AD Process Optimization Top Tips For Farmers
It is easy to see why anaerobic digestion (AD) has become so popular with farmers across a range of sectors, from livestock and dairy production to vegetable growers. Currently over 400 AD plants are generating more than 500 MW of energy – yet the potential for UK AD is as much as 80 TWh of methane (equivalent to 30% of the UK’s household gas demand) according to ADBA*.
AD utilises manures, crops and waste streams, providing an additional form of revenue, as well as increasing crop diversity and, in many cases, a valuable form of waste treatment. It can provide heat and electricity for on farm operations such as crop drying, export power for sale and also produces a valuable end product in the form of digestate biofertiliser.
However, although plants of all scales can provide these benefits, not all of them are as efficient as they could be. With recent changes to support schemes for renewable energy in the UK, it is important to make sure that every available watt of power is utilised in some way. That means that everything, from the obvious, such as the amount of biogas produced and the efficiency of combined heat and power (CHP) units, to the less obvious, like utilising wasted CHP heat to further improve the process, must be examined and maximised where possible.
Obviously the best time to do this is as early as possible in the planning process, so that equipment can be appropriately specified and the plant designed around an optimum layout. However, while this might be the ideal situation, chances are that farm managers and plant operators can improve the overall efficiency of almost any existing on-farm AD plant if key considerations are taken into account.
AD Process Optimisation Top Tips
- Take a good look at your entire process from harvesting energy crops and obtaining feedstock to exporting digestate. The first step of any optimisation process is to understand what you are (or will be) doing. Analyse each step to see where wastage may occur and how it could be improved. For example, could you pre-warm feedstock before it enters the digester or, if you are already doing so, could you re-use heat already generated elsewhere? Does the proportion of heat to electricity generated match your planned requirements? If you are still planning, would another unit suit your needs better? If you are already operating your plant, could you re-use any heat which may be wasted? Also look at your digestate – could you add value to it, perhaps through pasteurisation or concentration?
- Maximise the efficiency of all processes, from energy generation to heat exchange. Check the efficiencies of all processes, whether in terms of thermal transfer, electrical output or gas generation. Keeping equipment, particularly heat exchange surfaces, clean and maintained will help it to operate at its maximum efficiency.
When specifying or researching equipment, remember that over the operational life of plant, the effects of even a small difference in overall efficiency could be considerable. For example, a 0.25% increase in the electrical efficiency of a 500 kW CHP unit could be worth more than £7,000 over the life of a plant at current wholesale prices.
Likewise, reducing the volume of digestate produced by the plant may create other efficiencies, such as reducing the storage required (reducing capital costs) or transport and application requirements (lowering operating costs). For example, if you could apply the same level of nutrients from three-quarters of the field operations, what would be the benefit to your crop margins?
- Re-use heat which would otherwise be wasted. It may not seem obvious at first, but any large quantity of heat can be used for something, including the heat given off by radiators (such as the cooling loop of a CHP engine), heat exchangers or other parts of the process.
If you don’t already pre-heat your feedstock, doing so may increase the thermal efficiency of the digester, while if you do already pre-heat, using heat from elsewhere in the process will be cheaper (and greener) than a creating heat just for this purpose. Likewise, pasteurising the digestate using recycled heat circumvents the need to install an additional heat source such as a biomass boiler, which could add hundreds of thousands of pounds to a project.
Many pasteurisation systems on the market simply dump the heat used. HRS systems recycle the heat, making them up-to 70% more efficient than a typical ‘heat jacket’ type pasteuriser and allowing this heat to be reused, either elsewhere in the process (for digestate concentration for example) or exported to process or district heating circuits for farm cottages, packhouses or livestock housing.
- Aim for continuous processing. Having to stop and start parts of your plant can be inefficient, even if the process is automated. Using multi-tank pasteurisation or digestate concentration systems means that while one tank is being heat treated the other/s can be filling or emptying, ensuring that the rest of the AD process is not interrupted.
On top of this, HRS multi-tank systems use four level temperature probes in each tank, three in the bottom of each tank and one in the top – so that the tanks can be filled to half capacities should the demand or flow drop off, allowing for continuous and flexible production of digestate. Without these probes, the system would be put on a ‘go slow’ in order to wait for the tanks to fill, which would require more heat as well as incurring the expense of slowing production or shutting the system down and restarting.
- Maximise the quality and value of your digestate. Maximising the value of digestate in terms of nutrients, ease of use and quality will also help to increase its financial value to your business. Pasteurising not only complies with PAS110, but can act to reassure the buyers of your produce and comply with certification schemes.
Traceability is important at all stages of today’s food chain and choosing a pasteurisation or concentration system with inbuilt traceability software will underline the quality of your biofertiliser to potential customers while reducing the time and costs associated with manual record keeping and compliance.
- Minimise downtime and maintenance. If the plant is not running, then it isn’t generating energy or a return on your investment. While routine maintenance and servicing is essential to keep any equipment running well, you shouldn’t have to keep stopping to clean pipes or replace filters. You should also expect equipment to have a reasonable service life and not be prematurely damaged, for example by corrosion. HRS DTI double tube or tube-in-tube type heat exchangers are designed to reduce fouling which reduces the need for cleaning and ensures a long, trouble free operating life.
HRS Heat Exchangers has worked with a number of on-farm AD plants during all stages, from initial feasibility studies through to retrofitting equipment to established operational plants. It is possible to reduce waste and, at the same time, increase the value and efficiency of an AD plant, which in turn provides real bottom line results and maintains an environmental sound addition to farming businesses, despite an uncertain policy environment.
*ADBA – Anaerobic Digestion and Bioresources Association, figure as of October 2015
By Matt Hale, International Sales Manager, HRS Heat Exchangers