4.1. Suitability of Raw Materials and Press Cakes for Combustion
In comparison to coniferous wood [15
] taken as common solid fuel, concentrations of minerals in faeces and its mixtures were substantially higher. For combustion, elevated S and N concentrations in faeces and in faeces mixed with straw might be problematic due to potential NOx
emissions, as well as the corrosive effects in the case of S [15
]. Obernberger et al. [15
] suggested guiding values for the concentrations of N and S in solid biofuels which shall not be exceeded, namely 0.1% DM for S and 0.6% DM for N. N concentrations in faeces and mixtures were well above this value, though only slightly in the case of the mixture with shavings. Regarding S concentrations, values for pure faeces and the mixture with straw exceeded the guiding value, whereas the concentration of S in the mixture with shavings is classified as unproblematic.
A guiding value is also given for Cl (<0.1% DM, [15
]), which is due to the risk of corrosion and detrimental emissions. Faeces in mixture with straw exceeded this value; however, pure faeces and faeces with shavings were in accordance. Herbaceous material regularly contains considerable amounts of Cl. Hensgen et al. [25
], for instance, measured Cl concentrations of about 0.4% DM in semi-natural grassland and Aho et al. [26
] report Cl concentrations of about 0.5% DM in both wheat straw and corn stover.
Lundgren and Pettersson [7
] reported higher values for N, S, and Cl concentrations (about 0.85%, 0.13%, 0.24% DM, respectively, calculated from given data) in a mixture of horse faeces and shavings, although they assumed a higher share of shavings in the mixture. Whether this disparity results from a quality difference between the shavings or faeces cannot be determined, as data on the pure materials were not given. Yet, Li [12
] also found higher N and S concentrations (2.4% and 0.67% DM, respectively) in pure faeces than in our study. A further explanation for the deviation of results may be that urine was not included in our study since we used clean bedding. In contrast, Lundgren and Pettersson [7
] used bedding from stall boxes, which is at least partly soaked with urine. Additionally, Böske et al. [10
] showed that the type of fodder may influence the N concentration in horse manure (fed with hay: 1.36% DM, fed with silage: 0.96% DM), which makes a direct comparison of literature findings difficult.
It should be noted that high concentrations of K may lead to corrosion and a reduced ash melting point during combustion [15
]. Therefore, low concentrations are favourable. However, K concentration in horse faeces was high in comparison to the other investigated materials. Li [12
] detected lower K concentrations in pure faeces (0.9% DM) than in our study. Deviations in mineral concentration in faeces might be caused by different feeding strategies; however, no details are given by Li [12
] regarding horse feed, so this cannot be verified.
Although ash content was high in both the faeces and mixtures (7.86%–11.75% DM), which reduces the heating value of the solid fuel, the value for faeces with shavings is in accordance with the German standard for biofuels derived from herbaceous material (class B, ≤10% DM, [28
]. Lundgren and Pettersson [7
] found ash contents of 5%–7% DM in a mixture of faeces and shavings, composed of 40%–80% shavings on a DM basis. In comparison, we assumed a shavings share (in DM) of approximately 30%. Faeces mixed with shavings is not very problematic for combustion. The low mineral concentrations and the low ash content of wood shavings lead to tolerable values for N, S, and Cl. However, in pure faeces, N and S concentrations are alarming regarding the combustion process, while in faeces mixed with straw, N, S, and Cl concentrations exceed the mentioned guiding values for combustion. Therefore, a reduction of these elements prior to combustion seems favourable.
The IFBB process reduced N, S and Cl concentrations in biomass significantly and in a similar magnitude to former studies on different biomass types. By treating grass from roadside verges with the IFBB process, Piepenschneider et al. [29
] detected relative reduction rates for Cl (90%) and K (82%) which are in accordance with our study. However, their N concentration was only reduced by 29%, which is substantially below the reduction rate we could achieve with horse faeces and mixtures. This might indicate that N compounds in faeces are of higher water solubility than in herbaceous material. Reduction of ash content was in the range of the reduction detected for grass from roadside verges (35%, [29
This reduction resulted in a LHV of the press cake from faeces with shavings which is only about 1 MJ·kg−1
DM less than that of coniferous wood and identical with the LHV found in press cake from semi-natural grassland (17.9 MJ·kg−1
]). Lundgren and Pettersson [7
] found a LHV of 18.14 MJ·kg−1
DM in a mixture of faeces and shavings without pretreatment. However, the mixture contained higher shares of shavings (40–80% DM) than the mixture used in our experiment (about 30% shavings on a DM basis).
Additionally, S concentration in the press cake from pure faeces and the mixture of faeces and straw (0.085% and 0.068% DM, respectively) fell below the guiding value for an unproblematic combustion [15
]. In contrast, N concentrations in these materials were still too high, though only slightly. Advanced combustion techniques, e.g., air staging or secondary measures [15
], are able to cope with an N concentration in this range. Lundgren and Pettersson [7
] conducted combustion experiments with a mixture of faeces and shavings without pre-treatment using a boiler adapted to biomass. They reported NOx emissions below 400 mg·Nm−3
, which is below the German emission limiting value (500 mg·Nm−3
in combustion plants <10 MW, [31
]). Considerable reduction of Cl and K concentrations through the IFBB process leads to the conclusion that no harmful influence is expected from these elements. The Cl concentration in press cakes from the mixture of faeces with shavings (0.007% DM) was even in the range of coniferous wood. Furthermore, the IFBB process simultaneously increased the DM content in the resulting press cake, which is of particular importance, as drying of solid fuel prior to combustion is necessary and might require additional energy input if accomplished with thermal energy [2
4.2. Suitability of Raw Materials and Press Fluids for Anaerobic Digestion
The lower digestibility of faeces in mixture with shavings is in accordance with the higher concentration of lignin in this material compared to pure faeces or to faeces in mixture with straw. While the NDF concentration of pure faeces was similar to the findings of Li [12
], the measured ADL concentration was substantially lower in our study (9% DM vs. 15% DM [12
]). However, methane yields observed by Li [12
] were about 75 lN
VS higher than ours. Another study confirms ADL concentrations in pure faeces in the range of 8%–12% DM [13
]. Mönch-Tegeder et al. [13
] presented NDF concentrations of straw bedding in the range of 70%–80% DM and ADL concentrations of 7%–10% DM, which is in accordance with our findings. Specific methane yield from unused pure straw bedding was about 207 lN
]. Yet, the authors urge that the actual yield in practice is dependent on straw quality which, in turn, depends on time of harvest and weather conditions [13
Böske et al. [10
] reported methane yields of 235.4 lN
VS from digestion of horse manure (from horses fed with hay) in mixture with straw, which is nearly identical with our results. With 78.1 lN
VS, the methane yields from digestion of horse faeces with wood shavings were close to the methane yields in the present study. In accordance with our study, Böske et al. [10
] also used clean bedding material for their experiments. Digestion of spent wheat straw resulted in higher methane yields than digestion of clean straw, but total yield was only about 150 lN
]. The authors hypothesised that the aerobic degradation of organic compounds by fungal and bacterial microorganisms may facilitate the anaerobic digestion. In contrast, aeration did not enhance the methane yield in an experiment with a mixture of horse faeces and spent straw (without aeration: max. 192 lN
VS, with aeration: 140 lN
]. Equine urine contains about 7% organic matter [32
], which might be available for digestion. Therefore, anaerobic digestion of manure and mixtures may account for higher yields than reported in this study. Mönch-Tegeder et al. [13
] concluded that straw-based manure is a suitable input material for anaerobic digestion.
Methane yields from anaerobic digestion of press fluids in this study were lower than known values for digestion of press fluids from semi-natural grassland (up to 450 lN
]). The considerable deviation of digestion yields from press fluids versus raw materials suggests two possible explanations: On the one hand, volatile solids of press fluids might be less digestible than those contained in raw materials. This would imply that during mechanical dehydration, the digestible compounds are, at least partly, absorbed in the press cake. On the other hand, the digestion process might be hampered by substances that are washed into the press fluid. It is fairly well‑established that ammonia produced from the degradation of nitrogenous material can inhibit anaerobic digestion, with the C/N ratio of the substrate assumed as an indicator [12
]. The C/N ratio of horse faeces was found to be well below the value for grasses (18.32 and 40.27, respectively [12
]). As the majority of fibres remains in the press cake, it seems likely that the C/N ratio in the press fluid is lower than in the raw material. However, the pH value of digestate from the press fluid of pure faeces was 7.6 on average and, thus, did not indicate an inhibition by ammonium. Horse manure was found to lack trace elements (e.g., Ni, Fe, Co [13
]). If the few trace elements are primarily contained in the press cake, the digestion process of the press fluid might be inhibited. However, Piepenschneider et al. [29
] showed a considerable mass flow of trace elements into the press fluid when treating grass with the IFBB technique. Additionally, the inoculum provides trace elements. Horse manure is also rich in organic acids [12
], which are highly digestible. They might be transferred into the press fluid primarily and might have imbalanced the digestion process due to fast acetogenesis.