Substrate buffer storage facilities are intended primarily for buffering the quantities of substrate needed as digester feedstock for periods ranging from a few hours up to two days. The design of the storage facility depends on the types of substrate used. Footprint varies with the quantities that the facility will have to handle and the time periods for which substrate will have to be buffered. If co-substrates from off-site sources are used, contractual conditions such as agreed acceptance quantities and frequency of supply factor into the considerations. Using hygienically problematic co-substrates from industrial sources, for example, necessitates strictly segregating the receiving station from farming operations. Intermingling of hygienically problematic and hygienically acceptable substrates at any point prior to the former's discharge from hygienisation must be impossible.
There are other reasons besides legal considerations for using sealed storage facilities to minimise odours. Enclosure in sheds is one possibility, and structures of this nature can include spaces for receiving and preparing the substrates, along with storage as such. The spent air can be extracted and ducted through suitable cleaners (e.g. washers and/or biofilters). The sheds for waste-product digesters frequently have negative-pressure systems which, along with waste-air extraction, largely prevent odour emissions. Sheds have other advantages as well as the potential for odour emissions. They offer the equipment a measure of protection, and work and checks can be carried out irrespective of weather conditions. Enclosure can also be a means of achieving compliance with noise-abatement regulations .
|Sizing||Depends on: substrate arisings, digester capacity, length of time to be bridged between successive deliveries, land-use specifics and yield of co-substrates, supply contracts for substrates from off-site sources, possible disruptions in operation|
|Costs||Storage facilities are generally in place; when new builds are needed the price has to be calculated on a case-to-case basis factoring in the multiplicity of influencing variables indicated above|
The nature and extent of substrate preparation influence the general usability of substrates with regard to the proportion of entrained interfering substances, so they factor directly into the availability of plant technology. Moreover, a suitable preparation process can have a positive effect on the digestion-process transient, which in turn affects utilisation of the substrates' energy potential.
Sorting and Removal of Interfering Substances
The necessity for sorting and removal of interfering substances depends on the origin and the composition of the substrate. Stones are the most common; they generally settle out in the pre-digester pit, where from time to time they have to be removed from the bottom. Separators for dense materials are also used, generally sited directly in the substrate pipe in front of the feed conveyor. Other matter has to be removed manually at the point of substrate delivery or during filling of the feed hoppers. There is considerable likelihood that biowaste materials may contain interfering substances.
Whenever material of this nature is used as co-substrate, every effort should be made to ensure that it is not freighted with interfering substances. Most farming operations would not have the resources to install complex sorting facilities with mechanical lines or sorting boxes comparable with those in dedicated biowaste processing plants. Modular-box or garage digesters, by contrast, are virtually unaffected by interfering substances, because wheeled loaders and grabs are the primary means of substrate transport and there is no contact with pumps, valves or screw conveyors or other components of similar nature that would be easily damaged by interfering substances.
Comminution increases the aggregate substrate surface area available for biodegradation and consequently for methanisation. Broadly speaking, although breaking down the size of the particles effectively accelerates the rate of biodegradation, it does not necessarily increase gas yield. The interplay of dwell time and degree of comminution is one of the factors influencing methane production. Hence the importance of adopting the appropriate technology. The equipment for comminuting solid substrates can be sited externally upstream from the point of infeed, in the pre-digester pit, pipe or digester. The range of equipment includes chippers, mills, crushers and shafts and screw conveyors with rippers and cutters. Shafts with paddles and bladed screw conveyors are very common in combined receiving and metering units. Given the extent of their application, the properties of these comminution devices are summarised separately for handling direct solids metering by combined receiving and metering units and processing by mills and chippers.
By contrast with comminution of solids before transfer to pre-digester pit, pipeline or digester, liquids with solid or fibrous content can be comminuted directly in the pre-digester pit, in other mixing tanks or in the pipeline. This can be necessary in the case of substrates and substrate mixtures the consistency of which could threaten the operability of the feeder (generally a pump). Separate comminution agitators sited in the pit upstream of the digester constitute one means of comminution. In-pipe, directly linked comminution and pumping is common, however, and the same applies to combination comminution/ pumping units. These units are generally powered by electric motors, and some are designed to be driven off a tractor PTO.
Characteristic values and process parameters of comminutors in combined receiving and metering units
|Characteristic values||Standard commercially available units are capable of handling up to 50 m3 a day (the substrate receiving
or holding vessel can be sized for a much larger capacity)
Usual silages, CCM, animal (including poultry) manure, bread waste, vegetables • Toothed rollers or bladed worm-type mixers are more suitable for long-fibre substances
|Advantages & Disadvantages||
+ High throughput rates
For more detailed information check the FNR Guide to biogas.
Wetting Down to Mash, Homogenising
Substrates have to be wetted down to mash in the wetdigestion process to render them pumpable by increasing their water content, so that they can be pumped into the digester. This generally takes place in the predigester pit or other containers, just before the substrate is introduced into the digestion process. The liquid used for wetting down to mash is liquid manure, liquid digestate (from pressings), process water or – in exceptional cases – fresh water. Using liquid digestate can reduce fresh-water consumption. A further advantage is that even before it reaches the digester, the substrate is inoculated with seed bacteria from the digestion process. Consequently, this procedure can be applied to particularly good effect after hygienisation or in the process known as the plug flow process. The use of fresh water as make-up liquid should be avoided whenever possible, on account of the high costs involved. If water from cleaning processes is used as the make-up liquid for wetting down to mash, it has to be borne in mind that disinfectants can impede the digestion process because substances of this nature have a negative effect on the microorganism population inside the digester.
The homogeneity of the substrate is of major importance in terms of the stability of the digestion process. Severe fluctuations in loading and changes in substrate composition require the microorganisms to adapt to the variations in conditions, and this is generally linked to drops in gas yield. Pumpable substrates are usually homogenised by agitators in the pre-digester pit. However,
homogenisation can also take place inside the digester, if different substrates are pumped in directly and/or are introduced into the digester via a solids infeed.
Compliance with statutory criteria for some substance groups that are critical from the epidemiological and phytohygienic standpoint can necessitate integrating thermal pretreatment into the biogas plant. Pretreatment consists of heating the substances to a temperature of 70 °C for at least one hour. Autoclaving is another method of killing germs. In this process the substrate is pretreated for 20 minutes at 133 °C and a pressure of 3 bar. This method is much less common in the sector than hygienisation at 70 °C, however. The size of the vessels used for hygienisation depends on throughput rate, and the same applies to energy input,
so hygienically problematic co-substrates are generally hygienised before being fed into the digester. This is a simple way of ensuring that only the problematic substances are hygienised, so the hygienisation stage can be made more economical (partial-flow hygienisation). Full-flow hygienisation of all the feedstock or the predigested material is also possible. One advantage of
pre-digester hygienisation is a certain degree of thermal decomposition of the substrate, which subsequently – and depending on its properties – is more readily fermentable.
Hygienisation can be undertaken in airtight, heated stainless-steel tanks. Tanks of the conventional type for livestock fodder are often used. Hygienisation is monitored and documented by instrumentation for fill level, temperature and pressure. The post-hygienisation temperature of the substrate is higher than the process temperature prevailing inside the digester. Consequently,
the hygienised substrate can preheat other substrates or it can be fed directly into and thus heat the digester. If there is no provision for utilising the waste heat of the hygienised substrate, suitable means must be used to cool it to the digester's temperature level.
Transport and Infeed
From the standpoint of process biology, a continuous flow of substrate through the biogas plant constitutes the ideal for a stable digestion process. It is virtually impossible to achieve this in practice, so quasi-continuous substrate feeding into the digester is the norm. The substrate is added in a number of batches over the course of the day. Consequently, all the equipment needed for substrate transport is not in continuous operation. This is extremely important in terms of design.
The choice of technology for transporting and infeed depends primarily on the consistency of the substrate.
A distinction has to be drawn between the technology for pumpable and for stackable substrate. Substrate temperature has to be taken into account as far as infeed is concerned. Sizeable differences between material temperature and digester temperature (such as can occur in post-hygienisation infeed or when the digester is loaded during the winter) have a severe effect on process biology and this in turn can cause gas yield to diminish. Heat exchangers and heated pre-digester pits are two technical solutions adopted from time to time to counter these issues.
From the standpoint of process biology, a continuous flow of substrate through the biogas plant constitutes the ideal for a stable digestion process. It is virtually impossible to achieve this in practice, so quasi-continuous substrate feeding into the digester is the norm. The substrate is added in a number of batches over the course of the day. Consequently, all the equipment needed for substrate transport is not in continuous operation. This is extremely important in terms of design. The choice of technology for transporting and infeed depends primarily on the consistency of the substrate.
Transport of Pumpable Substrates
Pumps driven by electric motors are the most common means of transporting pumpable substrates in biogas plants. They can be controlled by timers or process- control computers, and in this way the overall process can be either fully or partially automated. In many instances, substrate transport within the biogas plant is handled in its entirety by one or two pumps centrally sited in a pump station or control cabin. The piping is routed in such a way that all operating situations (e.g. feeding, complete emptying of tanks, breakdowns,etc.) are controlled by means of readily accessible or automatic gate valves.
It is important to make sure that pumps are readily accessible, with sufficient working space kept clear all round. Even despite precautionary measures and good substrate pretreatment, pumps can still clog and need speedy clearing. Another point to bear in mind is that the moving parts of pumps are wear parts. Subject to harsh conditions in biogas plants, they have to be replaced from time to time without the necessity of shutting down the plant. Consequently, shut-off valves have to be installed so that the pumps can be isolated from the piping system for servicing. The pumps are virtually always of rotary or positive-displacement design, of the kind also used for pumping liquid manure .
Pump suitability in terms of power and delivery capability depends to a very large extent on the substrate, the degree of substrate preparation, and the dry matter content. Cutter or chopper comminutors and foreignmatter separators can be installed directly upstream to protect the pumps. Another possibility is to use pumps with pumping gear ready-tooled for comminution.
Transport of Stackable Substrates
The transport of stackable substrates is a feature of wet digestion plants through to material infeed or to the stage of wetting down to mash with make-up liquid. Most of the work can be done with loaders of conventional design. It is only when automated feeding takes over that scraper-floor feeders, overhead pushers and screw conveyors are used. Scraper-floor feeders and overhead pushers are able to move virtually all stackable substrates horizontally or up slightly inclined planes. They cannot be used for metering, however. They permit very large holding tanks to be used. Screw conveyors can transport stackable substrates in virtually any direction. The only prerequisites are the absence of large stones and comminution of the substrate to the extent that it can be gripped by the worm and fits inside the turns of the worm's conveyor mechanism. Automatic feeder systems for stackable substrates are often combined with the loading equipment to form a single unit in the biogas plant.
- Substrate Types and Management
- All Biogas Articles on energypedia
- FNR (2012): Guide to biogas – from production to use
- FNR, 2012: Guide to biogas