Food & Drink Process & Packaging Issue 7 2016 - Page 27

1 Assess your entire process from 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 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? 4 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. 2 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 a 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 amount of storage required (reducing capital costs) or transport requirements (lowering operating costs). 3 Re-use heat which would otherwise be wasted. It may not seem obvious at first, but any large quantity of heat has a use, 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 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 other processes. Carolyn Wheeler, Sales & Marketing Coordinator HRS House, 10-12 Caxton Way, Watford Business Park, Watford, Herts, WD18 8JY Tel: 01923 232 335 Direct Email: Email for publications: 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.