
Arcanite (K2SO4) follows similar trend as KCl. It has been identified in laboratory and fly ash but not in bottom ash. Similarly to KCl, the origin of potassium sulfate in fly ash is mostly attributed to the condensation of this compound on the existing particles in the flue gas. At an initial stage, when exiting the combustion chamber, these particles have quite similar structural characteristics to those ending as bottom ash. This initial similarity is, in some way, confirmed by the resemblance between the insoluble fractions of fly and bottom ash (Fig. 12). Later, some reactions take place, like carbonatation (in both types of ash) and condensation of volatile and soluble chlorides and sulfates (on fly ash). These are clearly identified in the original fly ash (Fig. 12) at 2 θ: 28.30º and 40.45º for KCl and at 2 θ: 29.70º and 30.80º for K2SO4.
Although the presence of NaCl has been observed on ash samples for November and December 1st, the most intense phase is still KCl in these samples. Chlorine has been widely considered as a promoter of alkalis release [31, 34]. Therefore, higher Cl content usually leads to higher alkali release, which is able to react with chlorine at the gaseous phase. Once at the gaseous phase, potassium remains in higher proportion than sodium, favoring the formation of KCl. Figure 13 presents the height of the highest peak associated with KCl along months for fly and laboratory ash samples. Highest peaks are associated with the abnormal months November and December 1st. The importance of KCl during the abnormal months on deposit formation has already been discussed above. However, the higher Na content on fly ash probably contributed as well, not only through the NaCl formation, but also causing the molten state of some structures on fly ash particles, which increase the capturing efficiency and the tenacity of the deposits, as have been probed in the previous study at this plant [21].
Table 5
Crystalline phases identified. (●) Identified in all ash samples (○) Identified in most of ash samples but not all
Crystalline phase
|
Ash
|
Phase name
|
Acronym
|
Chemical Formula
|
Laboratory
|
Fly
|
Bottom
|
Chlorides
|
Sylvite
|
Sy
|
KCl
|
●
|
●
|
○
|
Halite
|
Ha
|
NaCl
|
○(a)
|
○(a)
|
|
Silicates
|
Potassium aluminium silicate
|
Ks
|
KAlSiO4
|
|
●
|
●
|
Potassium magnesium silicate
|
KMg
|
K2MgSiO4
|
|
|
○
|
Talc
|
T
|
Mg3Si4O10(OH)2
|
●
|
|
|
Quartz
|
Q
|
SiO2
|
●
|
●
|
●
|
Carbonates & bicarbonates
|
Potassium carbonate
|
Kc
|
K2CO3
|
○
|
○
|
|
Potassium carbonate hydrated
|
Kh
|
K2CO3 (H2O)1.5
|
○
|
○
|
|
Kalicinite
|
K
|
KHCO3
|
○
|
|
○
|
Fairchildite
|
F
|
K2Ca(CO3)2
|
○(b)
|
|
|
Huntite
|
H
|
CaMg3(CO3)4
|
|
○
|
○
|
Calcite
|
Cc
|
CaCO3
|
●
|
○
|
○
|
Phosphates
|
Chloroapatite
|
ApCl
|
Ca5(PO4)3Cl0.9F0.1
Ca5(PO4)3Cl
|
|
●
|
○
|
Hydroxylapatite
|
ApH
|
Ca10(PO4)6(OH)2
|
|
●
|
○(c)
|
Potassium phospate
|
P
|
K4P2O7
|
|
|
○
|
Sulphates
|
Arcanite
|
Arc
|
K2SO4
|
●
|
●
|
|
Notes:
(a) Halite is identified in November and December 1st
(b) Fairchildite is identified in all laboratory ash samples except November 1st
(c) Hydroxylapatite is identified in all bottom ash samples except December 1st and April 2nd
|
Seasonal Study and Elements Correlations
Chemical composition of fly ash, collected along 23 months, was analyzed with the aim to check any seasonal trend. A possible relationship between the weather winter conditions and the high content on Na and Cl observed during the winter of the first year was suspected, and contamination with road salt treatment was presumed to be the cause. In addition, NaCl was detected with XRD. However, as seen in Fig. 14, the episode of high Cl and Na content did not happen during the second winter despite similar weather conditions. Therefore, the road salt treatment should not be assumed to explain the anomalously high Cl and Na contents. Furthermore, the seasonal evolution of the rest of elements was also studied. Figure 15 shows the evolution of Fe, Si and Al, as an example. As can be observed, no seasonal trend has been found since no composition is repeated between similar months or seasons. However, the seasonal study reveals interesting correlations between elements. As observed in Fig. 14 Cl and Na show similar trends. Likewise, clear relationships between Fe, Si and Al can be observed (Fig. 15). Figure 16 shows the best correlation between elements. Left panels (a) show the correlations considering November (1st year) and December (1st year) and the right panels (b) show the same correlations discarding these months. This distinction was done to show that the correlation between Cl and Na (R2 = 0.98 for laboratory ash and 0.96 for fly ash) is much better when considering November and December 1st year (a). On the contrary, this correlation is very weak (R2 = 0.41 for laboratory ash and 0.64 for fly ash) when these months were not considered (b), meaning that there is no significant correlation between these two elements when the usual untainted feedstocks are used, and thus adding consistency to the attributed contamination with NaCl during the aforementioned months. Nevertheless, the strong relationship between Si, Al and Fe (R2 = 0.8 to 0.9 for laboratory ash) is clear in the two scenarios (including and discarding November and December 1st year). As an indicative value, the average Al content on biomass basis for the samples measured in this study is 1754 ppm which is very high for a fruit-based biomass, and could be derived from soil contamination [37]. This hypothesis is consistent with the close relationship between Si, Al and Fe. Soil contamination is commonly derived from storage, handling and transport operations, and evidence of stones in the feedstock at their milling pretreatment have often been observed. Nevertheless, this probable soil contamination happens every month and has never led to troublesome deposit formation on the superheaters.
Ash Applications
Reuse and recycling of biomass ash as a new resource can enhance the development of a circular and sustainable economy. Knowing the quality of recovery ash, including its variability, is necessary to efficiently look for applications [38]. Therefore, this study provides useful information about the chemical and mineralogical composition and the variability of fly and bottom ash from orujillo. This information is useful for any user who must establish the ash quality control, the possible ash applications, and the ash pretreatment (removal of unburnt carbon, thermal treatment, blending with other kind of ash...) need to adapt it for its final use.
Considering the low acid “K” type classification [18] of the recovery ash obtained from orujillo under suspension boiler combustion, a wide variety of utilization possibilities can be foreseed. Some of these possible uses are: fluxing materials, detergents, activate carbons, recovery of water-soluble major, minor and trace elements, glass and glass ceramics, fertilization, insulators and others [39].
Ash from orujillo has been widely used as fertilizer because of its high potassium content [40]. In fact, fly ash from these kinds of plants have been usually sold to fertilizer companies. Other possible uses for this recovery ash are the use in the manufacture of construction materials. Fly [24] and bottom [25] ash from a twin plant (i.e. similar type of fly and bottom ash characterized here) have been studied as raw material for clay bricks. Bonet-Martínez et al. [24] found that up to 25 wt% of fly ash (similar to that studied here) could be incorporated in clay brick, being an economical and sustainable solution without losing quality and fulfilling current regulations. Moreover, Eliche-Quesada and Leite-Costa [25] concluded that up to 20 wt.% of bottom ash (and 10 % as optimum) could be employed in the brick formulation, fulfilling standard requirements for clay masonry units, and, at the same time, with better thermal insulation properties than bricks made only with clay. Fly ash from combustion of orujillo and olive pruning has recently been studied, with promising results, as an alkali activator for generating geopolymers, due its high potassium content [41]. An important point to be further evaluated is the energy saving that is expected when adding orujillo ash (with high content of the fluxing oxides, e.g. K2O) to the construction materials, as a consequence of the reduction in the firing temperature.
Another interesting application studied for orujillo ash [42–43], as well as of other kind of biomass ashes [44–45], is the use as adsorbent to remove metal ions from aqueous solutions.
It must be noticed that using ash with abnormal composition would require revising the proportions to be employed for typical ash. For example, the contamination shown in this study (Cl and Na) produces ash with an abnormally high content of sodium and KCl which significantly decrease their characteristic melting points [46] and thus, could decrease more than expected the paste sintering point in clay or ceramic bodies. Furthermore, high KCl is expected to cause scumming [47], and an excess of chlorine could cause harmful corrosion on the oven metal parts. Moreover, if used in the production of concrete, an excess in chlorine content could attack the oxide passivation layer of concrete, causing a negative effect on the durability and strength of concrete [48]. All these undesirable effects demonstrate the necessity to control both the fed orujillo and the derived ash, especially the Cl and Na contents, at the power plant and at the facility for ash recycling, respectively.