- Email: [email protected]
Manure-based energy generation and fertiliser production: Determination of calorific value and ash characteristics
b i o s y s t e m s e n g i n e e r i n g 1 1 3 ( 2 0 1 2 ) 1 6 6 e1 7 2
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/issn/15375110
Research Paper
Manure-based energy generation and fertiliser production: Determination of calorific value and ash characteristics Ole Thygesen a,*, Tina Johnsen b a
Institute of Chemical Engineering, Biotechnology and Environmental Technology, Faculty of Engineering, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark b Kommunekemi A/S, Lindholmvej 3 DK-5300 Nyborg, Denmark
article info
The potential for energy generation and phosphorus (P) fertiliser production through the
Article history:
combustion of manure fibre combined with ash recycling was examined. Manure fibre has
Received 15 February 2012
a positive calorific value and may be used as a CO2-neutral fuel for combustion. The ashes
Received in revised form
from combustion are also rich in P. Fibre samples from anaerobically digested manure
27 June 2012
produced by pigs, mink and cattle were collected from 32 slurry separation units around
Accepted 23 July 2012
Denmark, along with data on the separation technology used and the origin of the manure
Published online 14 August 2012
fibre. The mean gross calorific value of the wet manure fibres for the different sources was 5.2 MJ kg1 for anaerobically digested (AD), 5.3 MJ kg1 for pigs and 4.4 MJ kg1 for mink and cattle, with the latter relatively low value being due to high water content, suggesting drying of the manure fibre would be helpful. The mean P concentration in the ashes was 112, 123, 157, and 51 g kg1 for respectively AD, pigs, mink and cattle. Manure fibre ashes derived from cattle slurry contained too little P to be suitable for fertiliser production. Pig slurry, to which sulphuric acid had been added prior to separation, had a low P content in the ash and was not suitable for fertiliser production. The low solubility of P means the ashes should be treated before being used as a fertiliser. ª 2012 IAgrE. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The global energy and climate crises call for the development of new energy sources. At the same time the increased consumption of animal feed products and a growing global population enhances the need for plant nutrients to support an increased plant production. Animal manure is a source of energy and of plant nutrients (Otero, Sanchez, Go´mez, & Mora´n, 2010; Petersen et al., 2007). This study focuses on the use of manure fibre (the dry-matter-rich fraction obtained
from animal slurry separation) as a fuel in combination with production of an ash-based P fertiliser. To fight global warming the Danish government intends to phase out the use of fossil fuels. Their vision is that 25e65% of the energy consumption by 2050 should be provided by biomass, to which animal manure shall provide a significant fraction, primarily through anaerobic digestion plants (Green energy, 2010). Currently 90,000 t of manure fibre is produced per year from a separation of approximately 3% of the annual Danish animal slurry production (Birkmose & Thygesen,
* Corresponding author. Tel.: þ45 21356037. E-mail address: [email protected] (O. Thygesen). 1537-5110/$ e see front matter ª 2012 IAgrE. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biosystemseng.2012.07.004
b i o s y s t e m s e n g i n e e r i n g 1 1 3 ( 2 0 1 2 ) 1 6 6 e1 7 2
Nomenclature AD Al Cd Cr Cu DM EU15
Anaerobically digested Aluminium Cadmium Chromium Copper Dry matter The countries in the European Union prior to the accession of ten candidate countries on 1 May 2004
2010). It has been estimated that within a few years incineration of manure fibre could replace 3.6 PJ of coal energy, equivalent to 4.3& of the yearly Danish energy consumption. To achieve this, about one third of the Danish slurry needs to be separated (Statistics Denmark, 2012; Wittrup, 2010). P fertilisation is fundamental for the high crop yields obtained in intensive agriculture. The agricultural consumption of phosphates constitutes approximately 85% of the total yearly consumption of P (Steen, 1998). The P used today almost exclusively comes from the mining of phosphate rock, which is a limited non-renewable resource (Steen, 1998). Estimations of current reserves and future consumption lead to varying conclusions, but it seems evident that P scarcity will become increasingly critical within the next few years (Chesworth, 2008; Steen, 1998; USGS, 2012). Agriculture will therefore become more and more dependent on phosphate rock from Morocco, since it controls 70% of the known reserves (USGS, 2012). With this background P management and recycling has raised both political and technical interest. Application of surplus P is intimately linked to animal production1; manure applied according to EU standards simply contains more P than needed by crops. When separating animal slurry most of the P will end up in the manure fibre (Hjorth, Christensen, Christensen, & Sommer, 2010), thus by separating the manure better management of the plant nutrients in manure can be achieved. Producing a P-fertiliser from the manure fibre ash could facilitate the effective redistribution of slurry P. When incinerating the manure fibre nearly 100% of the P is recovered in the ash. Of this P, 80% is recovered in the bottom ash and 20% in the fly ash (Cohen, 2009; Møller, Jensen, Tobiasen, & Hansen, 2007). Different P concentrations have been recorded in ashes from differently sourced manure fibres, with a 7e15% concentration in ashes from manure fibres coming from pigs, 11% in ashes from manure fibres coming from anaerobically digested (AD) slurry and 3.6% in ashes from manure fibres coming from cattle slurry (Møller et al., 2007; Westborg, Johansen, & Christensen, 2010). Ash with P content below 10e12% is not considered suitable for fertiliser production. Manure fibre ashes therefore appear to be good candidates for such fertiliser production, but the lack of knowledge on ash quality has so far been an obstacle, since the large-scale combustion of manure fibre has not yet been implemented. 1 The surplus application of P to crops in EU15 adds up to 1 million tonnes P per year (OECD, 2004). This amount is equivalent to 5% of the global yearly P consumption.
GCV ICPeMS NCV NCV90 Ni P Pb Zn
167
Gross calorific value Inductively coupled plasma mass spectrometry Net calorific value Net calorific value at 90% DM Nickel Phosphorus Lead Zinc
The aim of the study was to provide technical insight into the quality of manure fibre and fibre ash by determining combustion and fertiliser relevant parameters based on manure fibre produced in Denmark.
2.
Materials and methods
2.1.
Manure fibre sampling
The animal slurry samples used in the study were collected in AprileJune 2010 from 32 commercial separation plants; these plants comprise approximately two-thirds of the separation plants currently operating in Denmark (Birkmose & Thygesen, 2010). Three additional AD slurry samples were included in the study; these were separated in the laboratory using a pressurised sieve to provide three manure fibre samples. These three samples were only evaluated for ash quality, since the fuel properties did not represent those from the commercial separation units. Sample information is given in Table 1, a detailed description of the different separation technologies can be found in Hjorth et al. (2010). The separated dry-matter-rich fractions were sampled at the outlet of the separator by collecting five subsamples, giving a total volume of 8e10 l. At three locations the separator could not be started at the time of sampling. At these sites the manure fibre samples were taken from five different sites of the storage heaps 10e20 cm below the surface, giving a total volume of 8e10 l. The samples were stored in airtight plastic containers during transportation to the laboratory. All samples were homogenised and divided into three subsamples immediately after arrival at the laboratory. One subsample of 1 l was stored at 18 C for later analysis, and one subsample of 1 l was stored at 6 C for analysis within a week of arriving at the laboratory. The remainder of the sample was dried and incinerated.
2.2.
Characterisation of the manure fibre samples
All measurements were carried out with samples at room temperature, i.e. 20 Ce22 C. The dry matter (DM) concentration and ash content were determined according to DS/EN 14346 (2007) and DS/EN 12879 (2001), respectively. Measurements were carried out in triplicate. Gross calorific value (GCV) was determined using a Parr 6300 calorimeter (Parr instrument company, IL, USA) in accordance with DIN 51900 (1989). Single measurements were carried out.
168
b i o s y s t e m s e n g i n e e r i n g 1 1 3 ( 2 0 1 2 ) 1 6 6 e1 7 2
Table 1 e Origin and treatment of the samples of manure fibre from livestock farms and biogas plants. Sample ID
Typea
1
AD
None
DC
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 17a 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
AD AD AD AD AD AD AD Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Mink Mink Mink Cattle Cattle Cattle
None None None None None None None None None None None None None None None None None None None None None None None 3 days None None None None None None 2 weeks None None None None
DC DC DC SP SI SI SI SI, SP, PO SI, SP, PO SI, SP, PO SI, SP, PO SI, SP, PO SI, SP, PO SI, SP SP SP SP, þ SF, SP, A SF, SP SP SP DC DC DC SI, SP, PO SF SI, SP, PO SI, SP, PO SI, SP, PO RD RD RD SP SP DC
Storage time
Type of separationa
Origin of the slurry treated 80% animal slurry (75% pig, 20% cattle and 5% mink) and 20% industrial wastes 15% fish slime and 85% pig slurry primarily pig slurry Sow, piglet and finishing slurry 50% pig and 50% cattle slurry 5% industrial waste and 95% cattle slurry Pig slurry Animal slurry and industrial waste Sow, piglet and finishing slurry Sow and finishing slurry Sow slurry Sow, piglet and finishing slurry Sow and piglet slurry Sow slurry Sow, piglet and finishing slurry Sow, piglet and finishing slurry Finishing slurry Finishing slurry Sow, piglet and finishing slurry Piglet and finishing slurry Sow slurry Sow and finishing slurry Sow slurry Sow slurry Sow slurry Finishing slurry Sow slurry Sow, piglet and finishing slurry Sow and piglet slurry Sow, piglet and finishing slurry Mink slurry Mink slurry Mink slurry Cattle slurry Cattle slurry Cattle slurry
a DC ¼ decanter centrifuge, þ ¼ pressed extra hard, A ¼ added sulphuric acid, RD ¼ rotating filter drum, SF ¼ shaking filter, SI ¼ sieve, SP ¼ screw press and PO ¼ polyacrylamide polymers added.
The net calorific value (NCV) of untreated samples and samples dried to 90% DM (NCV90) was calculated from the measured GCVs according to Dahlin (1985). In the calculations the hydrogen content was estimated to be 6% of the dry matter content in accordance with the findings of Westborg et al. (2010).
2.3.
Ash production and characterization
Ashes were produced by incinerating dried fibre samples at 850 C for 1 h in a laboratory oven. A temperature of 850 C was chosen to resemble the temperature used in waste incineration plants (Johnke, 1999, pp. 455e468). For the phosphate solubility tests the samples were prepared according to EN1482-2 (2007), the extractions in water and 2% citric acid were carried out in accordance with DS/CEN/TS 15958 (2010) and DS/CEN/TS 15920 (2009), respectively. The orthophosphate content of the filtrate was determined according to EN 1189 (1997) using Spectroquant phosphate tests from Merck (The Merck Group, Darmstadt, Germany). Measurements were carried out in duplicate.
Analyses of the fibre ash content of total P, aluminium and the heavy metals Zn, Cu, Cr, Ni, Pb and Cd were carried out using Inductively coupled plasma mass spectrometry (ICPeMS) (Thermo Scientific, x-series, Waltham, MA, USA) after acidic digestion of the fibre ash sample using nitric acid DS 259 (2003). Measurements were carried out in duplicate. For the heavy metals Cr, Ni and Pb, the detection value was 10 mg kg1 ash. For Cd, the detection value was 0.8 mg kg1.
2.4.
Statistical methods
Statistical analysis was carried out using Excel 2007 (Microsoft Corporation, Redmond, WA, USA). The analysis included a pair-wise F-test to assess when variance homogeneity could be assumed. ANOVA was applied in the cases where variance homogeneity was proven and pair-wise t-testing was used to test data without variance homogeneity. In some cases, a onesided test was used to compensate for a low degree of freedom, either due to a low number of samples or to the lack
169
b i o s y s t e m s e n g i n e e r i n g 1 1 3 ( 2 0 1 2 ) 1 6 6 e1 7 2
of variance homogeneity or both (Montgomery & Runger, 2003).
3.
Results and discussion
Due to the relatively low numbers of separation plants currently operating in Denmark, some groups of manure fibre contained few samples i.e. mink and cattle manure fibre. However, despite the low numbers of samples the groups still showed distinct differences from one another justifying the use of the small groups. The group of AD manure fibre samples was very heterogeneous, some samples came exclusively from pig slurry while others were produced from mainly cattle slurry, therefore the AD manure fibre group will not only be treated as one group, but the differences within the group will also be discussed. Regarding ash quality, a very simple set up was used to produce the ash. The ashes produced had a light grey or khaki colour, indicating a low content of unburned carbon. The content of chlorides in ash is highly dependent on combustion parameters; analysis of chloride content was therefore omitted.
3.1.
DM and ash content
Mink fibre had the highest DM content, due to the presence of bone residues in the diet, which have low water retention capacities. The AD and the pig fibre had similar dry matter concentrations. Cattle fibre had highest water content, reflecting the less efficient dewatering of cattle slurry than of pig and AD slurry (Hjorth et al., 2010) see Table 2. The ash content could after statistical analysis be ranked as follows: mink > AD > pig ¼ cattle. The ash content from mink is the highest, due to the high content of bone residues. The AD fibre samples have higher ash contents than pig and cattle fibre samples, since some of the organic matter is transformed to biogas during anaerobic digestion.
3.2.
Calorific values
Three calorific values were considered in the following: GCV and NCV of manure fibre samples as received and NCV90 (see Table 2).
Fig. 1 e The relative frequencies of each manure fibre group in the GCV intervals (as received).
The GCV of wet manure fibre varied between 3 and 8 MJ kg1 (Fig. 1), typically ranging between 4 and 6 MJ kg1. The GCV of cattle fibre was significantly lower than the GCV of pig fibre, but otherwise differences were not statistically significant. The GCV is related to the content of organic matter in the manure fibre and is low in mink dry-matter-rich fractions due to the high ash content. Cattle fibre had high water content and consequently a low content of organic matter, leading to low GCVs. The NCV is a measure of the maximum energy gain that can be obtained by combustion of the wet fibre without condensation. As can be seen in Table 2, the NCV varied between 1.5 and 5.4 MJ kg1 with mean values ranging from 2.4 MJ kg1 (cattle) to 3.4 MJ kg1 (AD and pigs). This is in line with the findings of Westborg et al. (2010). The distribution of NCV90s of mink, pig and AD slurry is shown in Fig. 2. The dried cattle fibre has the highest calorific value (17.8 MJ kg1), which may reflect a high straw content. Dried mink fibre had a low calorific value (11.0 MJ kg1) due to its high ash content. The NCV90 of AD fibre was not significantly different from pig fibre, despite AD having a 40% higher ash content. This may be due to the higher contents of lignin and neutral detergent soluble substances found in AD fibre
Table 2 e DM, ash content and calorific values of fibre samples. Standard error given in brackets. Minemax [ the lowest and highest measured value. Means followed by the same letter indicate the means are not significantly different from each other ( p < 0.05).
DM Minemax Ash Minemax GCV Minemax NCV Minemax NCV90 Minemax
g kg1 g kg1 g kg1 DM g kg1 DM MJ kg1 MJ kg1 MJ kg1 MJ kg1 MJ kg1 MJ kg1
AD
Pig
Mink
Cattle
28.3a (1.8) 21.5e31.8 22.6 (2.3) 15.6e26.8 5.2ab (0.42) 3.8e6.1 3.4a (0.58) 1.9e4.3 15.5ab (0.5) 13.9e16.8
28.1a (1.5) 12.3e47.1 16.1a (1.4) 6.3e40.2 5.3a (0.19) 3.7e7.2 3.4a (0.57) 1.5e5.4 16.3a (0.8) 10.5e20.6
34.5 (4.1) 26.2e39.4 43.4 (5.8) 35.3e54.7 4.4ab (0.66) 3.4e5.7 2.8ab (0.67) 1.9e4.1 11.0b (2.0) 7.1e13.4
20.6 (0.41) 19.8e21.2 12.6a (1.0) 10.6e14.0 4.4b (0.04) 4.3e4.4 2.4b (0.04) 2.3e2.5 17.8 (0.3) 17.4e18.5
170
b i o s y s t e m s e n g i n e e r i n g 1 1 3 ( 2 0 1 2 ) 1 6 6 e1 7 2
Fig. 3 e The relative frequencies of each manure fibre ash in the water solubility intervals. Fig. 2 e The relative frequencies of each manure fibre group in the NCV90 intervals.
(Jørgensen & Jensen, 2009). Lignin and various detergentsoluble lipids are more energy-rich than the cellulose and hemicellulose making up the rest of the organic carbon in manure fibre.
would transfer more P to the liquid fraction during the separation (Stumm & Morgan, 1996, Chapter 7). AD, pig and mink fibre ashes all contain similar amount of P to mined phosphate rock; the cattle fibre ash is so low in P that recovery is probably not be economically viable (Chesworth, 2008).
3.3.1. 3.3.
Phosphorus in ash
The general picture regarding P content in the manure fibre ashes is that cattle fibre ashes contained the lowest amount of P, AD fibre ashes had intermediate P contents similar to pig fibre ashes, and mink fibre ashes had high P contents (see Table 3). Two ash samples from AD manure fibre produced ashes with P concentrations below the 10%, which we set as a minimum requirement for the ash that could be used for P fertiliser production. The two plants were no. 1 with 86.4 g kg1 and no. 6 with 82.3 g kg1. From the feed specification in Table 1 it can be seen that these plants had the lowest percentage of P-rich slurries (pig and mink) in their input, explaining the lower P concentration in the ash. The sample of pig manure fibre from plant no. 18, which was the only sample included in the study that had sulphuric acid added to the slurry prior to separation, had a low P content in the ash and was unsuitable for fertiliser production; the lower P concentration could be due to a higher solubility of P compounds in the acidified slurry due to the lower pH, which
Solubility in water and 2% citric acid
The solubility in water of P from ashes is typically below 1% (see Fig. 3), and the solubility in 2% citric acid ranged between 4 and 7% (see Fig. 4). The low solubility in these two solvents illustrates the need for processing the ash before it can be applied as an effective fertiliser; the processing could include treatment with acid. A lower solubility of P in water than in 2% citric acid was expected (Rubæk, Stoholm, & Sørensen, 2006). This was in fact the case for all samples but one. The unusual sample was from cattle slurry from plant no. 34; this sample was equally soluble in both solvents. No single explanatory factor from an examination of the major cation content (data not shown) and data on the origin of the fibre sample could account for its high solubility in water.
3.4.
Heavy metals and aluminium in ash samples
Danish legislation (Executive Order no. 1650, 2006) sets the threshold values for heavy metals in waste used for agricultural application; threshold values for the heavy metals found in the highest concentrations are presented in Table 4 along with means of the measured heavy metal contents. Ash
Table 3 e Concentration of P in fibre ash, solubility of P in water and 2% citric acid. Standard error given in brackets. Minemax [ the lowest and highest measured value. Means followed by the same letter indicate the means are not significantly different from each other ( p < 0.05).
Total P Minemax P solubility in H2O Minemax P solubility in 2% citric acid Minemax
g kg1 g kg1 % % % %
AD
Pig
Mink
Cattle
112a (7.5) 82e143 0.43 (0.24) 0.004e1.96 5.49 (0.31) 4.29e7.39
123a (3.1) 86e149 0.16 (0.031) 0.003e0.688 5.10 (0.19) 3.74e7.27
157 (8.5) 147e188 0.11(0.035) 0.019e0.240 4.53 (0.19) 4.20e5.06
51 (10.0) 34e69 1.55 (1.35) 0.040e4.24 5.00 (0.71) 4.25e6.41
171
b i o s y s t e m s e n g i n e e r i n g 1 1 3 ( 2 0 1 2 ) 1 6 6 e1 7 2
slurry; the aluminium could originate from the fish mucus that has been reported to possess a strong affinity for aluminium (Ballance, Sheehan, Tkachenko, McCrohan, & White, 2002).
3.4.1. Problematic levels of copper and zinc in pig fibre ashes is associated with the use of polymers
Fig. 4 e The relative frequencies of each manure fibre ash in the citric acid solubility intervals.
samples from mink and cattle fibre ash did not exceed any threshold values. The ash derived from AD only exceeded the threshold values in the ash from plant no. 7, which exclusively uses pig manure. For pig manure the thresholds were exceeded in several ashes. Copper and zinc were found in all ash samples. Chromium and nickel were found in most samples. The fibre ash from plant no. 12 contained copper, zinc and chromium all exceeding the threshold values, but threshold values for chromium and nickel were not exceeded when applying the phosphate based threshold value for nickel given in the Danish legislation (Executive Order no. 1650, 2006). In all ash samples lead and cadmium concentrations were below the detection levels and below the threshold values which were 120 mg kg1 for lead and 0.8 mg kg1 for cadmium; this is probably due to the volatile nature of these two heavy metals. This indicates that bottom ash from combustion in the normal temperature range (850 C and above) will not contain problematic concentrations of cadmium and lead, but this may depend on the design and performance of the actual combustion plant. Fly ashes are known to accumulate cadmium, so special attention to cadmium should be given in the case of fly ash (Hansen, Pedersen, Ottosen, & Willumsen, 2001). Aluminium is unwanted in fertilisers as high concentrations of Al3þ are toxic to plants and can significantly reduce crop growth. Al3þ for example inhibits root growth and disrupts membranes and ATP-metabolism (Lack & Evans, 2001). In general, manure fibre ashes have low aluminium contents (Table 4), but biogas plant no. 2 stands out with its sample showing high aluminium content (106 mg g1). This plant adds 15% fish mucus from the nearby fishing industry to the pig
Copper and zinc salts are added to pig feeds due to their positive effect on productivity. Therefore copper and zinc are found in relatively high concentrations in manure fibre ashes. Many of the ashes from pig fibre exceeded the threshold values. The average concentrations for copper and zinc in 11 dry fodders for finishing pigs and sows used by the farmers in this study were 16.4 and 107 mg kg1, respectively, and 150 and 119 mg kg1, respectively, for piglets. This study found no relationship between the copper and zinc contents in feed and in fibre ashes from piglets and finishing pigs. As described by Møller et al. (2007), the addition of polyacrylamide polymers to the separation process leads to higher contents of the heavy metals. All nine pig fibre ashes produced using polymers during separation gave ash samples exceeding the copper threshold value defined in Danish legislation (Executive Order no. 1650, 2006) and seven exceeding the threshold value for zinc. In contrast, only two out of 12 pig fibre ashes produced without polymers exceeded the threshold values of copper or zinc. All farmers used similar polymers, the amount of polymers used by the farmers varied from 0.07 to 0.38 l m-3 slurry, but there seemed to be no correlation between the consumption of polymers and the concentration of zinc and copper in the ashes that derived from separation plants using polymers. This might be due to variations in the slurry. As the P content in the ash is not affected by the addition of polymers during slurry separation, the better ash quality, in terms of heavy metals and P content, is attained by not adding polymers to the animal slurry during separation.
4.
Conclusions
The water solubility of P in the ashes was generally low with an average solubility percentage of 0.31. The solubility of P in 2% citric acid was somewhat higher with an average of 5.1%, but still too low to be an effective P fertiliser. The low solubility means the ashes should be treated before being used as a fertiliser. Cattle manure fibre ash contained too little P to qualify as a P fertiliser candidate with a P content of only 5%. Mink fibre ash had the highest P content at an average 16% and AD and pigs had intermediate P contents of 11 and 12%, respectively.
Table 4 e Content of aluminium and heavy metals in fibre ash samples determined using ICPeMS. Standard error given in brackets. AD Al Zn Cu Cr Ni/P
g kg1 mg kg1 mg kg1 mg kg1 mg kg1
16.8 2632.5 785.5 27.5 360.3
(12.7) (254) (179.3) (3.6) (15.6)
a Given in Executive Order no. 1650 (2006).
Pig
Mink
Cattle
Threshold valuea
2.8 (0.20) 4293 (497) 1269 (190) 29 (6.4) 519.6 (66.9)
0.27 (0.07) 624 (43.4) 44 (4.3) e 73.9
3.4 (0.46) 959 362 19 415.7 (42.1)
4000 1000 100 2500
172
b i o s y s t e m s e n g i n e e r i n g 1 1 3 ( 2 0 1 2 ) 1 6 6 e1 7 2
Mink, AD and pig fibre ash are suitable candidates for a P fertiliser production. The aluminium concentration in these ashes was generally acceptable for fertiliser production, but one AD ash, which partly originated from fish waste, had an aluminium content of 11%, making its ash problematic. The content of copper and zinc in pig slurries was high, which could be related to a widespread application of polyacrylamide polymers in the separation process. When polymers were not used, pig fibre ashes only in a few cases contained copper or zinc in concentrations exceeding the threshold values defined in Danish legislation. Manure fibre has a positive calorific value and the wet fibre may be combusted with a support fuel before or after anaerobic digestion to get a net energy gain. Once dried, manure fibre has calorific values comparable to straw, but the energy requirement for drying reduces the net energy gain. The energy gain is therefore smaller compared to straw, making manure fibre less attractive than straw as a biomass for combustion. Fibre from anaerobic digestion plants has the advantage of being a more uniform fuel (AD NCV90 14e17 MJ kg1) than fibre from farm sites (Pig NCV90 11e21 MJ kg1). Calorific value was seemingly not lost during digestion. Also, uniform ash quality from AD fibre can be expected, making AD fibres prime candidates for combustion and ash-based fertilizer production, followed by mink and pig fibres, while cattle fibre due to its low P content is not considered a candidate.
Acknowledgements The study was supported by the ForskEL project ”Working up phosphates from ashes”, Kommunekemi a/s and a grant from the Danish Council for Strategic Research under the work program “Clean and environmentally friendly animal waste technologies for fertilizer and energy production (CLEANWASTE).”
references
Ballance, S., Sheehan, J. K., Tkachenko, A., McCrohan, C. R., & White, K. N. (2002). Interaction of mucus with freshly neutralised aluminum in freshwater. Journal of Inorganic Biochemistry, 92, 11e18. Birkmose, T., & Thygesen, O. (2010). Status of application of slurry separation in Denmark, (in Danish). Horticulture, No. 187. Skejby, Denmark: Danish Agricultural Advisory Centre. Chesworth, W. (2008). Encyclopaedia of soil science (1st ed.). Springer, ISBN: 978-1-4020-3994. Cohen, Y. (2009). Phosphorus dissolution from ash of incinerated sewage sludge and animal carcasses using sulphuric acid. Environmental Technology, 30, 1215e1226. Dahlin, J. (1985). Heating value calculations. Copenhagen: BP Coal (in Danish). DIN 51900 German standard. (November 1989). Testing of solid and liquid fuels. DS 259:2003 Danish standard. Determination of metals in water, sludge and sediments e General guidelines for determination by atomic absorption spectrophotometry in flame. DS/CEN/TS 15920:2009 European standard. Fertilizers e Extraction of phosphorus soluble in 2% citric acid. DS/CEN/TS 15958:2010 European standard. Fertilizers e Extraction of water soluble phosphorus.
DS/EN 12879:2001 European standard. Characterization of sludges e Determination of the loss on ignition of dry mass. DS/EN 14346:2007 European standard. Characterization of waste e Calculation of dry matter by determination of dry residue or water content. EN 1189:1997 European standard. Water quality. Determination of phosphorus. Ammonium molybdate spectrometric method. EN1482e2:2007 European standard. Fertilizers and liming materials e Sampling and sample preparation e Part 2: Sample preparation. Executive Order no. 1650 of 13/12/2006. (2006). Executive order regarding sludge (slambekendtgørelsen). Danish Ministry of the Environment (in Danish). Green energy. (2010). The road to a Danish energy system without fossil fuels. Danish Commission on Climate Change Policy, ISBN 978-87-7844-881-1. http://www.ens.dk/en-US/policy/danishclimate-and-energy-policy/danishclimatecommission/ greenenergy/Documents/green%20energy%20GB%20screen% 201page%20v2.pdf. Hansen, H. K., Pedersen, A. J., Ottosen, L. M., & Willumsen, A. (2001). Speciation and mobility of cadmium in straw and wood combustion fly ash. Chemosphere, 45, 123e128. Hjorth, M., Christensen, K. V., Christensen, M. L., & Sommer, S. G. (2010). Solid-liquid separation of animal slurry in theory and practice. A review. Agronomy for Sustainable Development, 30, 153e180. Johnke, B. (1999). Emissions from waste incineration. Background paper for expert meeting on good practice in inventory preparation: Emissions from waste. IPCC/OECD/IEA National Greenhouse Gas Inventories Programme. Jørgensen, K., & Jensen, L. S. (2009). Chemical and biochemical variation in animal manure solids separated using different commercial technologies. Bioresource Technology, 100, 3088e3096. Lack, A. J., & Evans, D. E. (2001). Plant biology (1st ed.). Oxford: BIOS Scientific Publishers Limited (Section G). Montgomery, D. C., & Runger, G. C. (2003). Applied Statistics and Probability for Engineers (3rd ed.). New York: John Wiley and Sons. Møller, H. B., Jensen, H. S., Tobiasen, L., & Hansen, M. N. (2007). Heavy metal phosphorus content of fractions from manure treatment and incineration. Environmental Technology, 28, 1403e1418. OECD. (2004). Average of 2000e2004 values from OECD phosphorus balances. http://stats.oecd.org Accessed June 2012. Otero, M., Sanchez, M. E., Go´mez, X., & Mora´n, A. (2010). Thermogravimetric analysis of biowastes during combustion. Waste Management, 30, 1183e1187. Petersen, S. O., Sommer, S. G., Be´line, F., Burton, C., Dach, J., Dourmad, J. Y., et al. (2007). Recycling of livestock manure in a whole-farm perspective. Livestock Science, 112, 180e191. Rubæk, G. H., Stoholm, P., & Sørensen, P. (2006). Availability of P and K in ash from thermal gasification of animal manure. In S. O. Petersen (Ed.), (2006). 12th Ramiran International conference, Technology for recycling of manure and organic residues in a whole-farm perspective, Vol. II, DIAS report no. 123, 177e180. Statistics Denmark, Available at http://statistikbanken.dk/ statbank5a/default.asp?w¼1466; Accessed June 2012. Steen, I. (1998). Phosphorus availability in the 21st century, management of a nonerenewable resource. Phosphorus & Potassium, 217, 25e31. Stumm, W., & Morgan, J. J. (1996). Aquatic chemistry (3rd ed.). New York: John Wiley & Sons. USGS. (2012). Phosphate rock statistics and information. US Geological Survey, Available at http://minerals.usgs.gov/minerals/pubs/ commodity/phosphate_rock/mcs-2012-phosp.pdf Accessed June 2012. Westborg, S., Johansen, L. P., & Christensen, B. H. (2010). Fuel characteristics of manure, Danish environmental protection agency, environmental project 1339. Wittrup, S. (2010). Power plant also wants animal slurry (in Danish). Available at http://ing.dk/artikel/109248-kraftvaerker- vilogsaa-have-gylle Accessed June 2012.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/issn/15375110
Research Paper
Manure-based energy generation and fertiliser production: Determination of calorific value and ash characteristics Ole Thygesen a,*, Tina Johnsen b a
Institute of Chemical Engineering, Biotechnology and Environmental Technology, Faculty of Engineering, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark b Kommunekemi A/S, Lindholmvej 3 DK-5300 Nyborg, Denmark
article info
The potential for energy generation and phosphorus (P) fertiliser production through the
Article history:
combustion of manure fibre combined with ash recycling was examined. Manure fibre has
Received 15 February 2012
a positive calorific value and may be used as a CO2-neutral fuel for combustion. The ashes
Received in revised form
from combustion are also rich in P. Fibre samples from anaerobically digested manure
27 June 2012
produced by pigs, mink and cattle were collected from 32 slurry separation units around
Accepted 23 July 2012
Denmark, along with data on the separation technology used and the origin of the manure
Published online 14 August 2012
fibre. The mean gross calorific value of the wet manure fibres for the different sources was 5.2 MJ kg1 for anaerobically digested (AD), 5.3 MJ kg1 for pigs and 4.4 MJ kg1 for mink and cattle, with the latter relatively low value being due to high water content, suggesting drying of the manure fibre would be helpful. The mean P concentration in the ashes was 112, 123, 157, and 51 g kg1 for respectively AD, pigs, mink and cattle. Manure fibre ashes derived from cattle slurry contained too little P to be suitable for fertiliser production. Pig slurry, to which sulphuric acid had been added prior to separation, had a low P content in the ash and was not suitable for fertiliser production. The low solubility of P means the ashes should be treated before being used as a fertiliser. ª 2012 IAgrE. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The global energy and climate crises call for the development of new energy sources. At the same time the increased consumption of animal feed products and a growing global population enhances the need for plant nutrients to support an increased plant production. Animal manure is a source of energy and of plant nutrients (Otero, Sanchez, Go´mez, & Mora´n, 2010; Petersen et al., 2007). This study focuses on the use of manure fibre (the dry-matter-rich fraction obtained
from animal slurry separation) as a fuel in combination with production of an ash-based P fertiliser. To fight global warming the Danish government intends to phase out the use of fossil fuels. Their vision is that 25e65% of the energy consumption by 2050 should be provided by biomass, to which animal manure shall provide a significant fraction, primarily through anaerobic digestion plants (Green energy, 2010). Currently 90,000 t of manure fibre is produced per year from a separation of approximately 3% of the annual Danish animal slurry production (Birkmose & Thygesen,
* Corresponding author. Tel.: þ45 21356037. E-mail address: [email protected] (O. Thygesen). 1537-5110/$ e see front matter ª 2012 IAgrE. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biosystemseng.2012.07.004
b i o s y s t e m s e n g i n e e r i n g 1 1 3 ( 2 0 1 2 ) 1 6 6 e1 7 2
Nomenclature AD Al Cd Cr Cu DM EU15
Anaerobically digested Aluminium Cadmium Chromium Copper Dry matter The countries in the European Union prior to the accession of ten candidate countries on 1 May 2004
2010). It has been estimated that within a few years incineration of manure fibre could replace 3.6 PJ of coal energy, equivalent to 4.3& of the yearly Danish energy consumption. To achieve this, about one third of the Danish slurry needs to be separated (Statistics Denmark, 2012; Wittrup, 2010). P fertilisation is fundamental for the high crop yields obtained in intensive agriculture. The agricultural consumption of phosphates constitutes approximately 85% of the total yearly consumption of P (Steen, 1998). The P used today almost exclusively comes from the mining of phosphate rock, which is a limited non-renewable resource (Steen, 1998). Estimations of current reserves and future consumption lead to varying conclusions, but it seems evident that P scarcity will become increasingly critical within the next few years (Chesworth, 2008; Steen, 1998; USGS, 2012). Agriculture will therefore become more and more dependent on phosphate rock from Morocco, since it controls 70% of the known reserves (USGS, 2012). With this background P management and recycling has raised both political and technical interest. Application of surplus P is intimately linked to animal production1; manure applied according to EU standards simply contains more P than needed by crops. When separating animal slurry most of the P will end up in the manure fibre (Hjorth, Christensen, Christensen, & Sommer, 2010), thus by separating the manure better management of the plant nutrients in manure can be achieved. Producing a P-fertiliser from the manure fibre ash could facilitate the effective redistribution of slurry P. When incinerating the manure fibre nearly 100% of the P is recovered in the ash. Of this P, 80% is recovered in the bottom ash and 20% in the fly ash (Cohen, 2009; Møller, Jensen, Tobiasen, & Hansen, 2007). Different P concentrations have been recorded in ashes from differently sourced manure fibres, with a 7e15% concentration in ashes from manure fibres coming from pigs, 11% in ashes from manure fibres coming from anaerobically digested (AD) slurry and 3.6% in ashes from manure fibres coming from cattle slurry (Møller et al., 2007; Westborg, Johansen, & Christensen, 2010). Ash with P content below 10e12% is not considered suitable for fertiliser production. Manure fibre ashes therefore appear to be good candidates for such fertiliser production, but the lack of knowledge on ash quality has so far been an obstacle, since the large-scale combustion of manure fibre has not yet been implemented. 1 The surplus application of P to crops in EU15 adds up to 1 million tonnes P per year (OECD, 2004). This amount is equivalent to 5% of the global yearly P consumption.
GCV ICPeMS NCV NCV90 Ni P Pb Zn
167
Gross calorific value Inductively coupled plasma mass spectrometry Net calorific value Net calorific value at 90% DM Nickel Phosphorus Lead Zinc
The aim of the study was to provide technical insight into the quality of manure fibre and fibre ash by determining combustion and fertiliser relevant parameters based on manure fibre produced in Denmark.
2.
Materials and methods
2.1.
Manure fibre sampling
The animal slurry samples used in the study were collected in AprileJune 2010 from 32 commercial separation plants; these plants comprise approximately two-thirds of the separation plants currently operating in Denmark (Birkmose & Thygesen, 2010). Three additional AD slurry samples were included in the study; these were separated in the laboratory using a pressurised sieve to provide three manure fibre samples. These three samples were only evaluated for ash quality, since the fuel properties did not represent those from the commercial separation units. Sample information is given in Table 1, a detailed description of the different separation technologies can be found in Hjorth et al. (2010). The separated dry-matter-rich fractions were sampled at the outlet of the separator by collecting five subsamples, giving a total volume of 8e10 l. At three locations the separator could not be started at the time of sampling. At these sites the manure fibre samples were taken from five different sites of the storage heaps 10e20 cm below the surface, giving a total volume of 8e10 l. The samples were stored in airtight plastic containers during transportation to the laboratory. All samples were homogenised and divided into three subsamples immediately after arrival at the laboratory. One subsample of 1 l was stored at 18 C for later analysis, and one subsample of 1 l was stored at 6 C for analysis within a week of arriving at the laboratory. The remainder of the sample was dried and incinerated.
2.2.
Characterisation of the manure fibre samples
All measurements were carried out with samples at room temperature, i.e. 20 Ce22 C. The dry matter (DM) concentration and ash content were determined according to DS/EN 14346 (2007) and DS/EN 12879 (2001), respectively. Measurements were carried out in triplicate. Gross calorific value (GCV) was determined using a Parr 6300 calorimeter (Parr instrument company, IL, USA) in accordance with DIN 51900 (1989). Single measurements were carried out.
168
b i o s y s t e m s e n g i n e e r i n g 1 1 3 ( 2 0 1 2 ) 1 6 6 e1 7 2
Table 1 e Origin and treatment of the samples of manure fibre from livestock farms and biogas plants. Sample ID
Typea
1
AD
None
DC
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 17a 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
AD AD AD AD AD AD AD Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Pig Mink Mink Mink Cattle Cattle Cattle
None None None None None None None None None None None None None None None None None None None None None None None 3 days None None None None None None 2 weeks None None None None
DC DC DC SP SI SI SI SI, SP, PO SI, SP, PO SI, SP, PO SI, SP, PO SI, SP, PO SI, SP, PO SI, SP SP SP SP, þ SF, SP, A SF, SP SP SP DC DC DC SI, SP, PO SF SI, SP, PO SI, SP, PO SI, SP, PO RD RD RD SP SP DC
Storage time
Type of separationa
Origin of the slurry treated 80% animal slurry (75% pig, 20% cattle and 5% mink) and 20% industrial wastes 15% fish slime and 85% pig slurry primarily pig slurry Sow, piglet and finishing slurry 50% pig and 50% cattle slurry 5% industrial waste and 95% cattle slurry Pig slurry Animal slurry and industrial waste Sow, piglet and finishing slurry Sow and finishing slurry Sow slurry Sow, piglet and finishing slurry Sow and piglet slurry Sow slurry Sow, piglet and finishing slurry Sow, piglet and finishing slurry Finishing slurry Finishing slurry Sow, piglet and finishing slurry Piglet and finishing slurry Sow slurry Sow and finishing slurry Sow slurry Sow slurry Sow slurry Finishing slurry Sow slurry Sow, piglet and finishing slurry Sow and piglet slurry Sow, piglet and finishing slurry Mink slurry Mink slurry Mink slurry Cattle slurry Cattle slurry Cattle slurry
a DC ¼ decanter centrifuge, þ ¼ pressed extra hard, A ¼ added sulphuric acid, RD ¼ rotating filter drum, SF ¼ shaking filter, SI ¼ sieve, SP ¼ screw press and PO ¼ polyacrylamide polymers added.
The net calorific value (NCV) of untreated samples and samples dried to 90% DM (NCV90) was calculated from the measured GCVs according to Dahlin (1985). In the calculations the hydrogen content was estimated to be 6% of the dry matter content in accordance with the findings of Westborg et al. (2010).
2.3.
Ash production and characterization
Ashes were produced by incinerating dried fibre samples at 850 C for 1 h in a laboratory oven. A temperature of 850 C was chosen to resemble the temperature used in waste incineration plants (Johnke, 1999, pp. 455e468). For the phosphate solubility tests the samples were prepared according to EN1482-2 (2007), the extractions in water and 2% citric acid were carried out in accordance with DS/CEN/TS 15958 (2010) and DS/CEN/TS 15920 (2009), respectively. The orthophosphate content of the filtrate was determined according to EN 1189 (1997) using Spectroquant phosphate tests from Merck (The Merck Group, Darmstadt, Germany). Measurements were carried out in duplicate.
Analyses of the fibre ash content of total P, aluminium and the heavy metals Zn, Cu, Cr, Ni, Pb and Cd were carried out using Inductively coupled plasma mass spectrometry (ICPeMS) (Thermo Scientific, x-series, Waltham, MA, USA) after acidic digestion of the fibre ash sample using nitric acid DS 259 (2003). Measurements were carried out in duplicate. For the heavy metals Cr, Ni and Pb, the detection value was 10 mg kg1 ash. For Cd, the detection value was 0.8 mg kg1.
2.4.
Statistical methods
Statistical analysis was carried out using Excel 2007 (Microsoft Corporation, Redmond, WA, USA). The analysis included a pair-wise F-test to assess when variance homogeneity could be assumed. ANOVA was applied in the cases where variance homogeneity was proven and pair-wise t-testing was used to test data without variance homogeneity. In some cases, a onesided test was used to compensate for a low degree of freedom, either due to a low number of samples or to the lack
169
b i o s y s t e m s e n g i n e e r i n g 1 1 3 ( 2 0 1 2 ) 1 6 6 e1 7 2
of variance homogeneity or both (Montgomery & Runger, 2003).
3.
Results and discussion
Due to the relatively low numbers of separation plants currently operating in Denmark, some groups of manure fibre contained few samples i.e. mink and cattle manure fibre. However, despite the low numbers of samples the groups still showed distinct differences from one another justifying the use of the small groups. The group of AD manure fibre samples was very heterogeneous, some samples came exclusively from pig slurry while others were produced from mainly cattle slurry, therefore the AD manure fibre group will not only be treated as one group, but the differences within the group will also be discussed. Regarding ash quality, a very simple set up was used to produce the ash. The ashes produced had a light grey or khaki colour, indicating a low content of unburned carbon. The content of chlorides in ash is highly dependent on combustion parameters; analysis of chloride content was therefore omitted.
3.1.
DM and ash content
Mink fibre had the highest DM content, due to the presence of bone residues in the diet, which have low water retention capacities. The AD and the pig fibre had similar dry matter concentrations. Cattle fibre had highest water content, reflecting the less efficient dewatering of cattle slurry than of pig and AD slurry (Hjorth et al., 2010) see Table 2. The ash content could after statistical analysis be ranked as follows: mink > AD > pig ¼ cattle. The ash content from mink is the highest, due to the high content of bone residues. The AD fibre samples have higher ash contents than pig and cattle fibre samples, since some of the organic matter is transformed to biogas during anaerobic digestion.
3.2.
Calorific values
Three calorific values were considered in the following: GCV and NCV of manure fibre samples as received and NCV90 (see Table 2).
Fig. 1 e The relative frequencies of each manure fibre group in the GCV intervals (as received).
The GCV of wet manure fibre varied between 3 and 8 MJ kg1 (Fig. 1), typically ranging between 4 and 6 MJ kg1. The GCV of cattle fibre was significantly lower than the GCV of pig fibre, but otherwise differences were not statistically significant. The GCV is related to the content of organic matter in the manure fibre and is low in mink dry-matter-rich fractions due to the high ash content. Cattle fibre had high water content and consequently a low content of organic matter, leading to low GCVs. The NCV is a measure of the maximum energy gain that can be obtained by combustion of the wet fibre without condensation. As can be seen in Table 2, the NCV varied between 1.5 and 5.4 MJ kg1 with mean values ranging from 2.4 MJ kg1 (cattle) to 3.4 MJ kg1 (AD and pigs). This is in line with the findings of Westborg et al. (2010). The distribution of NCV90s of mink, pig and AD slurry is shown in Fig. 2. The dried cattle fibre has the highest calorific value (17.8 MJ kg1), which may reflect a high straw content. Dried mink fibre had a low calorific value (11.0 MJ kg1) due to its high ash content. The NCV90 of AD fibre was not significantly different from pig fibre, despite AD having a 40% higher ash content. This may be due to the higher contents of lignin and neutral detergent soluble substances found in AD fibre
Table 2 e DM, ash content and calorific values of fibre samples. Standard error given in brackets. Minemax [ the lowest and highest measured value. Means followed by the same letter indicate the means are not significantly different from each other ( p < 0.05).
DM Minemax Ash Minemax GCV Minemax NCV Minemax NCV90 Minemax
g kg1 g kg1 g kg1 DM g kg1 DM MJ kg1 MJ kg1 MJ kg1 MJ kg1 MJ kg1 MJ kg1
AD
Pig
Mink
Cattle
28.3a (1.8) 21.5e31.8 22.6 (2.3) 15.6e26.8 5.2ab (0.42) 3.8e6.1 3.4a (0.58) 1.9e4.3 15.5ab (0.5) 13.9e16.8
28.1a (1.5) 12.3e47.1 16.1a (1.4) 6.3e40.2 5.3a (0.19) 3.7e7.2 3.4a (0.57) 1.5e5.4 16.3a (0.8) 10.5e20.6
34.5 (4.1) 26.2e39.4 43.4 (5.8) 35.3e54.7 4.4ab (0.66) 3.4e5.7 2.8ab (0.67) 1.9e4.1 11.0b (2.0) 7.1e13.4
20.6 (0.41) 19.8e21.2 12.6a (1.0) 10.6e14.0 4.4b (0.04) 4.3e4.4 2.4b (0.04) 2.3e2.5 17.8 (0.3) 17.4e18.5
170
b i o s y s t e m s e n g i n e e r i n g 1 1 3 ( 2 0 1 2 ) 1 6 6 e1 7 2
Fig. 3 e The relative frequencies of each manure fibre ash in the water solubility intervals. Fig. 2 e The relative frequencies of each manure fibre group in the NCV90 intervals.
(Jørgensen & Jensen, 2009). Lignin and various detergentsoluble lipids are more energy-rich than the cellulose and hemicellulose making up the rest of the organic carbon in manure fibre.
would transfer more P to the liquid fraction during the separation (Stumm & Morgan, 1996, Chapter 7). AD, pig and mink fibre ashes all contain similar amount of P to mined phosphate rock; the cattle fibre ash is so low in P that recovery is probably not be economically viable (Chesworth, 2008).
3.3.1. 3.3.
Phosphorus in ash
The general picture regarding P content in the manure fibre ashes is that cattle fibre ashes contained the lowest amount of P, AD fibre ashes had intermediate P contents similar to pig fibre ashes, and mink fibre ashes had high P contents (see Table 3). Two ash samples from AD manure fibre produced ashes with P concentrations below the 10%, which we set as a minimum requirement for the ash that could be used for P fertiliser production. The two plants were no. 1 with 86.4 g kg1 and no. 6 with 82.3 g kg1. From the feed specification in Table 1 it can be seen that these plants had the lowest percentage of P-rich slurries (pig and mink) in their input, explaining the lower P concentration in the ash. The sample of pig manure fibre from plant no. 18, which was the only sample included in the study that had sulphuric acid added to the slurry prior to separation, had a low P content in the ash and was unsuitable for fertiliser production; the lower P concentration could be due to a higher solubility of P compounds in the acidified slurry due to the lower pH, which
Solubility in water and 2% citric acid
The solubility in water of P from ashes is typically below 1% (see Fig. 3), and the solubility in 2% citric acid ranged between 4 and 7% (see Fig. 4). The low solubility in these two solvents illustrates the need for processing the ash before it can be applied as an effective fertiliser; the processing could include treatment with acid. A lower solubility of P in water than in 2% citric acid was expected (Rubæk, Stoholm, & Sørensen, 2006). This was in fact the case for all samples but one. The unusual sample was from cattle slurry from plant no. 34; this sample was equally soluble in both solvents. No single explanatory factor from an examination of the major cation content (data not shown) and data on the origin of the fibre sample could account for its high solubility in water.
3.4.
Heavy metals and aluminium in ash samples
Danish legislation (Executive Order no. 1650, 2006) sets the threshold values for heavy metals in waste used for agricultural application; threshold values for the heavy metals found in the highest concentrations are presented in Table 4 along with means of the measured heavy metal contents. Ash
Table 3 e Concentration of P in fibre ash, solubility of P in water and 2% citric acid. Standard error given in brackets. Minemax [ the lowest and highest measured value. Means followed by the same letter indicate the means are not significantly different from each other ( p < 0.05).
Total P Minemax P solubility in H2O Minemax P solubility in 2% citric acid Minemax
g kg1 g kg1 % % % %
AD
Pig
Mink
Cattle
112a (7.5) 82e143 0.43 (0.24) 0.004e1.96 5.49 (0.31) 4.29e7.39
123a (3.1) 86e149 0.16 (0.031) 0.003e0.688 5.10 (0.19) 3.74e7.27
157 (8.5) 147e188 0.11(0.035) 0.019e0.240 4.53 (0.19) 4.20e5.06
51 (10.0) 34e69 1.55 (1.35) 0.040e4.24 5.00 (0.71) 4.25e6.41
171
b i o s y s t e m s e n g i n e e r i n g 1 1 3 ( 2 0 1 2 ) 1 6 6 e1 7 2
slurry; the aluminium could originate from the fish mucus that has been reported to possess a strong affinity for aluminium (Ballance, Sheehan, Tkachenko, McCrohan, & White, 2002).
3.4.1. Problematic levels of copper and zinc in pig fibre ashes is associated with the use of polymers
Fig. 4 e The relative frequencies of each manure fibre ash in the citric acid solubility intervals.
samples from mink and cattle fibre ash did not exceed any threshold values. The ash derived from AD only exceeded the threshold values in the ash from plant no. 7, which exclusively uses pig manure. For pig manure the thresholds were exceeded in several ashes. Copper and zinc were found in all ash samples. Chromium and nickel were found in most samples. The fibre ash from plant no. 12 contained copper, zinc and chromium all exceeding the threshold values, but threshold values for chromium and nickel were not exceeded when applying the phosphate based threshold value for nickel given in the Danish legislation (Executive Order no. 1650, 2006). In all ash samples lead and cadmium concentrations were below the detection levels and below the threshold values which were 120 mg kg1 for lead and 0.8 mg kg1 for cadmium; this is probably due to the volatile nature of these two heavy metals. This indicates that bottom ash from combustion in the normal temperature range (850 C and above) will not contain problematic concentrations of cadmium and lead, but this may depend on the design and performance of the actual combustion plant. Fly ashes are known to accumulate cadmium, so special attention to cadmium should be given in the case of fly ash (Hansen, Pedersen, Ottosen, & Willumsen, 2001). Aluminium is unwanted in fertilisers as high concentrations of Al3þ are toxic to plants and can significantly reduce crop growth. Al3þ for example inhibits root growth and disrupts membranes and ATP-metabolism (Lack & Evans, 2001). In general, manure fibre ashes have low aluminium contents (Table 4), but biogas plant no. 2 stands out with its sample showing high aluminium content (106 mg g1). This plant adds 15% fish mucus from the nearby fishing industry to the pig
Copper and zinc salts are added to pig feeds due to their positive effect on productivity. Therefore copper and zinc are found in relatively high concentrations in manure fibre ashes. Many of the ashes from pig fibre exceeded the threshold values. The average concentrations for copper and zinc in 11 dry fodders for finishing pigs and sows used by the farmers in this study were 16.4 and 107 mg kg1, respectively, and 150 and 119 mg kg1, respectively, for piglets. This study found no relationship between the copper and zinc contents in feed and in fibre ashes from piglets and finishing pigs. As described by Møller et al. (2007), the addition of polyacrylamide polymers to the separation process leads to higher contents of the heavy metals. All nine pig fibre ashes produced using polymers during separation gave ash samples exceeding the copper threshold value defined in Danish legislation (Executive Order no. 1650, 2006) and seven exceeding the threshold value for zinc. In contrast, only two out of 12 pig fibre ashes produced without polymers exceeded the threshold values of copper or zinc. All farmers used similar polymers, the amount of polymers used by the farmers varied from 0.07 to 0.38 l m-3 slurry, but there seemed to be no correlation between the consumption of polymers and the concentration of zinc and copper in the ashes that derived from separation plants using polymers. This might be due to variations in the slurry. As the P content in the ash is not affected by the addition of polymers during slurry separation, the better ash quality, in terms of heavy metals and P content, is attained by not adding polymers to the animal slurry during separation.
4.
Conclusions
The water solubility of P in the ashes was generally low with an average solubility percentage of 0.31. The solubility of P in 2% citric acid was somewhat higher with an average of 5.1%, but still too low to be an effective P fertiliser. The low solubility means the ashes should be treated before being used as a fertiliser. Cattle manure fibre ash contained too little P to qualify as a P fertiliser candidate with a P content of only 5%. Mink fibre ash had the highest P content at an average 16% and AD and pigs had intermediate P contents of 11 and 12%, respectively.
Table 4 e Content of aluminium and heavy metals in fibre ash samples determined using ICPeMS. Standard error given in brackets. AD Al Zn Cu Cr Ni/P
g kg1 mg kg1 mg kg1 mg kg1 mg kg1
16.8 2632.5 785.5 27.5 360.3
(12.7) (254) (179.3) (3.6) (15.6)
a Given in Executive Order no. 1650 (2006).
Pig
Mink
Cattle
Threshold valuea
2.8 (0.20) 4293 (497) 1269 (190) 29 (6.4) 519.6 (66.9)
0.27 (0.07) 624 (43.4) 44 (4.3) e 73.9
3.4 (0.46) 959 362 19 415.7 (42.1)
4000 1000 100 2500
172
b i o s y s t e m s e n g i n e e r i n g 1 1 3 ( 2 0 1 2 ) 1 6 6 e1 7 2
Mink, AD and pig fibre ash are suitable candidates for a P fertiliser production. The aluminium concentration in these ashes was generally acceptable for fertiliser production, but one AD ash, which partly originated from fish waste, had an aluminium content of 11%, making its ash problematic. The content of copper and zinc in pig slurries was high, which could be related to a widespread application of polyacrylamide polymers in the separation process. When polymers were not used, pig fibre ashes only in a few cases contained copper or zinc in concentrations exceeding the threshold values defined in Danish legislation. Manure fibre has a positive calorific value and the wet fibre may be combusted with a support fuel before or after anaerobic digestion to get a net energy gain. Once dried, manure fibre has calorific values comparable to straw, but the energy requirement for drying reduces the net energy gain. The energy gain is therefore smaller compared to straw, making manure fibre less attractive than straw as a biomass for combustion. Fibre from anaerobic digestion plants has the advantage of being a more uniform fuel (AD NCV90 14e17 MJ kg1) than fibre from farm sites (Pig NCV90 11e21 MJ kg1). Calorific value was seemingly not lost during digestion. Also, uniform ash quality from AD fibre can be expected, making AD fibres prime candidates for combustion and ash-based fertilizer production, followed by mink and pig fibres, while cattle fibre due to its low P content is not considered a candidate.
Acknowledgements The study was supported by the ForskEL project ”Working up phosphates from ashes”, Kommunekemi a/s and a grant from the Danish Council for Strategic Research under the work program “Clean and environmentally friendly animal waste technologies for fertilizer and energy production (CLEANWASTE).”
references
Ballance, S., Sheehan, J. K., Tkachenko, A., McCrohan, C. R., & White, K. N. (2002). Interaction of mucus with freshly neutralised aluminum in freshwater. Journal of Inorganic Biochemistry, 92, 11e18. Birkmose, T., & Thygesen, O. (2010). Status of application of slurry separation in Denmark, (in Danish). Horticulture, No. 187. Skejby, Denmark: Danish Agricultural Advisory Centre. Chesworth, W. (2008). Encyclopaedia of soil science (1st ed.). Springer, ISBN: 978-1-4020-3994. Cohen, Y. (2009). Phosphorus dissolution from ash of incinerated sewage sludge and animal carcasses using sulphuric acid. Environmental Technology, 30, 1215e1226. Dahlin, J. (1985). Heating value calculations. Copenhagen: BP Coal (in Danish). DIN 51900 German standard. (November 1989). Testing of solid and liquid fuels. DS 259:2003 Danish standard. Determination of metals in water, sludge and sediments e General guidelines for determination by atomic absorption spectrophotometry in flame. DS/CEN/TS 15920:2009 European standard. Fertilizers e Extraction of phosphorus soluble in 2% citric acid. DS/CEN/TS 15958:2010 European standard. Fertilizers e Extraction of water soluble phosphorus.
DS/EN 12879:2001 European standard. Characterization of sludges e Determination of the loss on ignition of dry mass. DS/EN 14346:2007 European standard. Characterization of waste e Calculation of dry matter by determination of dry residue or water content. EN 1189:1997 European standard. Water quality. Determination of phosphorus. Ammonium molybdate spectrometric method. EN1482e2:2007 European standard. Fertilizers and liming materials e Sampling and sample preparation e Part 2: Sample preparation. Executive Order no. 1650 of 13/12/2006. (2006). Executive order regarding sludge (slambekendtgørelsen). Danish Ministry of the Environment (in Danish). Green energy. (2010). The road to a Danish energy system without fossil fuels. Danish Commission on Climate Change Policy, ISBN 978-87-7844-881-1. http://www.ens.dk/en-US/policy/danishclimate-and-energy-policy/danishclimatecommission/ greenenergy/Documents/green%20energy%20GB%20screen% 201page%20v2.pdf. Hansen, H. K., Pedersen, A. J., Ottosen, L. M., & Willumsen, A. (2001). Speciation and mobility of cadmium in straw and wood combustion fly ash. Chemosphere, 45, 123e128. Hjorth, M., Christensen, K. V., Christensen, M. L., & Sommer, S. G. (2010). Solid-liquid separation of animal slurry in theory and practice. A review. Agronomy for Sustainable Development, 30, 153e180. Johnke, B. (1999). Emissions from waste incineration. Background paper for expert meeting on good practice in inventory preparation: Emissions from waste. IPCC/OECD/IEA National Greenhouse Gas Inventories Programme. Jørgensen, K., & Jensen, L. S. (2009). Chemical and biochemical variation in animal manure solids separated using different commercial technologies. Bioresource Technology, 100, 3088e3096. Lack, A. J., & Evans, D. E. (2001). Plant biology (1st ed.). Oxford: BIOS Scientific Publishers Limited (Section G). Montgomery, D. C., & Runger, G. C. (2003). Applied Statistics and Probability for Engineers (3rd ed.). New York: John Wiley and Sons. Møller, H. B., Jensen, H. S., Tobiasen, L., & Hansen, M. N. (2007). Heavy metal phosphorus content of fractions from manure treatment and incineration. Environmental Technology, 28, 1403e1418. OECD. (2004). Average of 2000e2004 values from OECD phosphorus balances. http://stats.oecd.org Accessed June 2012. Otero, M., Sanchez, M. E., Go´mez, X., & Mora´n, A. (2010). Thermogravimetric analysis of biowastes during combustion. Waste Management, 30, 1183e1187. Petersen, S. O., Sommer, S. G., Be´line, F., Burton, C., Dach, J., Dourmad, J. Y., et al. (2007). Recycling of livestock manure in a whole-farm perspective. Livestock Science, 112, 180e191. Rubæk, G. H., Stoholm, P., & Sørensen, P. (2006). Availability of P and K in ash from thermal gasification of animal manure. In S. O. Petersen (Ed.), (2006). 12th Ramiran International conference, Technology for recycling of manure and organic residues in a whole-farm perspective, Vol. II, DIAS report no. 123, 177e180. Statistics Denmark, Available at http://statistikbanken.dk/ statbank5a/default.asp?w¼1466; Accessed June 2012. Steen, I. (1998). Phosphorus availability in the 21st century, management of a nonerenewable resource. Phosphorus & Potassium, 217, 25e31. Stumm, W., & Morgan, J. J. (1996). Aquatic chemistry (3rd ed.). New York: John Wiley & Sons. USGS. (2012). Phosphate rock statistics and information. US Geological Survey, Available at http://minerals.usgs.gov/minerals/pubs/ commodity/phosphate_rock/mcs-2012-phosp.pdf Accessed June 2012. Westborg, S., Johansen, L. P., & Christensen, B. H. (2010). Fuel characteristics of manure, Danish environmental protection agency, environmental project 1339. Wittrup, S. (2010). Power plant also wants animal slurry (in Danish). Available at http://ing.dk/artikel/109248-kraftvaerker- vilogsaa-have-gylle Accessed June 2012.