Extracting Energy From Sludge in the UK, Recent Experience

Transcription

1 Extracting Energy From Sludge in the UK, Recent Experience Keith Panter* Dr Bill Barber** * Ebcor Ltd, UK ( keith@becor.freeserve.co.uk) ** Cambi Inc ( bill.barber@cambi.com ) Abstract In the UK, a combination of strict financial regulation, high energy costs, and various renewable energy incentives have led the Water Industry to continue investing heavily in advanced anaerobic digestion. A trend of recent years has seen the shut down and decommissioning of drying plants deemed unnecessary with respect of best practice environmental option (BPEO) of land application of cake, expensive to operate and high carbon footprint. In addition, the number of existing incineration facilities has dwindled significantly as they have suffered increasingly expensive operation, and are being replaced by anaerobic digestion plants combined with biosolids use in agriculture. In the meantime, existing digestion facilities are being upgraded to enhance performance further and enable them to be used more intensely. As a consequence, the UK Water Industry leads the world in the use of advanced anaerobic digestion. With 21 full-scale facilities, UK has the highest proliferation of thermal hydrolysis plants that generate 60 MWs power continuously from 1500 metric dry t/day and account for nearly 30% of all sludge in UK. This is nearly half of all renewable energy produced from biogas generated from sewage sludge in the UK. In addition, there are a number of advanced biological hydrolysis systems, acid phase, Enzymic and Enhanced Enzymic Hydrolysis (EH and EEH); EH has 9 project accounting for another 250 tds/day. This paper describes the latest findings from the UK with respect to energy recovery from sludge and looks at how this experience can be used elsewhere. What is notable from the UK is the following: Combined Heat and Power (CHP) output has increased by 249% with a digestion capacity increase of 113%. This asset sweating has been achieved by a combination of increased VS loading, increased VSR% and additional CHP installations at sludge centres. There has been a wholesale move away from energy intensive/high carbon footprint process, such as incineration, thermal drying and lime stabilisation. 92% of all sludge is now digested with CHP and biosolids to land. There has been an increase in the quality of biosolids in terms of odour, stackability and pathogens that has assisted this switch away from high carbon footprint technologies. Nearly all dryers and incinerators in UK have been de-commissioned and lime stabilisation is now very rare except in remote communities. Keywords: Renewable energy, anaerobic digestion, CHP 550

2 THE LAST TEN YEARS IN UK. The rapid growth of anaerobic digestion and CHP stems from the mid-2000s when there was a rise in world energy prices due the Gulf crisis. This coincided with the introduction of Renewable Obligation Credits (ROCs), similar to RECs in USA. These ROCs still trade for about $60/MWh, though in later years the value of ROCs has been reduced by 50% for WWTPs with new projects and other forms of incentive now in place are leading to gas to grid projects (stimulated by Russian dominance of gas pipelines in Europe). The latter can be considered the German model that is leading to so many biowaste/manure projects in parts of Europe. Therefore in the UK, a combination of strict financial regulation, high energy costs, and ROCs have led the Water Industry to continue investing heavily in advanced anaerobic digestion. A notable feature of the UK is the organisation into 10 water companies (England and Wales) and 2 Water Authorities (Scotland and N. Ireland) that have seen multiple benefits from regionalisation. In this context, the main effect has been to see the formation of sludge centres. Sludge centres are locations where Anaerobic Digestion (AD) is centralised and raw sludge is trucked to the receiving site either as cake or liquid and digested. The advantages of sludge centres are as follows: 1. Digestion and CHP expertise is focussed on one location, 2. By doubling solids load the WWTP becomes self-sufficient in energy 3. Nutrient management is centralised also 4. Sludge pre-treatment becomes more affordable 5. Product quality can be enhanced 6. Expensive lime stabilisation systems are shut down A trend of recent years has seen the shut down and decommissioning of drying plants deemed unnecessary with respect of best practice environmental option (BPEO) of land application of cake, expensive to operate and high carbon footprint. In addition, the number of existing incineration facilities has dwindled significantly as they have suffered increasingly expensive operation, and are being replaced by anaerobic digestion plants combined with biosolids use in agriculture. Surprisingly one of the major spin-offs of the advanced AD approach has been a rapid increase in cake quality with respect to both dry solids and stabilisation (particularly so following thermal hydrolysis pre-treatment) so that products are easy to store and distribute without nuisance and safety concerns. This success has been replicated recently in the US with DC Water project at Blue Plains. SITUATION COMPARISON BETWEEN 2015 AND PRESENT TIME The UK Water Industry leads the world in the use of advanced anaerobic digestion. With 21 full-scale facilities, UK has the highest proliferation of thermal hydrolysis plants anywhere in the world, that treat over a quarter of the UK s sludge production so that 1500 metric dry t/d, equivalent to 19 million p.e., and generating 60 MWs of electrical energy continuously. In addition, there are numerous advanced biological hydrolysis systems, of these the most popular technology is Enzymic Hydrolysis that has 9 operating facilities treating about 250 metric dry t/d, though there was a drop in uptake of this technology in this decade. These two technologies account for about one third of all UK sludge between them. See table 1 for the largest project in the UK that are in the range of 3 12 MWs generation. 551

3 Table 1. Top 10 renewable energy generators in the UK Water Industry Site Pre-treatment 1 Davyhulme CHP at Davyhulme WWTW Thermal Hydrolysis 2 Minworth Generating Station Thermal Hydrolysis added 2016/7 3 Great Billing STW CHP Plant (RR) Enzymic Hydrolysis 4 Mogden STW (RA) Conventional 5 Avonmouth STW CHP Generation Enzymic Hydrolysis 6 Bran Sands (RSTC Advanced Digestion) Facility Thermal Hydrolysis 7 Cardiff East WWTW CHP Thermal Hydrolysis 8 Howdon AAD Thermal Hydrolysis 9 Longreach STW (RB) Thermal Hydrolysis Separate maize digestion from attached Stoke Bardolph STW Generating Station 10 sludge farm + conventional sludge A notable feature of these two technologies is the ability to operate at higher organic loading. Cambi THP plants are operating at anywhere between 4 and 8 kg VS/m3/day and enzymic plants more commonly in the 3-4 kg VS/m3/day. Compared to the UK average of about 2 kg VS/m3 /day (equivalent to.155 lb VS/cuft/day) that is more typical of US digester loading rates PARADIGM SHIFT IN TREATMENT OF SLUDGE FOR ENGLISH WATER COMPANIES The tables below compare the overall situation for the 10 English Water companies in 2005 and the changes made to date in the application of digestion and CHP. see table 2 for the estimated situation in 2005 for the English water companies (telephone survey by the Author). Table 2. Estimate of CHP from AD in English water companies 2005 Situation in 2005 % generated pop M TDA raw digested % digested M3 capacity MWs gen Theoretical from biogas Comments Southern % % Gas used for drying Thames % % 2 raw sludge incinerators - CHP not optimised? Anglian % % programme to digest tds more sludge under AMP4 Yorkshire % % focus on incineration NWL % % Raw sludge drying UU % % raw sludge incineration proposed DCWW % % raw sludge drying and lime ST Water % % digested sludge incineration Wessex % % CHP not optimised, I raw sludge dryer SWW % % raw sludge to lime - low cost Total % % of total sludge theoretical MWs tonnes VS VS conversion average digester loading % 1.97 Kg VS/m3/day 552

4 Now compare this with the situation that is planned for the end of the investment period in 2019 in Table 3 below. In 10 years there has been a paradigm shift in the way the water industry treats sludge. Table3. Estimate of CHP from AD in English water companies 2019 Planned position in 2019 % generated pop M TDA raw digested % digested M3 capacity MWs gen Theoretical from biogas Comments taking advantage of green credits, stopping Southern % % drying Thames % % diverting sludge from incineration to digestion Anglian % % closing all raw sludge operations and maximising digestion Yorkshire % % moving away from incineration over the next 5 years NWL % % stopped raw sludge drying in favour of THP, digestion CHP UU % % new incinerator abandoned in favour of THP, digestion CHP DCWW % % completing regionalsiation of sludge centres withthp/digestion CHP ST Water % % Closed all Incinerators, adding THP to some plants and gas to grid Wessex % % some optimisation of digestion CHP SWW % % no change Total % % of total sludge average digester loading 113% 249% 92% 96% of digested sludge VSR 52% 2.75 Kg VS/m3/day The following can be seen: total generation has increased from 64 MWs to 155 MWs an increase of 249% but digestion capacity has only risen by 113% 44% of sludge was digested and biogas used in 2005 this will rise to 92% when all current projects are complete in 2019 Organic loading has increased from 1.97 kgvs/m3/day to 2.75 kgvs/m3/day a 40% increase Comments show that there has been a wholesale move away from thermal drying, incineration and lime stabilisation that are all energy intensive and have high carbon footprints The water industry has sweated its assets by increasing VS loading, VSR% and adding CHP to sludge centres so that MWs generated have increased by 249% while digestion stock has only increased 113%. Typical power yields for thermal hydrolysis are between 0.62 and 1.01 MWh electrical per dry metric dry ton 1,2, showing a wide variation due to feedstock differences, biogas bypass and CHP availability. Nearly all the digested sludge is going to farmland as biosolids with a large increase in Class A and high DS% cake 2,3 with a typical content of 28 to 34% dry solids for thermally hydrolysed sludge. The result of this is that all the thermal drying plants in UK have been shut down or are about to be though 3 still operate in Scotland. Also, nearly all the incineration plants have been shut down and the remaining few are under review. This is a triumph for beautiful biosolids! Conclusions There has been a paradigm shift in the UK away from energy intensive/ high carbon footprint process such as incineration, thermal drying and lime stabilisation to energy producing processes using advanced digestion. This has been achieved by sweating existing digestion assets mainly and making sludge centres. The overall cost of sludge management has gone down with the sludge as a fertiliser and the sludge centre as an energy factory. 553

5 References 1. Merry J, Oliver B, Proc of 19 th European Biosolids Conference, Nov 2014, Manchester UK 2. Panter K, Fountain P, Shana A, Ringoot, D, Proc 21 st European Biosolids Conference Nov 2016 Edinburgh UK 3. Panter K, Proc WEFTEC 2009, session 91, October 2009, Orlando, Fla 554