By Dr Nigel Horan and Dr David Tompkins
The past decade has seen a dramatic increase in the number of anaerobic digesters treating food and farming residues in the UK, rising from just 24 plants in 2010 to around 200 today. These digesters allow the processing of over five million tonnes of material, including: food waste, manures and slurries, crop residues and purpose-grown crops. With installed electrical generating capacity at these plants in the order of 216 MW, it means that more than 1 TWh of energy is recovered. Whilst the electricity has a commercial value, this is enhanced by subsidies in the form of ROCs or FiTs. The biogas itself attracts also subsidies in the form of the RHI, whilst waste accepted as feedstocks can attract a gate fee. However, the process produces large quantities of liquid end-product or digestate. This material is virtually all applied to agricultural land as a biofertiliser – although its fertilizing value does not usually cover the costs associated with its haulage and spreading. And of course as the number of digesters increase, then competition for suitable feedstocks also increases, and this is having the effect of driving down the gate fee (figure 1). Whilst median gate fees appear relatively stable, the low price has fallen rapidly – and in a number of cases operators have indicated that it has already fallen below zero.
Figure 1. Trends in gate fees for organic waste processed by anaerobic digestion (Data courtesy of WRAP).
The need to innovate
During the digestion process the aim is to convert organic matter (volatile solids) to biogas and an efficient digestion process can result in the destruction of from 60 to 90% of the volatile solids. However any water or inorganic material is conserved, although the chemical form may have changed. For example, organic nitrogen is converted to ammoniacal nitrogen, but the total nitrogen concentration should not change from beginning to end of the digestion process. Thus for a typical feedstock fed at 12% dry solids with a 70% volatile solids fraction, to a digester that destroys 70% of the volatile solids, then the digestate exiting the digester will have a dry solids of 6.5% and a VS of 41%. The conservative nature of the process means that typically, for every 1 tonne of feedstock added then 0.94 tonnes of digestate will be produced.
Whilst various innovative approaches to digestate valorization have been suggested, none of these has achieved any significant market penetration, beyond separation of whole digestate into fibre and liquor fractions. However techniques to dry digestate are increasingly common, due to RHI support. Whereas separation can increase the range of potential markets for digestate fibre, it still results in large volumes of dewatering liquor that must still be further treated or hauled.
Digestate valorization normally focusses on the nutrient aspects of the material, although some attempts have been made to combust the separated fibre fraction. Combusting the liquor fraction is energetically nonsensical, but an emerging thermal technique could offer a viable alternative.
A wide range of thermal treatment options are available, operating at different temperatures and pressures, which can convert organic matter to energy-rich gaseous, liquid and solid forms (table 1).
Table 1. The temperatures and pressures associated with different thermal processes
|180 – 250400 – 500500 – >600
600 – 1,000
800 – 1,000
|2 -101 – 20
1 – 30
Thermal processes for energy intensification are not new and as early as 1790 the production of town gas from coal, wood and oil feedstocks by heating them to around 400°C in the absence of air, was practiced in the UK. In 1807 Pall Mall was fully illuminated by town gas, and by 1870 over 400 Statutory Gas Undertakings had been authorized by Parliament. The by-products from this process were coke, which was used in the steel industry, and coal tar, which provided a wide range of high value chemicals of which paracetamol is still in use today; an early example of resource efficiency.
For many thermal processes a dry (>80% dry solids) feedstock is essential for successful carbonization. However hydrothermal carbonization (HTC), which takes place at temperatures between 185 and 250°C can be successfully applied to organic material with a solids content as low as 20%. The system operates at autogenous pressures, in other words the pressure that derives naturally in a closed system in response to the temperature, as defined by the gas laws. As a result it is a relatively simple system to design and operate, particularly in batch mode.
The products of the HTC process are not dissimilar to those of the gasworks of the 19th century with over 80% of the carbon in the feedstock being recovered in various forms, depending on the operating temperature. Typically up to 8% of the carbon is converted to carbon dioxide, while 75% can be recovered as a hydrochar with an energy density of up to 16 MJ/kg (the equivalent of lignite coal). The remaining carbon is recovered as a high-strength organic liquid of varying biodegradability, which can be recirculated to the digester as a feedstock diluent to improve energy recovery.
Applications of HTC to Digestate Management
The potential of HTC within an existing AD flow scheme is summarised in figure 2. The technology can be applied at facilities where dewatering is practiced and will accept the dewatered cake directly, but it can also be applied where dewatering is not available, simply by diverting some of the feedstock away from digestion in order to augment the solids content of the digestate prior to HTC.
Whilst showing clear promise at both bench and commercial scale, there are still a number of research questions to be addressed, and in particular:
i) What is the most economic temperature to operate HTC, to maximize carbon conversion to readily usable forms?
ii) How much energy can be recovered from the HTC liquor during anaerobic digestion and does this liquor present any inhibition problems?
iii) In overall life cycle terms, is hydrochar more valuable as an energy source or as a soil amendment?
iv) What might be the typical payback periods for medium and large AD facilities?
v) Given the incompatibility between recycling and energy recovery for some AD feedstocks, does HTC lend itself more to specific AD approaches?
vi) Following HTC what is the dewaterability of the resultant material
Figure 2. Flow schemes for HTC in the MAD process
Our current programme of work is attempting to provide answers to these questions but all the signs are that digestate will soon cease to be a drain on the AD process and become a valuable material, providing an additional income stream and further enhancing resource recovery through the digestion process.
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