ENVIRONMENTAL AND ECONOMIC EVALUATION OF SLAGHTERHOUSE WASTE USED AS A SOURCE OF BIOMASS FOR ENERGY PRODUCTION

BALDINI, C.¹, BORGONOVO, F.², TULLO, E.¹, GUARINO, M.¹

1Department of Environmental Science and Policy, Università degli Studi di Milano, Milan, Italy

2Department of Health, Animal Science and Food Safety, Università degli Studi di Milano, Milan, Italy

ABSTRACT: This work aims to valorise the residue of anaerobic digestion of slaughterhouse waste using an alternative approach. A demonstrative-scale plant for the drying and combustion of digestate was evaluated in order to understand its capability in terms of environmental and economic performance. The solution appears feasible when the lower heating value of digestate is 17000-18000 kJ kgTS-1, the digestate production is more than 10 t d^-1 or the cost of digestate disposal is higher than 50 € t^-1.

Keywords: Digestate, Waste Combustion, Environmental evaluation.

INTRODUCTION: The amount of organic waste related to meat industries is increasing worldwide (Pagés-Díaz et al., 2014). A suitable solution for its treatment is the anaerobic digestion (AD), that enables to recover energy through biogas production (Palatsi et al., 2011). Nevertheless, the AD produces relevant amount of a solid residue, that contains compounds that cannot be converted into biogas. For its high content of nutrients and stabilized organic matter, digestate can be used for agricultural purposes as organic fertilizer. However, its application on soil is controversial when the AD involves mixtures containing organic waste. This kind of digestate is often considered as waste and disposed in landfills or used as fuel in solid waste incinerators. Otherwise, post-treatments are required with subsequent further investments. This work suggests an alternative solution considering the in-situ combustion of digestate (Kratzeisen et al., 2010), evaluating its environmental and economic performance. A demonstrative-scale plant was installed and operated for two years in an important slaughterhouse, enabling the recovery of energy and of a fraction of the nitrogen for agricultural purposes.

1. MATERIAL AND METHODS:

1.1. Demonstrative scale plant: A demonstrative-scale plant for the in-situ combustion of digestate was installed downstream of an anaerobic digester serving an important cattle slaughterhouse of northern Italy. Figure 13 shows the scheme of the whole process.

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Figure 13. Scheme of the scale plant.SC=scrubber, HE1 and HE2=heat exchangers, BF1 and BF2=baghouse filters.

1.1.1. Drying unit: A third (4.2±0.2 t d^-1) of the overall daily production of the solid fraction of digestate (obtained with a centrifugal separator) was treated in a moving conveyor dryer, operated at mid-low range of air temperature (70°C) and with a thermal power consumption of 150 Kw. Digestate humidity was continuously measured in order to set consequently the time duration of the process (approximately assumed around 40-50 min, maximum humidity of 15%). The optimal air flow rate was empirically established at 6,400 Nm³ h^-1, which assure a maximum water evaporation of 130 kg h^-1. The ammonia and dust particles removal in the output gas was achieved with a vertical scrubber, using a solution of sulphuric acid. The pH of the resulting ammonium sulphate solution ((NH4)2SO4) is maintained at a value below 4, adding acid to the recirculation water. The empty bed contact time of the reactor was 2 seconds, with a gas velocity of 2 m s^-1. The overall electric power consumption of the drying unit was 11.5 kW (for the movement of digestate and the ventilation system).

1.1.2. Combustion unit: The combustion unit was designed in order to obtain a self-sufficient process, producing 150 kW of thermal energy from the dried digestate combustion. The burner was externally constituted of a triple-layer steel chassis and, internally, of a monolithic ring of refractory concrete (internal Ø 60 cm, external Ø 80 cm, depth 50 cm) with a high content of alumina (>50%) in order to assure corrosion resistance. The energy transfer from the combustion chamber to the dryer was obtained through a countercurrent heat exchanger with an efficiency of 70% and an electric power consumption of 2 kW. The water temperature in the heating circuit ranged between 70-90°C. An aqueous solution of urea (32.5%) was directly injected in the

Emissions of Gas and Dust from Livestock – Saint-Malo, France – May 21-24, 2017 170

Standard Methods [15]. Lower heating value (LHV) was measured according to UNI EN 15400:2011. Total nitrogen was measured according to UNI 10780:1998 method.

Analyses were carried out in duplicate, two-three times per month. Gases characteristics were determined according to UNI 10169/01, UNI EN 13284-1/03 and EPA CTM-34, at least one times per month.

2. RESULTS AND DISCUSSION: The main characteristics of digestate entering the scale plant, of the dried digestate and of ashes are shown in Table 3.

Table 4 reports the measured parameters of the flue gases coming from the drying and combustion processes.

Table 3. Characteristics of digestate (solid fraction), dried digestate and ashes.

Parameters U.M. Digestate (solid fraction) Dried digestate Ashes

TS % 25.6±1.5 92.8±2.8 98.9±0.2

VS % TS 71.5±7.8 71.7±10.4 0.4±0.1

LHV kJ kgTS-1

- 17 398±1 751 -

Nitrogen (total) g kg^-1 9.6±2.0 9.7±1.8 0.50±0.7

Table 4. Characteristics of the flue gases from drying and combustion processes and their emissions limits (dry gas, reference O2=5%).

* Under detection limit, ^** Total Particulate Matter

2.1. Mass balance: It can be observed that, after the drying process, the digestate lost 72% of its weight (as a consequence of water evaporation, 125-130 kg h^-1). At the same time, the 70% of the nitrogen initially present in the digestate was stripped as ammonia.

In fact almost all the nitrogen present into the dried digestate was in the organic form (92%). Finally the consumption of sulfuric acid for the ammonia abatement was measured at 130 kg d^-1 (3.65 kgH2SO4 kgNH3,removed-1

, +27% with respect to stoichiometric ratio) and the removal efficiency of the acid scrubber reached 97%. The vertical scrubber produced about 140 kg d^-1 of (NH₄)₂SO₄. Thereafter, the combustion process significantly reduced the initial mass of digestate (about 93%) producing ashes and oxidation gases. At the same time, nitrogen was almost completely converted to NOx.

The non-catalytic process for the abatement of NOx requests the dosage of urea. The consumption of urea was estimated at 47.3 kg d^-1 (1.58 kgCO(NH2)2 kgNO2,removed-1, +102%

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with respect to stoichiometric ratio). The removal of SOx implied an average consumption of Ca(OH)₂ of 43.8 kg d^-1 (2.41 kg_Ca(OH)2 kgSO2,removed-1, +117% with respect to stoichiometric ratio) and the process efficiency reached 80%. The plant produced about 72 kg d^-1 of CaSO3.

2.2. Energy balance: The LHV of dried digestate was measured equal to 17,398±1,751 kJ kgTS-1

. This value is equivalent to other common low-value fuels (such as lignite or wood pellets). The combustion generated a thermal power of about 778±80 MJ h^-1. The hot combustion flue gases flux represents a heat loss and a thermal flux of 171±39 MJ h^-1 with the flue gases can be estimated. The dryer requests a thermal power of 150 kW, which corresponds to 540 MJ h^-1. Therefore, the overall average energy balance appears to be positive but this value is overestimated, since the thermal flux through the surfaces was not considered. Nevertheless, the plant was self-sufficient for at least the 85% of the operation time. In winter, when the external air temperature was below 5°C, the heat exchanger was no able to reach the correct water temperature. Thus, for about 45 days per year, it was necessary to integrate the dried digestate with wood pellets in order to compensate the heat losses (2.5 kg h^-1, which assured an additional heat flux of about 50 MJ h^-1). A better thermal insulation of the structure, or a more efficient heat exchanger, could avoid the need of additional fuel.

2.3. Economic balance: The cost analyses refer to Italian prices and conditions and are reported in Table 5 (the monitored demonstration-scale plant, treating a third of the digestate production and a designed full-scale plant). A life-span of 15 years, with a replacement of the baghouse filters after 7.5 years and a 5% discount rate for the capital costs were assumed. The studied demonstrative-scale plant treats a third of the overall production of digestate at a cost of 85,980 € y^-1. Currently, the slaughterhouse spends 270,000 € y^-1 for the digestate in agriculture (90,000 € y^-1 considering the same quantity of digestate treated in the scale plant). Thus, the drying-combustion solution appears to be not particularly interesting. On the contrary, the designed full-scale plant would allow an estimated saving of 60,000 € y^-1, with a specific treatment cost of about 45-50 € t^-1. In this balance are not included the products of the process, (NH4)2SO4 and CaSO3. These products could have a destination in, respectively, fertilizers and cements production. However, due to the presence of impurities, it is much more plausible that these materials will have no economic value on the market.

Emissions of Gas and Dust from Livestock – Saint-Malo, France – May 21-24, 2017 172 Table 5. Economic evaluation of the demonstrative plant (real data) and of the full scale plant (data

extrapolated from the results of the demonstrative plant).

Demonstrative plant (real) Full plant (designed) can be reduced recovering, at the same time, a fraction of nitrogen under a chemically stable form (ammonium sulfate). The process can be self-sufficient and avoid the need of additional fuel, if the LHV of dried digestate is higher than 17,000-18,000 kJ kg^-1. An efficient thermal insulation is also required in order to minimize any heat loss.

Nevertheless, this solution appears feasible only for large anaerobic plants and/or when digestate cannot be used for agronomic purposes and has to be disposed as waste, since the system is expensive. From the economical evaluation of the present study it was observed that the drying-combustion process is affordable for a digestate production higher than 10 t d^-1 and for a conventional digestate disposal cost higher than 50 € t^-1. REFERENCES:

Palatsi J., Viñas M., Guivernau M., Fernandez B., Flotats X., 2011. Anaerobic digestion of slaughterhouse waste: Main process limitations and microbial community interactions. Bioresour. Technol., 2219-2227.

Pagés-Díaz J., Pereda-Reyes I., Taherzadeh M.J., Sárvári-Horváth I., Lundin M., 2014.

Anaerobic co-digestion of solid slaughterhouse wastes with agro-residues: Synergistic and antagonistic interactions determined in batch digestion assays. Chem. Eng. J., 89-98.

Kratzeisen M., Starcevic N., Martinov M., Maurer C., Müller J., 2010. Applicability of

biogas digestate as solid fuel. Fuel, 2544-2548.

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