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Sustainable paths for managing solid and liquid waste from distilleries and breweries
Accepted Manuscript Sustainable paths for managing solid and liquid waste from distilleries and breweries Bernd Weber, Ernst A. Stadlbauer PII:
S0959-6526(17)30268-8
DOI:
10.1016/j.jclepro.2017.02.054
Reference:
JCLP 8980
To appear in:
Journal of Cleaner Production
Received Date: 14 August 2016 Revised Date:
4 February 2017
Accepted Date: 7 February 2017
Please cite this article as: Weber B, Stadlbauer EA, Sustainable paths for managing solid and liquid waste from distilleries and breweries, Journal of Cleaner Production (2017), doi: 10.1016/ j.jclepro.2017.02.054. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Sustainable Paths for Managing Solid and Liquid Waste from Distilleries and Breweries Bernd Webera, Ernst A. Stadlbauerb a
Bernd Weber, Universidad Autónoma del Estado de México, Toluca, C.P. 03310, [email protected] b Ernst A. Stadlbauer, University of Applied Sciences, THM, Giessen, C.P. 35390, Germany
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Key Words: Spent grain, Vinasse, Hydro-Thermal Carbonization, Anaerobic Digester, Biogas
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Dr. Bernd Weber Facultad de Ingeniería Universidad Autónoma del Estado de México Cerro de Coatepec s/n C.P. 50130 Toluca, Edo. Mex. Mexico Tel. 0052722 2140855 Ext. 1064 FAX: 0052722 2140855 Ext. 1064 E-mail: [email protected]
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Abstract:
Attempts towards a more sustainable food industry focus on reducing the energy intensity of biomass conversion processes and finding better alternatives for the management of the large amounts of agro-industrial waste produced in such industries. While various sustainable practices for managing waste streams have been set forth in recent studies, their integration into established production schemes is lagging due the lack of an assessment of their technical feasibility and economic practicality. This study investigates the use of anaerobic digestion (AD) and
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Corresponding author:
hydrothermal carbonization (HTC) on wastewater and solid waste streams generated by the brewing and distilling industry to be considered for further implementation in a German brewery, which has already implemented AD for treating wastewater generated in brewing and distilling processes. The layout of the HTC reactor is based on previous
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experimental studies, which, when converting 895 kg h
-1
of wet spent grains, produces 257 kW of excess heat
available to cover energy demands in the brewing process. Exploitation of solid wastes by AD and HTC enables energy conversion in combined heat and power (CHP) systems which can meet more than 40% of electricity demands and more the 23% of thermal energy demands. Moreover, the mass of remaining solids is reduced to 43% of initial mass in the case of AD and 58% in the case of HTC treatment, which reduces energy demand for dehydrating solids significantly.
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1. Introduction:
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Transition to a low carbon economy has put focus both on the use of renewable or nuclear energy as well as the
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While there is a rising interest in further using these by-products, the challenge is in finding value adding processes or
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These valuable industrial techniques show promise for more efficient use of all resources in the food and beverage
increase in energy efficiency and reduction of resource consumption for industrial processes (DDPP, 2015). Basic ecostrategies for breweries include improving insulation and implementing heat recovery measures (Sturm et al., 2013). By-product management is an augmented approach to achieve lower carbon dioxide emissions. Presently, waste, such
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as liquid and solid wastes from breweries and distilleries, plays a variety of important roles in strategies aimed at sustainably managing the technical, economic, ecologic, and social aspects of the alcohol production industry. High strength brewery effluents are traditionally treated on-site in anaerobic sludge blanket reactors, fixed film reactors, and/or completely stirred tank reactors (Pettigrew et al., 2015). The current industry practice is to dispose of distillery molasses and sell brewery grain mash for use as an animal (Cook, 2011; Fillaudeau et al., 2005) or human (Rosa and
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Beloborodko, 2015) nutrition supplement, however, this industrial waste is increasingly being viewed as a useful by-
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product, from a more molecular point of view.
tailored applications for more energy efficient uses of industrial wastes. Some studies have reported on the feasibility of using spent grains from breweries as a nutrient base for actinobacteria or mushroom cultivation (Mussatto et al., 2006; Philippoussis, 2009). Extraction technologies applied to brewery spent grains focus on obtaining protein, in particular zein, which is used as feedstock for the production of coatings, plastics, textiles, and adhesives (Anderson and Lamsal, 2011), or the recovery of polyphenols as a feedstock for the antioxidant industry (Menesses et al., 2013). Alternatively, due to the high remaining lignocellulose in these wastes, other studies have been realized at the laboratory scale for
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alternative processes such as second generation bioethanol production (White et al., 2008), xylose to xylitol bioconversion (Mussato et al. 2005) or lactic acid fermentation to be used in bio-based plastics manufacturing (Shindo and Tachibana, 2004; Mussatto et al. 2008). At the same time, spent grains offer exceptional benefits with respect to decreasing energy dependency within breweries and the brewing process by providing a substrate for on-site anaerobic digestion (AD), combustion, or bio-char formation and thus promoting energy self-sufficiency. As an
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example, small anaerobic digesters are already available and are being used on high strength brewery streams as reported by Cummings (2015). The same trend has been observed for the use of spent grains in industrial incineration systems. Cook reports that more than 100 combinations of mechanical drying and combustion are already being
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implemented worldwide (Cook, 2011). Similarly, spent grains can undergo low temperature conversion at 350 °C to 500 °C to generate bio-oil and bio-char (Sanna et al., 2011) or may be subjected to hydrothermal carbonization (HTC) to produce bio-char with chemical characteristics (C/O) similar to lignite for on-site use (Poerschmann et al., 2014). For most procedures the reaction temperature is between 180 °C to 260 °C with heat released due to exother mic nature of the reaction (Kruse et al., 2012). As such, the process can be driven auto-thermally. The carbonaceous materials obtained in the process show a reduction in mass while at the same time exhibiting an increase in net calorific value (Stengl et al., 2012).
sector. On the one hand, energy demand for processing foods is considerably high, therefore, increasing plant
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efficiencies has the potential to be very beneficial. In developed countries alone, food processing accounts for 6% of
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With the intent of achieving sustainable development of industrial processes, the objective of this study is to evaluate
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Starting with the hypothesis that the energy demand of a facility producing beer and wine could be met by processing
the industrial primary energy demand (IEA, 2016). Meat and dairy production is considered the largest share of the food processing industry by commercial value, however, the production of alcoholic beverages is important as well. For example, in Germany the production of alcoholic beverages is fourth in the industry by market volume, generating 56 billion Euros (Statista, 2016). The production of alcoholic beverages is divided into small, medium, and big facilities, each with a different specific energy demand for processing their products. About 30% of beer production in Germany
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is carried out in medium sized breweries, which have a production rate of 5 000 to 500 000 hL per year (Marx, 2009). On the other hand, running a high efficiency production plant that effectively manages secondary products in a more -1
intelligent way is an on-going challenge. The specific energy intensity of brewing processes is about 150 MJ hL , which is showing a decreasing tendency over time with the ongoing implementation of technical and management improvements (Sturm et al., 2013). The energy recovery from spent grains by anaerobic fermentation can reach 36 MJ hL
-1
beer produced (Muster-Slawitsch et al., 2011). Decisions on which alternatives are chosen to promote more
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energy efficient production schemes are normally driven by economic considerations.
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the potential of energy recovery by AD of the effluent mixture of a medium sized brewery (90 000 hL y ) and wine -1
distillery (1 000 hL y ) in combination with the use of an HTC unit for conditioning spent grains. To achieve the aim of this study, (1) the design and performance of biogas fermenters in pilot and technical scale are described to ensure operational digester stability, (2) the principal design parameters of the HTC-Reactor are outlined, and (3) changes made to the initial energy management scheme to integrate excess heat are discussed. The assessment is based on a comparison between the existing brewery, which as a first step towards more sustainable production has already implemented AD for treating liquid waste streams (Strategy A), with two treatment alternatives for solid waste streams
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to promote energy recovery management: AD extended to wet spent grains, denominated as Strategy B, and the proposed novel integration of HTC for treating spent grains, termed Strategy C. Finally, conclusions are drawn on the role of AD and HTC as sustainable tools for effective energy use and greenhouse gas reduction.
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2. Methodology:
secondary organic streams in an intelligent manner, different technologies which convert organic matter into usable
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energy resources were evaluated. Fig. 1 shows possible solid waste management strategies for a medium sized brewery. Strategy A, corresponding to the existing brewery, is considered a typical brewery employing state of the art technology, where spent grains are commercialized and processed outside of production facility and wastewater is treated in an anaerobic digester. Strategy B consists of an anaerobic digester for treating liquid waste streams and a second digester for treating spent grains. Strategy C comprises an HTC-Reactor for treating spent grains. The brewery in this study also distills wine, therefore, vinasse management is considered as part of all the above process schemes (Strategies A, B and C).
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Secondary side streams in breweries and distilleries mainly consist of wastewater, vinasse, and spent grains. The energy content of the liquid secondary streams generated was assessed in the course of an investigation conducted on a local brewery in Germany. Over several weeks the wastewater generated and the organic contamination of specific flows, as measured by the chemical oxygen demand (COD), were quantified at selected points in the
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production facility. The volume of vinasse generated was taken from a logbook which registered vinasse transportation by tank vehicles. The organic content of the vinasse was also quantified by the COD. Based on the potential energy recovery from these wastes, AD was proposed as an appropriate process for treating such contaminated wastewater. The choice of treating high concentrations of dissolved organic matter in wastewater with high performance digesters, 3
some of which can reach a production capacity of more than 10 m of biogas per cubic meter of digester volume, is
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supported in the literature (Moletta, 2010).
The main stream of organic solid wastes in breweries is the generation of spent grains. The quantity of spent grains is
=
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determined using the overall brewhouse yield (OBY): ∑
(1)
The waste stream consists of spent grains, which is a wet solid product, and a small amount of wastewater, which is ignored. Under this assumption the yield of spent grains (SGY) is: = 1 −
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Evaluation of the potential for energy recovery from wastewater and spent grains
(2)
The chemical energy content of the solid waste streams can be assessed by the organic dry matter content (oDM). AD of spent grains (Strategy B) is a widely applied method that reduces oDM by producing biogas. Kafle and Kim (2013) 3
-1
reported that when organic matter is digested in co-fermentation, biogas yield (YBiogas) is approximately 0.55 m kg of oDM with a methane concentration of about 59%, which is the biogas generation potential used in energy balances in
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2.1.
this study.
For HTC treatment within the solids treatment process, energy content of the spent grains comprises the process heat released during thermal conversion, the biogas generated from the wastewater using AD, and residual solid product.
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The heat released during HTC treatment is determined by an enthalpy balance of process input and output flows. Because the exact chemical composition of spent grains is unknown, the enthalpy balance is alternatively based on the gross calorific values of its substrates and products as described by the following equation: ∆ℎ = ∙
+ " ∙
#
−
#$
(3)
Here, yL is the liquid phase yield of organic matter; yS is the solid phase yield; and HL, HS, and HSP are the gross calorific value of the liquid phase, solid phase and spent grains, respectively.
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30 31 32
produced during the carbonization of cellulosic matter, conceptualized as a dehydration reaction in combination with decarboxylation steps. The composition of the liquid phase has been described previously by the authors and includes acetic acid as the most prevalent compound, making up approximately 30% of the total organic matter found in the reaction water (Weber et al., 2013). The high organic content is well suited for AD and thus the overall biogas potential 3
is increased. For this kind of wastewater, a specific biogas production of 0.55 m kg
oDM was used for energy
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3
-1
balances, which results in a biogas production potential of 0.099 m of biogas per kg of wet spent grains (fresh mass, FM) processed. The biogas released during the AD process stage in a high performance digester in Strategy C is the sum of the wastewater generated in beer production, the vinasse discharged, and the additional wastewater from the HTC process.
The three process alternatives, Strategies A, B, and C, produce final solid products, each with a different specific
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generation potential and moisture content. A dehydrating step for each of the three alternatives was used as a comparable standard. Mass flow into the dehydrating process is different for each treatment because the mass loss depends on the specific reaction paths (e.g. formation of biogas during AD of spent grains). In the case of AD of spent
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grains, mass loss and solids concentration in the anaerobic effluent is determined with the following equations, taking into account specific biogas potential, elemental analysis of spent grains (C: 45.6%; H: 8.2%; N: 2.5%; S: 0.2% Ash: 4.4% and O: 39.1% - latter by difference), and water needed for hydrolysis of organic matter according to Symons and Buswell (1933):
%&.()(
* &.+,- .&.&++ &.&&*
+ 0.0812
+
→ 0.190 %+ + 0.274 %
(
+ 0.022 .
6
Where the water needed for hydrolysis, described by ;8 :,=>? @A>"B" 789 : = 9 ∆;@CD -1
is 0.125 kg kg
+ 0.001
+
(4),
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mashing. The water in the HTC process originates from the initial moisture present in spent grains and the water
oDM of spent grains. From the chemical equation, a methane concentration close to 60% was
determined. The remaining component was assumed to be CO2 since sulfur elimination is required as a plant
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In addition, HTC produces wastewater with mainly organic constituents since inorganics are mostly washed out during
precondition for running CHP systems. With respect to ammonia, the equilibrium is mainly shifted toward the -3
ammonium side due to pH in the range of 7. This corresponds to a biogas density ( ) of 1.21 kg m under standard conditions. The mass balance based on the water needed for hydrolysis must be equal to the produced biogas
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calculated with specific biogas yield. The following equation shows this balance: ∆;#E,CD,FGG? ∙ H1 + 789 : I = ;#E,@CD,FGG? ∙ JB@KL" ∙ MJB@KL"
(5)
Using ∆mSG,oDM,feed = mSG,oDM,feed - mSG,oDM,out, the residual fraction of digestate flowing out of digester is expressed by the following equation: ;NO,OP = ;,QP ∙ (1 − S,
2
) ∙ US#E,V"=
+ H1 − S#E,V"= I ∙ W1 −
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X ∙ MX (1 + 7
2
)
YZ
(6)
ACCEPTED MANUSCRIPT Where φSG,H2O and φSG,Ash are the fractions of moisture and ash in SG. The moisture content of anaerobic digestate is SNO,
=
φSG,H2O and φSG,Ash:
1 − (1 − S,
∙
X
∙ MX
(1 + 7
2
)
YZ
(8)
Finally the fraction of fresh mass in the final product from anaerobic digestion of spent grains is calculated with equation (9): ;NO,OP
1 − SNO,
2
(9)
The reactions occurring during the HTC process can be classified into a dehydration reaction with water as a reaction product and decarboxylation steps with CO2 as a reaction product, which is partially released into the gas phase. Based on the yield of HTC char from spent grains the concentration of solids in suspension leaving the HTC reactor (Fig. 1) is determined with the following equations: ;
^%,OP
S
= ;,QP ∙ \1 − S#E,8 : ] ∙ %ℎ , 9 ^%, 2
=
1 − ,
17 ^%,QP
=
^%
^%
− \1 − S#E,8 : ] ∙ %ℎ , 9 1 − ,
;
^%
^%
^%,OP
1−S
(10)
(11)
(12)
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;
18 19 20 21 22 23 24 25
2
) ∙ H1 − S#E,V"= I ∙ X ∙ MX
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2
7
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=
] ∙ US#E,V"= + H1 − S#E,V"= I ∙ W1 + 2
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2
1 − \1 − S,
;NO,QP =
10 11 12 13 14 15
(7)
and, using this information, the moisture content of the anaerobic digestate is expressed as a function of YBiogas, µH2O ,
SNO,
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;VC,[D − ;VC,CD ;#E,[D − ;JB@KL"
assessed with the following mass balance:
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^%, 2
Similar to anaerobic digestion of spent grains (Equations 6, 8, and 9) equations 10, 11, and 12 calculate the fraction of HTC char produced, the moisture content (at the outlet of corresponding treatments), and the fresh mass fraction, which has to be dehydrated. The dehydration of the solid products to a residual moisture content of 12% was realized by subjecting the solid products to a mechanical step which reduced moisture content to 60% followed by a subsequent step in a convection -1
dryer. The specific energy of 40 kWh t of fresh mass needed for mechanical dehydration in a screw press was taken from BMU (2012). Weger et al. (2014) reports similar energy requirements for mechanical dewatering of spent grains.
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spent grains. This study assumes the same specific energy for all three solid streams. The specific energy required for thermal drying of spent grains (2.93 MJ t
-1
of water removed) was taken from Dorset (2016) and also used for the
digestate and carbonized product.
2.2.
Hydrothermal carbonization (HTC) treatment
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HTC of spent grains is considered as an innovative treatment of waste streams and is carried out to obtain a carbonaceous material with an enriched net calorific value and an improved dewatering ability. In this form, the converted waste is viewed as a value added product, which has market potential as an alternative fuel or soil conditioner (Libra et al., 2010; Zhengang et al., 2013). The conversion of spent grains was realized at a laboratory scale under various temperature conditions between 180 °C and 260 °C with a constant reaction time of 3 hours (Weber et al., 2013). It is worth mentioning that longer reaction times on a technical scale are not feasible from an
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economic point of view.
This study outlines the design of a HTC reactor for processing spent grains originating in the local brewery with a reaction temperature of 220 °C and a reaction time of 3 hours. Under these reaction conditions the HTC conversion
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transforms 18% of the organic matter into the liquid phase (Weber et al., 2013). The HTC Process can be realized in batch or continuous operation. A current version of the operation in batch mode is demonstrated in a pilot plant at the Institute of Technology in Karlsruhe, Germany (Kruse et al., 2013). Alternatively, the continuous operation has the advantage of needing only one screw feeder to move the substrate. The reactor dimensions and energy balance for such a configuration are calculated in this study. The screw feeder is divided into three zones: Zone I provides the energy to heat the substrate, Zone II serves as the reaction section, and Zone III acts to cool the carbonaceous
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product (Fig. 2).
A pressureless thermal oil system should be used as an intermediate heat carrier for all heat transfers realized throughout the tube reactor. The principal design parameters of the HTC-Reactor ensure heat transfer through the tube
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There is a lack of information about the specific energy needed for mechanical dehydrating of digested or carbonized
shell (about 20 mm thick) in the different zones and the residence time of the substrate in the reaction zone (3 hours). The convective heat transfer coefficient from the reactor shell to the biomass is calculated with the dimensionless Nusselt number using Equation 13:
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*
. = _` ∙ ab c ∙ d 6 (
7 &.*( ) 7eLAA
(13)
where the Reynolds number is: ab =
∙ f ∙ + 7
(14)
In Liepe (1982), values of cNu and m for screw mixers are given as 0.4 and 0.62, respectively. These values result in -2
-1
-2
-1
total heat transfer coefficients from 100 W m K to 150 W m K as screw diameters vary from 0.325 m to 0.625 m.
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ACCEPTED MANUSCRIPT Such heat transfer coefficients are in accordance with experimental arrangements for applications of screw heat exchangers in sludge drying in the sticky phase with about 30 to 50% solid content (Koellemann, 2016). The specific surface per meter of reactor length can be increased with parallel operation of various screw reactors, which also allows for more flexible production. The authors used a method of trial and error to determine temperatures and the length of the three zones of the reactor acting as heat exchangers which is described as follows. Zones I and III function as counter flow heat exchangers
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while Zone II, with respect to the outer jacket, functions as a parallel flow heat exchanger. For a given length of Zone III the effluent temperature inside the screw was determined under the following conditions: product temperature at the outlet at 80 °C to 90 °C, thermal oil temperature a t the inlet at 80 °C, and thermal oil temperature a t the jacket outlet at 182 °C. For Zone III, the heat capacity rates of th e thermal oil along the screw and through the jacket were determined using the energy balance for the same conditions. The flow inside the screw was set as constant and the temperature between Zone I and II was set at 180°C, the tempera ture at which the speed of the reaction starts increasing rapidly.
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Thus, the temperature at the outlet of the screw in Zone I was selected such that the thermal oil flow inside the screw is equal in Zone I, II and III. Using a chosen outlet temperature for thermal oil in Zone II in the jacket of 214°C, which must be lower than the maximum reaction temperature, the inlet temperature of the thermal oil into the jacket was
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calculated under the condition that heat transferred to the heat carrier medium is equal to the heat of reaction released. With the inlet and outlet temperatures determined for the jacket in Zone II, the heat capacity rate of the flow was determined from the energy balance. The heat capacity rate in the Zone I jacket is higher than in the coupled Zone III because a fraction of the thermal oil at its highest temperature (in Zone II) is combined with that of Zone III before being injected into the jacket of Zone I. Next, the reactor length of Zone I and thermal oil effluent flow in the jacket were adjusted such that heat transferred is set equal to the temperature increment of the product from 50 °C to 180 °C and the mass flow of thermal oil balances with the mass flows of the thermal oil in Zones II and III. Finally, the temperatures
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of the thermal oil must be equal in interconnected sections. The energy available for the consumer at high and low temperature levels was then determined. Thus, the energy available for consumers at high and low temperature levels is a function of the fraction of thermal oil from Zone II combined with that of Zone III before entering into Zone I.
Full scale anaerobic digester
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2.3.
The design of the full scale anaerobic digester for liquid effluents was based on extensive experimental data from 3
-3
running a 1.5 m pilot plant. At the pilot-scale, an 80% reduction in COD with an organic loading rate of 10 ± 3 kg m d 1
-
was obtained (Stadlbauer et al., 2001). The full scale anaerobic digester was then built as a combination of a
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continuous stirred tank reactor (CSTR) and an up-flow biofilm reactor. The volume of the CSTR reactor was 21 m and the volume of the biofilm reactor for methanization was 14 m3. This configuration was chosen to incorporate sulfate reduction in the hydrolysis reactor. As pretreatment for solids separation, sieves with 4 mm, 1.5 mm, and 1 mm mesh size were installed at the inlet. Between the hydrolysis tank and the methanization tank, a two stage pH titration system was installed. Effluents from the methanization reactor were then passed through a sedimentation tank and a percolation column where FeCl3 was added and biogas was blown counter-current through the wastewater effluent to remove H2S. The biogas was burnt with a Dreizler burner in a Vitola Biferral VB040 boiler from the Viessmann Company, Germany. The thermal energy released was used for the reactor heating and bottle washing stages of the beer production process. Operation of the plant was controlled using analytical measurements throughout the process
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to ensure the stability of the digesters. The pilot plant was operated for 50 weeks to allow sufficient time for the growth
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3.1.
of anaerobic microorganisms on the support material in the anaerobic filter after initial seeding with sludge originating from the anaerobic digester of a neighboring brewery. After 24 weeks, sulfate containing vinasse was fed into the digester and the performance of the digesters was evaluated during a subsequent adaptation phase to assure stabilization. Initializing digesters and substrate changes should be realized slowly due to slow bacterial growth of the
2.4.
Energy demand for beer and distilled wine production
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anaerobic microorganisms (Barber and Stukey, 1999).
Beer production requires high quantities of thermal and electric energy, especially during malt extraction, wort boiling, and fermentation. In the literature there is a wealth of information about specific energy demands of the various stages of beer production (Sattler, 2000; Kubule et al., 2016), which was compiled to estimate the energy demands of the
2.5.
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brewery.
Balance between energy demand of production facility and conversion potential of secondary streams
The overall energy balance of the brewery and distillery was determined by the demands of the different production
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steps and the energy recovery by running CHP units with biogas obtained from anaerobic digestion of the liquid and -3
solid waste streams. The analysis for all scenarios was based on a net calorific value of 6.8 kWh m for the biogas, electric efficiency of 38% for the CHP system, and recovery of 80% for the thermal energy. The internal power demand of the biogas digesters was assumed to be 3% of the electric energy produced for the liquid stream fermenter and 5% for the solid stream fermenter (VDI, 4631). Estimation of energy loss through the fermenter shell was based on an -2
-1
overall heat transfer coefficient of 0.8 W m K and an average temperature difference between the fermenter and ambient temperature of 26 °C. Because spent grains and most wastewaters are produced at higher temperatures, an
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energy demand for heating up the substrate was not considered in the balance. The operational change as a result of HTC treatment integration (Strategy C) produces thermal energy at a high temperature level which can also be used in production processes.
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3. Results and Discussion:
Energy demand of brewing and wine distillation
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The implementation of more energy efficient production methods requires the identification of possible measures to reduce energy as a first step. A compilation of beer production energy requirements reported in the literature is shown in Table 1 (Sattler, 2000). Table 1 shows that scaling factors play a role in energy requirements, as bigger production facilities are more efficient (Russel et al., 2008). Specific thermal energy demand exhibits a steeper drop from the first category to the second than from the second category to the third. This is because smaller plants with a production -1
lower than 200 000 hL y have to switch to discontinuous production, thus losing more opportunities for heat recovery.
35
S0959-6526(17)30268-8
DOI:
10.1016/j.jclepro.2017.02.054
Reference:
JCLP 8980
To appear in:
Journal of Cleaner Production
Received Date: 14 August 2016 Revised Date:
4 February 2017
Accepted Date: 7 February 2017
Please cite this article as: Weber B, Stadlbauer EA, Sustainable paths for managing solid and liquid waste from distilleries and breweries, Journal of Cleaner Production (2017), doi: 10.1016/ j.jclepro.2017.02.054. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Sustainable Paths for Managing Solid and Liquid Waste from Distilleries and Breweries Bernd Webera, Ernst A. Stadlbauerb a
Bernd Weber, Universidad Autónoma del Estado de México, Toluca, C.P. 03310, [email protected] b Ernst A. Stadlbauer, University of Applied Sciences, THM, Giessen, C.P. 35390, Germany
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Key Words: Spent grain, Vinasse, Hydro-Thermal Carbonization, Anaerobic Digester, Biogas
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Dr. Bernd Weber Facultad de Ingeniería Universidad Autónoma del Estado de México Cerro de Coatepec s/n C.P. 50130 Toluca, Edo. Mex. Mexico Tel. 0052722 2140855 Ext. 1064 FAX: 0052722 2140855 Ext. 1064 E-mail: [email protected]
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Abstract:
Attempts towards a more sustainable food industry focus on reducing the energy intensity of biomass conversion processes and finding better alternatives for the management of the large amounts of agro-industrial waste produced in such industries. While various sustainable practices for managing waste streams have been set forth in recent studies, their integration into established production schemes is lagging due the lack of an assessment of their technical feasibility and economic practicality. This study investigates the use of anaerobic digestion (AD) and
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3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Corresponding author:
hydrothermal carbonization (HTC) on wastewater and solid waste streams generated by the brewing and distilling industry to be considered for further implementation in a German brewery, which has already implemented AD for treating wastewater generated in brewing and distilling processes. The layout of the HTC reactor is based on previous
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2
experimental studies, which, when converting 895 kg h
-1
of wet spent grains, produces 257 kW of excess heat
available to cover energy demands in the brewing process. Exploitation of solid wastes by AD and HTC enables energy conversion in combined heat and power (CHP) systems which can meet more than 40% of electricity demands and more the 23% of thermal energy demands. Moreover, the mass of remaining solids is reduced to 43% of initial mass in the case of AD and 58% in the case of HTC treatment, which reduces energy demand for dehydrating solids significantly.
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1. Introduction:
3 4 5 6 7 8 9 10 11 12 13
Transition to a low carbon economy has put focus both on the use of renewable or nuclear energy as well as the
14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
While there is a rising interest in further using these by-products, the challenge is in finding value adding processes or
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These valuable industrial techniques show promise for more efficient use of all resources in the food and beverage
increase in energy efficiency and reduction of resource consumption for industrial processes (DDPP, 2015). Basic ecostrategies for breweries include improving insulation and implementing heat recovery measures (Sturm et al., 2013). By-product management is an augmented approach to achieve lower carbon dioxide emissions. Presently, waste, such
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as liquid and solid wastes from breweries and distilleries, plays a variety of important roles in strategies aimed at sustainably managing the technical, economic, ecologic, and social aspects of the alcohol production industry. High strength brewery effluents are traditionally treated on-site in anaerobic sludge blanket reactors, fixed film reactors, and/or completely stirred tank reactors (Pettigrew et al., 2015). The current industry practice is to dispose of distillery molasses and sell brewery grain mash for use as an animal (Cook, 2011; Fillaudeau et al., 2005) or human (Rosa and
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Beloborodko, 2015) nutrition supplement, however, this industrial waste is increasingly being viewed as a useful by-
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product, from a more molecular point of view.
tailored applications for more energy efficient uses of industrial wastes. Some studies have reported on the feasibility of using spent grains from breweries as a nutrient base for actinobacteria or mushroom cultivation (Mussatto et al., 2006; Philippoussis, 2009). Extraction technologies applied to brewery spent grains focus on obtaining protein, in particular zein, which is used as feedstock for the production of coatings, plastics, textiles, and adhesives (Anderson and Lamsal, 2011), or the recovery of polyphenols as a feedstock for the antioxidant industry (Menesses et al., 2013). Alternatively, due to the high remaining lignocellulose in these wastes, other studies have been realized at the laboratory scale for
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alternative processes such as second generation bioethanol production (White et al., 2008), xylose to xylitol bioconversion (Mussato et al. 2005) or lactic acid fermentation to be used in bio-based plastics manufacturing (Shindo and Tachibana, 2004; Mussatto et al. 2008). At the same time, spent grains offer exceptional benefits with respect to decreasing energy dependency within breweries and the brewing process by providing a substrate for on-site anaerobic digestion (AD), combustion, or bio-char formation and thus promoting energy self-sufficiency. As an
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example, small anaerobic digesters are already available and are being used on high strength brewery streams as reported by Cummings (2015). The same trend has been observed for the use of spent grains in industrial incineration systems. Cook reports that more than 100 combinations of mechanical drying and combustion are already being
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implemented worldwide (Cook, 2011). Similarly, spent grains can undergo low temperature conversion at 350 °C to 500 °C to generate bio-oil and bio-char (Sanna et al., 2011) or may be subjected to hydrothermal carbonization (HTC) to produce bio-char with chemical characteristics (C/O) similar to lignite for on-site use (Poerschmann et al., 2014). For most procedures the reaction temperature is between 180 °C to 260 °C with heat released due to exother mic nature of the reaction (Kruse et al., 2012). As such, the process can be driven auto-thermally. The carbonaceous materials obtained in the process show a reduction in mass while at the same time exhibiting an increase in net calorific value (Stengl et al., 2012).
sector. On the one hand, energy demand for processing foods is considerably high, therefore, increasing plant
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efficiencies has the potential to be very beneficial. In developed countries alone, food processing accounts for 6% of
14 15 16 17 18 19 20 21 22 23 24 25 26
With the intent of achieving sustainable development of industrial processes, the objective of this study is to evaluate
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Starting with the hypothesis that the energy demand of a facility producing beer and wine could be met by processing
the industrial primary energy demand (IEA, 2016). Meat and dairy production is considered the largest share of the food processing industry by commercial value, however, the production of alcoholic beverages is important as well. For example, in Germany the production of alcoholic beverages is fourth in the industry by market volume, generating 56 billion Euros (Statista, 2016). The production of alcoholic beverages is divided into small, medium, and big facilities, each with a different specific energy demand for processing their products. About 30% of beer production in Germany
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is carried out in medium sized breweries, which have a production rate of 5 000 to 500 000 hL per year (Marx, 2009). On the other hand, running a high efficiency production plant that effectively manages secondary products in a more -1
intelligent way is an on-going challenge. The specific energy intensity of brewing processes is about 150 MJ hL , which is showing a decreasing tendency over time with the ongoing implementation of technical and management improvements (Sturm et al., 2013). The energy recovery from spent grains by anaerobic fermentation can reach 36 MJ hL
-1
beer produced (Muster-Slawitsch et al., 2011). Decisions on which alternatives are chosen to promote more
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energy efficient production schemes are normally driven by economic considerations.
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the potential of energy recovery by AD of the effluent mixture of a medium sized brewery (90 000 hL y ) and wine -1
distillery (1 000 hL y ) in combination with the use of an HTC unit for conditioning spent grains. To achieve the aim of this study, (1) the design and performance of biogas fermenters in pilot and technical scale are described to ensure operational digester stability, (2) the principal design parameters of the HTC-Reactor are outlined, and (3) changes made to the initial energy management scheme to integrate excess heat are discussed. The assessment is based on a comparison between the existing brewery, which as a first step towards more sustainable production has already implemented AD for treating liquid waste streams (Strategy A), with two treatment alternatives for solid waste streams
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to promote energy recovery management: AD extended to wet spent grains, denominated as Strategy B, and the proposed novel integration of HTC for treating spent grains, termed Strategy C. Finally, conclusions are drawn on the role of AD and HTC as sustainable tools for effective energy use and greenhouse gas reduction.
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2. Methodology:
secondary organic streams in an intelligent manner, different technologies which convert organic matter into usable
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energy resources were evaluated. Fig. 1 shows possible solid waste management strategies for a medium sized brewery. Strategy A, corresponding to the existing brewery, is considered a typical brewery employing state of the art technology, where spent grains are commercialized and processed outside of production facility and wastewater is treated in an anaerobic digester. Strategy B consists of an anaerobic digester for treating liquid waste streams and a second digester for treating spent grains. Strategy C comprises an HTC-Reactor for treating spent grains. The brewery in this study also distills wine, therefore, vinasse management is considered as part of all the above process schemes (Strategies A, B and C).
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Secondary side streams in breweries and distilleries mainly consist of wastewater, vinasse, and spent grains. The energy content of the liquid secondary streams generated was assessed in the course of an investigation conducted on a local brewery in Germany. Over several weeks the wastewater generated and the organic contamination of specific flows, as measured by the chemical oxygen demand (COD), were quantified at selected points in the
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production facility. The volume of vinasse generated was taken from a logbook which registered vinasse transportation by tank vehicles. The organic content of the vinasse was also quantified by the COD. Based on the potential energy recovery from these wastes, AD was proposed as an appropriate process for treating such contaminated wastewater. The choice of treating high concentrations of dissolved organic matter in wastewater with high performance digesters, 3
some of which can reach a production capacity of more than 10 m of biogas per cubic meter of digester volume, is
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supported in the literature (Moletta, 2010).
The main stream of organic solid wastes in breweries is the generation of spent grains. The quantity of spent grains is
=
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determined using the overall brewhouse yield (OBY): ∑
(1)
The waste stream consists of spent grains, which is a wet solid product, and a small amount of wastewater, which is ignored. Under this assumption the yield of spent grains (SGY) is: = 1 −
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Evaluation of the potential for energy recovery from wastewater and spent grains
(2)
The chemical energy content of the solid waste streams can be assessed by the organic dry matter content (oDM). AD of spent grains (Strategy B) is a widely applied method that reduces oDM by producing biogas. Kafle and Kim (2013) 3
-1
reported that when organic matter is digested in co-fermentation, biogas yield (YBiogas) is approximately 0.55 m kg of oDM with a methane concentration of about 59%, which is the biogas generation potential used in energy balances in
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17 18 19 20
2.1.
this study.
For HTC treatment within the solids treatment process, energy content of the spent grains comprises the process heat released during thermal conversion, the biogas generated from the wastewater using AD, and residual solid product.
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The heat released during HTC treatment is determined by an enthalpy balance of process input and output flows. Because the exact chemical composition of spent grains is unknown, the enthalpy balance is alternatively based on the gross calorific values of its substrates and products as described by the following equation: ∆ℎ = ∙
+ " ∙
#
−
#$
(3)
Here, yL is the liquid phase yield of organic matter; yS is the solid phase yield; and HL, HS, and HSP are the gross calorific value of the liquid phase, solid phase and spent grains, respectively.
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30 31 32
produced during the carbonization of cellulosic matter, conceptualized as a dehydration reaction in combination with decarboxylation steps. The composition of the liquid phase has been described previously by the authors and includes acetic acid as the most prevalent compound, making up approximately 30% of the total organic matter found in the reaction water (Weber et al., 2013). The high organic content is well suited for AD and thus the overall biogas potential 3
is increased. For this kind of wastewater, a specific biogas production of 0.55 m kg
oDM was used for energy
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3
-1
balances, which results in a biogas production potential of 0.099 m of biogas per kg of wet spent grains (fresh mass, FM) processed. The biogas released during the AD process stage in a high performance digester in Strategy C is the sum of the wastewater generated in beer production, the vinasse discharged, and the additional wastewater from the HTC process.
The three process alternatives, Strategies A, B, and C, produce final solid products, each with a different specific
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generation potential and moisture content. A dehydrating step for each of the three alternatives was used as a comparable standard. Mass flow into the dehydrating process is different for each treatment because the mass loss depends on the specific reaction paths (e.g. formation of biogas during AD of spent grains). In the case of AD of spent
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grains, mass loss and solids concentration in the anaerobic effluent is determined with the following equations, taking into account specific biogas potential, elemental analysis of spent grains (C: 45.6%; H: 8.2%; N: 2.5%; S: 0.2% Ash: 4.4% and O: 39.1% - latter by difference), and water needed for hydrolysis of organic matter according to Symons and Buswell (1933):
%&.()(
* &.+,- .&.&++ &.&&*
+ 0.0812
+
→ 0.190 %+ + 0.274 %
(
+ 0.022 .
6
Where the water needed for hydrolysis, described by ;8 :,=>? @A>"B" 789 : = 9 ∆;@CD -1
is 0.125 kg kg
+ 0.001
+
(4),
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mashing. The water in the HTC process originates from the initial moisture present in spent grains and the water
oDM of spent grains. From the chemical equation, a methane concentration close to 60% was
determined. The remaining component was assumed to be CO2 since sulfur elimination is required as a plant
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In addition, HTC produces wastewater with mainly organic constituents since inorganics are mostly washed out during
precondition for running CHP systems. With respect to ammonia, the equilibrium is mainly shifted toward the -3
ammonium side due to pH in the range of 7. This corresponds to a biogas density ( ) of 1.21 kg m under standard conditions. The mass balance based on the water needed for hydrolysis must be equal to the produced biogas
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calculated with specific biogas yield. The following equation shows this balance: ∆;#E,CD,FGG? ∙ H1 + 789 : I = ;#E,@CD,FGG? ∙ JB@KL" ∙ MJB@KL"
(5)
Using ∆mSG,oDM,feed = mSG,oDM,feed - mSG,oDM,out, the residual fraction of digestate flowing out of digester is expressed by the following equation: ;NO,OP = ;,QP ∙ (1 − S,
2
) ∙ US#E,V"=
+ H1 − S#E,V"= I ∙ W1 −
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X ∙ MX (1 + 7
2
)
YZ
(6)
ACCEPTED MANUSCRIPT Where φSG,H2O and φSG,Ash are the fractions of moisture and ash in SG. The moisture content of anaerobic digestate is SNO,
=
φSG,H2O and φSG,Ash:
1 − (1 − S,
∙
X
∙ MX
(1 + 7
2
)
YZ
(8)
Finally the fraction of fresh mass in the final product from anaerobic digestion of spent grains is calculated with equation (9): ;NO,OP
1 − SNO,
2
(9)
The reactions occurring during the HTC process can be classified into a dehydration reaction with water as a reaction product and decarboxylation steps with CO2 as a reaction product, which is partially released into the gas phase. Based on the yield of HTC char from spent grains the concentration of solids in suspension leaving the HTC reactor (Fig. 1) is determined with the following equations: ;
^%,OP
S
= ;,QP ∙ \1 − S#E,8 : ] ∙ %ℎ , 9 ^%, 2
=
1 − ,
17 ^%,QP
=
^%
^%
− \1 − S#E,8 : ] ∙ %ℎ , 9 1 − ,
;
^%
^%
^%,OP
1−S
(10)
(11)
(12)
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;
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2
) ∙ H1 − S#E,V"= I ∙ X ∙ MX
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2
7
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=
] ∙ US#E,V"= + H1 − S#E,V"= I ∙ W1 + 2
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2
1 − \1 − S,
;NO,QP =
10 11 12 13 14 15
(7)
and, using this information, the moisture content of the anaerobic digestate is expressed as a function of YBiogas, µH2O ,
SNO,
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2
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3 4 5
;VC,[D − ;VC,CD ;#E,[D − ;JB@KL"
assessed with the following mass balance:
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1 2
^%, 2
Similar to anaerobic digestion of spent grains (Equations 6, 8, and 9) equations 10, 11, and 12 calculate the fraction of HTC char produced, the moisture content (at the outlet of corresponding treatments), and the fresh mass fraction, which has to be dehydrated. The dehydration of the solid products to a residual moisture content of 12% was realized by subjecting the solid products to a mechanical step which reduced moisture content to 60% followed by a subsequent step in a convection -1
dryer. The specific energy of 40 kWh t of fresh mass needed for mechanical dehydration in a screw press was taken from BMU (2012). Weger et al. (2014) reports similar energy requirements for mechanical dewatering of spent grains.
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spent grains. This study assumes the same specific energy for all three solid streams. The specific energy required for thermal drying of spent grains (2.93 MJ t
-1
of water removed) was taken from Dorset (2016) and also used for the
digestate and carbonized product.
2.2.
Hydrothermal carbonization (HTC) treatment
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HTC of spent grains is considered as an innovative treatment of waste streams and is carried out to obtain a carbonaceous material with an enriched net calorific value and an improved dewatering ability. In this form, the converted waste is viewed as a value added product, which has market potential as an alternative fuel or soil conditioner (Libra et al., 2010; Zhengang et al., 2013). The conversion of spent grains was realized at a laboratory scale under various temperature conditions between 180 °C and 260 °C with a constant reaction time of 3 hours (Weber et al., 2013). It is worth mentioning that longer reaction times on a technical scale are not feasible from an
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economic point of view.
This study outlines the design of a HTC reactor for processing spent grains originating in the local brewery with a reaction temperature of 220 °C and a reaction time of 3 hours. Under these reaction conditions the HTC conversion
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transforms 18% of the organic matter into the liquid phase (Weber et al., 2013). The HTC Process can be realized in batch or continuous operation. A current version of the operation in batch mode is demonstrated in a pilot plant at the Institute of Technology in Karlsruhe, Germany (Kruse et al., 2013). Alternatively, the continuous operation has the advantage of needing only one screw feeder to move the substrate. The reactor dimensions and energy balance for such a configuration are calculated in this study. The screw feeder is divided into three zones: Zone I provides the energy to heat the substrate, Zone II serves as the reaction section, and Zone III acts to cool the carbonaceous
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product (Fig. 2).
A pressureless thermal oil system should be used as an intermediate heat carrier for all heat transfers realized throughout the tube reactor. The principal design parameters of the HTC-Reactor ensure heat transfer through the tube
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There is a lack of information about the specific energy needed for mechanical dehydrating of digested or carbonized
shell (about 20 mm thick) in the different zones and the residence time of the substrate in the reaction zone (3 hours). The convective heat transfer coefficient from the reactor shell to the biomass is calculated with the dimensionless Nusselt number using Equation 13:
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*
. = _` ∙ ab c ∙ d 6 (
7 &.*( ) 7eLAA
(13)
where the Reynolds number is: ab =
∙ f ∙ + 7
(14)
In Liepe (1982), values of cNu and m for screw mixers are given as 0.4 and 0.62, respectively. These values result in -2
-1
-2
-1
total heat transfer coefficients from 100 W m K to 150 W m K as screw diameters vary from 0.325 m to 0.625 m.
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ACCEPTED MANUSCRIPT Such heat transfer coefficients are in accordance with experimental arrangements for applications of screw heat exchangers in sludge drying in the sticky phase with about 30 to 50% solid content (Koellemann, 2016). The specific surface per meter of reactor length can be increased with parallel operation of various screw reactors, which also allows for more flexible production. The authors used a method of trial and error to determine temperatures and the length of the three zones of the reactor acting as heat exchangers which is described as follows. Zones I and III function as counter flow heat exchangers
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while Zone II, with respect to the outer jacket, functions as a parallel flow heat exchanger. For a given length of Zone III the effluent temperature inside the screw was determined under the following conditions: product temperature at the outlet at 80 °C to 90 °C, thermal oil temperature a t the inlet at 80 °C, and thermal oil temperature a t the jacket outlet at 182 °C. For Zone III, the heat capacity rates of th e thermal oil along the screw and through the jacket were determined using the energy balance for the same conditions. The flow inside the screw was set as constant and the temperature between Zone I and II was set at 180°C, the tempera ture at which the speed of the reaction starts increasing rapidly.
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Thus, the temperature at the outlet of the screw in Zone I was selected such that the thermal oil flow inside the screw is equal in Zone I, II and III. Using a chosen outlet temperature for thermal oil in Zone II in the jacket of 214°C, which must be lower than the maximum reaction temperature, the inlet temperature of the thermal oil into the jacket was
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calculated under the condition that heat transferred to the heat carrier medium is equal to the heat of reaction released. With the inlet and outlet temperatures determined for the jacket in Zone II, the heat capacity rate of the flow was determined from the energy balance. The heat capacity rate in the Zone I jacket is higher than in the coupled Zone III because a fraction of the thermal oil at its highest temperature (in Zone II) is combined with that of Zone III before being injected into the jacket of Zone I. Next, the reactor length of Zone I and thermal oil effluent flow in the jacket were adjusted such that heat transferred is set equal to the temperature increment of the product from 50 °C to 180 °C and the mass flow of thermal oil balances with the mass flows of the thermal oil in Zones II and III. Finally, the temperatures
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of the thermal oil must be equal in interconnected sections. The energy available for the consumer at high and low temperature levels was then determined. Thus, the energy available for consumers at high and low temperature levels is a function of the fraction of thermal oil from Zone II combined with that of Zone III before entering into Zone I.
Full scale anaerobic digester
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2.3.
The design of the full scale anaerobic digester for liquid effluents was based on extensive experimental data from 3
-3
running a 1.5 m pilot plant. At the pilot-scale, an 80% reduction in COD with an organic loading rate of 10 ± 3 kg m d 1
-
was obtained (Stadlbauer et al., 2001). The full scale anaerobic digester was then built as a combination of a
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3
continuous stirred tank reactor (CSTR) and an up-flow biofilm reactor. The volume of the CSTR reactor was 21 m and the volume of the biofilm reactor for methanization was 14 m3. This configuration was chosen to incorporate sulfate reduction in the hydrolysis reactor. As pretreatment for solids separation, sieves with 4 mm, 1.5 mm, and 1 mm mesh size were installed at the inlet. Between the hydrolysis tank and the methanization tank, a two stage pH titration system was installed. Effluents from the methanization reactor were then passed through a sedimentation tank and a percolation column where FeCl3 was added and biogas was blown counter-current through the wastewater effluent to remove H2S. The biogas was burnt with a Dreizler burner in a Vitola Biferral VB040 boiler from the Viessmann Company, Germany. The thermal energy released was used for the reactor heating and bottle washing stages of the beer production process. Operation of the plant was controlled using analytical measurements throughout the process
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to ensure the stability of the digesters. The pilot plant was operated for 50 weeks to allow sufficient time for the growth
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3.1.
of anaerobic microorganisms on the support material in the anaerobic filter after initial seeding with sludge originating from the anaerobic digester of a neighboring brewery. After 24 weeks, sulfate containing vinasse was fed into the digester and the performance of the digesters was evaluated during a subsequent adaptation phase to assure stabilization. Initializing digesters and substrate changes should be realized slowly due to slow bacterial growth of the
2.4.
Energy demand for beer and distilled wine production
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anaerobic microorganisms (Barber and Stukey, 1999).
Beer production requires high quantities of thermal and electric energy, especially during malt extraction, wort boiling, and fermentation. In the literature there is a wealth of information about specific energy demands of the various stages of beer production (Sattler, 2000; Kubule et al., 2016), which was compiled to estimate the energy demands of the
2.5.
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brewery.
Balance between energy demand of production facility and conversion potential of secondary streams
The overall energy balance of the brewery and distillery was determined by the demands of the different production
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steps and the energy recovery by running CHP units with biogas obtained from anaerobic digestion of the liquid and -3
solid waste streams. The analysis for all scenarios was based on a net calorific value of 6.8 kWh m for the biogas, electric efficiency of 38% for the CHP system, and recovery of 80% for the thermal energy. The internal power demand of the biogas digesters was assumed to be 3% of the electric energy produced for the liquid stream fermenter and 5% for the solid stream fermenter (VDI, 4631). Estimation of energy loss through the fermenter shell was based on an -2
-1
overall heat transfer coefficient of 0.8 W m K and an average temperature difference between the fermenter and ambient temperature of 26 °C. Because spent grains and most wastewaters are produced at higher temperatures, an
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energy demand for heating up the substrate was not considered in the balance. The operational change as a result of HTC treatment integration (Strategy C) produces thermal energy at a high temperature level which can also be used in production processes.
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3. Results and Discussion:
Energy demand of brewing and wine distillation
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The implementation of more energy efficient production methods requires the identification of possible measures to reduce energy as a first step. A compilation of beer production energy requirements reported in the literature is shown in Table 1 (Sattler, 2000). Table 1 shows that scaling factors play a role in energy requirements, as bigger production facilities are more efficient (Russel et al., 2008). Specific thermal energy demand exhibits a steeper drop from the first category to the second than from the second category to the third. This is because smaller plants with a production -1
lower than 200 000 hL y have to switch to discontinuous production, thus losing more opportunities for heat recovery.
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