Bioenergy in milk production
- 1 Description
- 2 Biomass combustion
- 2.1 Fixed bed combustion
- 2.2 Combustion technologies for industrial systems
- 2.3 Direct combustion CHP systems
- 3 Anaerobic digestion
- 3.1 Biogas from waste streams
- 3.2 Biogas from waste water
- 4 Additional renewable technologies in the food sector
- 5 References
Among renewable energy sources, biomass plays the most important role and creates with about 80% by far the biggest share of the energy generated by renewable sources. The main difference between biomass and other renewable is the possibility of its utilization as a fuel. Biomass is the only carbon based renewable energy source which can directly substitute fossil fuels. Biomass is also the only one renewable energy source that can be stored and applied to produce heating, electricity and fuels when they are needed. The environmental benefit of using biomass as energy source is the ability to reduce CO2 emissions. The liquid or solid biomass feedstock can be directly used to provide energy or converted to more convenient energy carriers. Basically the processes utilized for energy production from biomass can be divided into three main categories: thermochemical, physical-chemical and biological conversion routes. The thermochemical conversion processes of biomass include: combustion, pyrolysis and gasification. The physical-chemical conversion processes can be applied for the production of biodiesel. The biological conversion routes of biomass are used to convert biomass into biogas or bioethanol .
Among the technologies for energy production from solid biomass, combustion is the most advanced and market-proven application. Consequently combustion of biomass has been found to be the most promising method for biomass utilization. Modern biomass combustion plants and biomass boilers achieve an efficiency of over 90% and utilize sophisticated technology to control the process in order to minimize its environmental effects and promote efficiency . In the recent years utilization of biomass as a fuel in combustion appliances has been of great interest as an alternative to burning fossil fuels . However, there are still unsolved problems related to the complexity of the biomass burning process, which include among others relatively low ash melting temperature or, variation in the fuel properties. Furthermore, efficiency enhancement and economic feasibility of bioenergy projects are often crucial issues . One of the problems with using solid biomass as a fuel is the varying fuel parameters like moisture content, calorific value and geometric shape and size .
Fixed bed combustion
Fixed bed wood combustion boilers have been developed to produce heat with the aim of optimised energy generation. Modern biomass combustion systems ensure complete combustion with small amounts of pollutant emissions. The biomass combustion appliances are producing small quantities of ash, which can be disposed of with relative ease for example for use as a fertiliser.
Wood pellet appliances and burners
Pellets are small cylindrical pieces made by compressing sawdust and wood shavings. Pellets can be used for automatically charged stoves because they are flowable and well suited to automated feed systems. Another advantages of pellets are their high energy content, low moisture and high density which allow a significant reduce of the transport and storage costs. Due to their advantages pellet burning appliances offer an ease of operation which is similar to fossil-fuel-fired boilers.
Figure 1: Wood pellet combustion system 
Pellet boilers are automatic combustion systems, which range in size from a few kilowatts up to large megawatt size units. They allow a continuous combustion appliance operation with high quality fuels and a system output performance controlled by the rate of the fuel supply. The operation of a pellet boiler can be controlled relatively easily and the system maintenance is similar to fossil-fuelled boilers. Pellet-fired systems offer a high fuel conversion efficiency, which can achieve values of over 80 per cent. Even at part-load operation the combustion efficiency remains considerably high.
Wood chips are mainly produced from round wood or by chipping wood residues containing bark by using specialised wood chippers. The chippers are designed to produce wood pieces with a uniform size, with a longest dimension of 20-50 mm, that work well in automated fuel feeding systems. The production of wood chips requires less effort than pellets and the handling of wood chips requires less effort than wood logs. Since the ease of maintenance and operation of wood chip furnaces is similar to fossil fuel boilers and therefore they can be the most attractive and energy efficient use of biomass in combustion appliances. Other advantages of wood chip combustion systems are the relatively high combustion efficiency, modern appliances achieve efficiencies of over 80% and a relatively low pollutant emissions.
Figure 2: Woodchip combustion appliance 
Wood chip boilers are fed by an automatic feed of chipped wood and their heat release rate is controlled by the use of the fuel feed rate. The size of wood chip boilers ranges from small domestic systems of few kilowatt up to power station boiler of 100 MW and more. Wood chip combustion appliances are most appropriate for medium scale and large scale bioenergy projects. Good quality wood chips with relatively low moisture contents (between 15 and 30%) are burned usually in smaller appliances with underfeed stokers. Large scale appliances, which can deal with varying fuel properties, use moved or stepped grate systems.
Combustion technologies for industrial systems
Combustion technologies for industrial systems are using fixed bed combustion (underfeed stoker and grate furnaces) as well as fluidised bed combustion (BFB and CFB furnaces).
Underfeed stokers are an economical and technically mature biomass combustion technology which can be applied up to a power capacity of 6 MW. In this type of furnace, a screw conveyor feeds the fuel from below. Primary air is supplied through the grate, where the fuel drying, gasification and charcoal oxidation occur. The secondary air is introduced to the combustion gas mixture at the entrance of the secondary combustion chamber in order to ensure complete fuel oxidation .
Figure 3: Underfeed stoker for biomass combustion 
Underfeed stokers are suitable for combustion of woody biomass with a water content between 5 and 50%, however, the design of the combustion chamber has to be adapted to a certain fuel water content. The underfeed stoker burning appliances offer the advantage of efficient part load operation and relatively simple load control. The output performance of the combustion system can be controlled relatively easily, because the amount of fuel in the combustion chamber is relatively low .
Table 1: Underfeed stoker characteristics 
|low investment costs for plants < 6MWth||low flexibility in regard to particle size|
|simple and good load control due to continuous fuel feeding and low fuel mass in the furnace||sustainable only for biomass fuels with low ash content and high ash-melting point (wood fuels) (<50mm)|
|-low emissions at partial load operation due to good fuel dosing|
Grate firing is one of the mainly used technologies for biomass combustion as it combines relatively high efficiency with reasonable investment and operating costs. Biomass grate furnaces can deal with varying fuel properties and thus represents a promising alternative for energy generation from biomass. In large-scale biomass combustion appliances separation of different stages of thermal decomposition of the fuel on the grate as well as staged air combustion is applied, where the combustion air supply in the grate zone is divided into sections according to the requirements of the individual steps of thermal decomposition of the fuel on the grate. This complex combustion air management allows smooth operation of grate furnaces at partial load down to the 25% of the nominal loadFehler! Verweisquelle konnte nicht gefunden werden.
Figure 4: Grate furnace for biomass combustion 
In modern biomass furnaces staged air combustion is also applied in order to simultaneously reduce both the emissions from incomplete combustion and NOX emissions. The combustion chamber is divided into primary and secondary zone. In the primary zone thermal decomposition of fuel on the grate occurs in fuel-rich conditions (λ<1), where the heat release from the fuel bed is determined by the amount of air fed to the grate zone. Due to the sub-stoichiometric conditions unburned fuel components leave the bed and are transported with the gas flow to the secondary combustion zone. The hot gases from the primary zone are oxidised in the secondary zone where the mixing of the combustible gases and secondary air should be as complete as possible. If good mixing is ascertained the emissions from incomplete combustion can be close to zero.
Table 2: Grate furnace characteristics 
|low investment costs for plants < 20 MWth||usually no mixing of wood fuels and herbaceous fuels possible (only special constructions can cope with such fuel mixtures)|
|low operating costs||efficient NOX reduction requires special technologies (combination of primary and secondary measures)|
|low dust load in the flue gases||high excess oxygen (5-8vol%) decreases efficiency|
|less sensitive to slagging than fluidised bed furnaces||combustion conditions not as homogenous as in fluidised bed furnaces|
|low emission levels at partial load operation require a sophisticated process control|
Bubbling fluidized bed (BFB) combustion
Bubbling fluidised bed (BFB) combustors are interesting for larger boiler capacities of over 20 MW. BFB furnaces are suitable for combustion of fuels with particle size of up to 100 mm and this furnace type allows achieving a complete fuel oxidation. The bed material consists of particles with the size of about 0.5-1 mm and is located at the bottom of the combustion chamber. The primary air is supplied from below over a distribution plate and fluidises the bed. The temperature of the fuel bed can be controlled by heat exchangers that are integrated in the fluidised bed. Nozzles located in the upper part of the combustion chamber introduce the secondary air in order to achieve complete combustion .
Figure 5: BFB furnace 
The advantage of BFB furnaces is the possibility to combust fuel with different particle sizes and moisture. Mixtures of different kinds of biomass fuels can be burned in BFB furnaces. Another advantage of BFB combustors is the possibility to operate with a relatively low fuel bed temperature (650-850°C) which allows to burn fuels with low ash melting temperature .
Table 3: BFB furnace characteristics 
|no moving parts in the hot combustion chamber||high investment costs, interesting only for plants > 20MWth|
|NOX reduction by air staging works well||high operating costs|
|high flexibility concerning moisture content and kind of biomass fuel used||reduced flexibility with regard to particle size (< 80mm)|
|low excess oxygen (3-4 Vol%) raises efficiency and decreases flue gas flow||utilisation of high alkali biomass fuels (e.g. straw) is critical due to possible bed agglomeration without special measures|
|high dust load in the flue gas|
|loss of bed material with the ash without special measures|
Circulating fluidised bed combustion
In circulating fluidised bed (CFB) furnaces the bed particles are carried with the flue gases, separated and fed back into fluidised bed. The temperature of the fluidised bed is controlled by heat exchanger which are cooling the bed particles. Due to the better heat and mass transfer conditions in the fuel bed, optimal combustion reaction conditions can be achieved. CFB furnaces are applied to combust a wide variety of biomass fuels and this type of combustion appliances is frequently used to burn residues .
Figure 6: CFB furnace 
The CFB furnaces have, however, the disadvantage of larger size and higher investment cost. The fuel size has to be small (0.1-40 mm), which often causes higher investments in fuel pretreatment. Due to their high combustion efficiency and lower gas flow produced, CFB furnaces can be an interesting option for plants of more than 30 MW thermal output .
Table 4: CFB furnace characteristics 
|no moving parts in the combustion chamber||high investment costs, interesting only for plants > 30MWth|
|NOX reduction by air staging works well||high operating costs|
|high flexibility concerning moisture content and kinds of biomass fuels used||low flexibility with regard to particle size (< 40mm)|
|homogenous combustion conditions in the furnace if several fuel injectors are used||utilisation of high alkali biomass fuels (e.g. straw) is critical due to possible bed agglomeration|
|high specific heat transfer capacity due to high turbulence||high dust load in the flue gas|
|use of additives easy||loss of bed material with the ash without special measures|
|very low excess oxygen (1-2 vol%) raises efficiency and decreases flue gas flow||high sensitivity concerning ash slagging|
Direct combustion CHP systems
In the recent years rapid technological progress has been made in the research and development of biomass combustion equipment with respect to the combustion efficiency and reduction of the environmental impact of biomass burning appliances. If operated in a sustainable manner decentralised energy generation systems based on biomass combustion can contribute to an improvement of the quality of environment in the respective regions. The sustainability of energy generation from biomass can in particular be achieved by using natural biomass fuel sources in combination with newly developed control systems of modern biomass boilers . The relatively low cost of fossil fuels and the relatively high investment cost of biomass combustion based plants are the main reasons why bioenergy does not contribute a higher proportion of the energy generated in the EU.
Among market available technologies the ORC-modules (Organic Rankine Cycle) and Stirling engines are the only commercialized technologies for heat and power production based on biomass combustion in decentralized cogeneration plants.
The most well-proven and commercial available technology for decentralized cogeneration based on biomass combustion is the Organic Rankine Cycle with its main advantages, which are excellent part-load operation and reduced operating costs. The principle of energy generation with the ORC technology is similar to the classical water-steam process, with the main difference that instead of water an organic fluid (silicone-oil) is used as the working medium. The utilization of silicone oil as the working fluid has the advantage that electricity can be generated at much lower temperature and pressure in comparison to the water-steam process.
Figure 7: ORC cogeneration plants based on biomass combustion.
The ORC plant is a closed system which is connected to a heat source. Thermal-oil is mainly used as a heat transport medium between the heat source and the evaporator of the cogeneration module. Thermal energy produced in a biomass combustion appliance is used to generate superheated organic steam in the evaporator. The steam flows to the turbine which is connected to a generator. The condensation of steam takes place in the condenser and the waste heat from the electricity production process is utilized as district heat. The electrical efficiency of ORC systems ranges between 6 and 20% and is linked with the maximum heat recovery and the thermal efficiency of the boiler .
Table 5: Technical specifications of ORC plants
|ORC plant parameters|
|Typical performance range||300 kW to 10 MW|
|Energy output||Hot water 90°C/60°C
Electricity (η el ca. 15%)
|Fuel||Wood-chips, pellets, sawdust|
Stirling engines are indirectly heated gas engines operating by cyclic gas compression and expansion, with air, helium or hydrogen as the working medium. Stirling engines enable to generate energy in a closed cycle with the advantage that various heat sources can be used to power the system. The engine can be driven by almost any kind of heat source with a flue gas temperature of approximately 1000 °C with low levels of particles and ashes. The typical electrical capacities of Stirling plants range between 1 kW and 100 kW and the efficiency of power production is in the range of 12-30 per cent . In comparison to the internal combustion engine Stirling engine can be characterized by relatively low maintenance requirements, high electrical efficiency, good part load performance, low emission level, low vibration and low noise level .
Figure 8: Biomass fuelled Stirling CHP plant
The cost of Stirling plants is relatively high; equating to approx. 3300 – 3900€ per kW electric output which is higher than comparable ORC systems. Another drawback of Stirling engines is the relatively low electrical efficiency; when solid fuels are used as a fuel the power generation efficiency can be as low as 15% .
Table 6: Technical specifications of Stirling plants
Figure 9 : Working principle of a biogas plant  The power capacity of biogas plants lies in the range between 100 and 2500 kW. The electrical efficiency of biogas plants in relation to the biomass energy at input is relatively high for decentralized plants and lies in the range between 20 and 40%. Table 7 : Technical specifications of biogas plants
|Stirling CHP plant parameters|
|Typical performance range||1 kW to 100 kW|
|Energy output||Hot water 90°C/60°C
Electricity (η el ca. 12-30%)
|Fuel||Wood-chips, pellets, solid biomass|
Biogas is produced during a biological decomposition of biomass in the absence of oxygen. The production of biogas from controlled anaerobic digestion is one of the principal advantages of the process. Anaerobic digestion involves the bacterial decomposition of organic material in absence of oxygen. One of the main limits on the anaerobic digestion process is its inability to degrade lignin. Anaerobic digestion is used in industry to handle very high COD wastes and as a treatment process for sewage sludge after an aerobic treatment of the waste. Anaerobic processes may be used to directly treat liquid wastes, the biological sludge generated by an earlier aerobic stage, organic solids and sludges. The inclusion of other feedstocks, such as sewage sludge, alters the resulting digestate. In the process, carbon from incoming organics is mostly converted to methane and carbon dioxide, and then released as biogas, which is capable of being combusted to generate energy . The proportion of methane to carbon dioxide will vary with the waste stream and the temperature of the system.    
Biogas from waste streams
The properties of biogas which have significant impact on the selection of end-use conversion technology are mainly influenced by the type of digestion process and substrates use. The gas mixture produced by means of anaerobic digestion is considered as a CO2-neutral biofuel and as a versatile renewable energy source for the production of electricity or/and heat. Mostly the biogas is utlizied in CHP- Plants.
Biogas generation is very sensitive to the feedstock, one plant found volumes ranging from 80 to 120 Nm3 per ton depending on the waste input. The system needs to have a balanced feed to maximise methane production. Installations usually target carbon rich wastes that will make use of the available nitrogen (and probably the extra required through bioaugmentation). Aside from the production of heat and electricity, biogas systems offer a possibility to reduce manure volume, produce a nutrient reach effluent and reduce methane emissions, which has a global warming potential of 21 times that of carbon dioxide.    
Figure 9 : Working principle of a biogas plant 
The power capacity of biogas plants lies in the range between 100 and 2500 kW. The electrical efficiency of biogas plants in relation to the biomass energy at input is relatively high for decentralized plants and lies in the range between 20 and 40%.
Table 7 : Technical specifications of biogas plants
|Biogas plant parameters|
|Typical performance range||100 to 2500 kW|
|Energy output||Biomethane (natural gas substitute)
Hot water 90°C/60°C
Substrates of farm origin
Table 8: Biogasproduction by substrate 
|Biogas [m³/t FM]||Methan [m³/t FM]||TS [%]||Methan concentration|
Biogas from waste water
Upflow anaerobic sludge blanket (UASB)
In the UASB system, the waste water is directed to the bottom of the reactor for uniform distribution. The waste water passes through a blanket of naturally formed bacterial granules with good settling characteristics so that they are not easily washed out of the system. The bacteria carry out the reactions and then natural convection raises a mixture of gas, treated waste water and sludge granules to the top of the reactor. Patented three-phase separator arrangements are used to separate the final waste water from the solids (biomass) and the biogas. The BOD and COD levels are reduced by an UASB-Reaktor. Loadings of up to 60 kg COD/m3 per day have been reported, but a more typical loading rate is a rate of 10 kg COD/m3 per day with a hydraulic retention time of 4 hours. The main disadvantage of the UASB reactor is the sensitivity to FOG. The Fat levels have to be below 50 mg/l in the waste water. On the other hand, a particular advantage of the process is the formation of pellets. This permits not only rapid reactivation after month-long breaks in operation, but also the sale of surplus sludge pellets, e.g. for the inoculation of new systems.
Table 9: Performance of a UASB reactor in the brewing sector reported  (Further treatment is necessary to discharge waste water with these concentrations to receiving waters)
|Initial load(kg COD/m3 per day)||5 – 10|
|Final COD level(mg/ml)||500 – 1000|
|Sludge generated per kg of COD removed (SS/kg)||0.04 – 0.08|
Types of UASB Reaktors
Internal circulation (IC) reactors
There is a special configuration of the UASB process .the IC reactor, in which two UASB reactor compartments can be put on top of each other, one high loaded and one low loaded. The biogas collected in the first stage drives a gas-lift resulting in an internal recirculation of the waste water and sludge, hence the process name. One of the main advantages of the IC reactor is that it can undergo a certain amount of selfregulation, irrespective of the variations in incoming flows and loads. As the load increases, the quantity of methane generated also increases, and further increases the degree of recirculation and hence dilution of the incoming load.
loading rate: 15 – 35 kg COD/m³ per day
Hybrid USAB reactors
The hybrid process is a variation of the conventional UASB. This incorporates a packed media zone above the main open zone. This allows for the collection and retaining of non-granulated bacteria which, in conventional UASB reactors, would be lost from the process. The lower sludge zone acts in exactly the same way as within a conventional UASB reactor and is responsible for the majority of the biodegradation of the organic material. The role of the micro-organisms and media in the packed zone is to provide a certain amount of polishing treatment, to hold biological solids in reserve, and to prevent the biomass from washing out of the reactor.
loading rate: 10 - 25 kg COD/m3 per day.
Fluidised and expanded bed reactors
These reactors are similar to the anaerobic filters. If the particles and biomass are completely mixed, the process is known as a fluidised bed, whereas a partially mixed system is known as an expanded bed. In the fluidised bed reactor the carrier material is constantly in motion, with a bed expansion of 50 % or more. The carrier material (usually sand but sometimes pumice or plastic pellets) is kept in suspension by means of high recirculation rates. The recirculation must be strong enough to keep the carrier material in suspension, but care must be taken to ensure that excessive circulation does not cause the biomass to become detached from the carrier material. The expanded bed reactor also incorporates support media, often no more than sand or synthetic plastic materials. Light materials are often used to minimise the up-flow velocities required to fluidise the beds, particle sizes are typically in the range 0.3 – 1.0 mm. The fluidised and expanded bed reactors are applicable to waste water of low pollution loads with an average COD of between 1500 and 3600 mg/l. To achieve high volume-time yields of 15 – 35 kg COD/m3 per day, it is absolutely essential to fill the methane reactors with as constant a volume of adequately acidified solids-free waste water as possible. For this reason, all large scale systems have been built as two-stage systems with for example a separate acidification stage.
Loading rate: 15 – 35 kg COD/m3 per day
Expanded granular sludge bed reactors (EGSB)
EGSB reactors use granular sludge of the type found in UASB reactors but they operate with a much greater depth of granular sludge and a higher water rise rate. The water rise rate is typically of 3 m/h, compared to 1 m/h for a UASB. The initial commissioning/acclimatisation phase is not long for EGSB reactors. The digester uses recirculated treated water and is fitted with a three-phase (solid, liquid, gas) separator.Reduced BOD/COD and nitrogen levels.
Loading rate: up to 30 kg COD/m3 per day
Table 10 : Performance of a EGSB reactor in the brewing sector reported  (Further treatment is necessary to discharge wastewater with these concentrations to receiving waters)
|Initial load(kg COD/m3 per day)||15 – 10|
|Final COD level(mg/ml)||500 – 1000|
|Sludge generated per kg of COD removed (SS/kg)||0.04 – 0.08|
In an example molasses distillery, an EGSB reactor treats the condensed vapours from the condensation unit and the singlings from distillation/rectification. The reactor reduces the COD and nitrogen load in the downstream activated sludge unit. The methane gas produced is burned in a CHP plant, to generate electricity and heat. The high efficiency of the reactor, results in the production of only small quantities of surplus aerobic sludge. In this example, it is concentrated in a decanter and used for agricultural purposes or disposed of to a MWWTP. Driving force for implementation is the reduction in waste water treatment costs and reliable compliance with discharge limits.
Additional renewable technologies in the food sector
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