Integration of CHP Into Waste Water Treatment Processes London 5 th November 2014 Peter Bense

2 Typical Waste Water Treatment Process Effluent Influent Cake Digester Primary Sedimentation Activated sludge Final Sedimentation Dewatering Return liquors Liquor Treatment Plant Sludge Disposal Biogas

3 Biogas Production  Historically the focus was on waste treatment and was driven by the need to meet strict compliance limits  Digestion was incorporated into the process primarily to reduce odour and toxicity of the sludge  Digestion produces biogas which is typically:  60 – 65% CH 4  40 – 35% CO 2  Traces of H 2 S, NH 3 & Ammonia  Saturated with water vapour  Biogas has a Calorific Value of around 20 Mj/Nm 3  The waste from 1 million people produces around 600Nm3./hr of biogas - equivalent to ~ 3.3 MW of fuel energy.

4 Traditional Application of CHP  Biogas was considered a useful by product of the treatment process.  The biogas was used in reciprocating engines to generate electricity.  Waste heat was recovered from the engine cooling jacket and the exhaust gas and used to heat traditional Mesophilic (MAD) digesters operating at around 40 o C  As the requirement for heat is at low temperature, typically < 80 o C, the system is energy efficient.

5 Traditional WwTW CHP Installation

6 CHP Operating Problems  Typically WwTW CHP Engines suffer from poor reliability and incur high maintenance costs.  The main reasons are impurities in the biogas, typically siloxanes and hydrogen sulphide.  This means that multiple units are often necessary to ensure there is reliable capacity to dispose of all of the biogas produced.  Gas clean up equipment is expensive and has its own operating cost penalty.

7 Developments in Treatment Technology  Current thinking is to move towards Advanced Anaerobic Digestion Processes, such as Thermal Hydrolysis, to increase throughput and improve the digestate quality.  THP gives higher gas yields but requires high grade heat in the form of steam. No digester heating is needed.  Only around 14% of the energy input into an engine can be recovered as steam.  The heat available from the engines alone is not sufficient to provide all the heat to THP processes. Supplementary fuel is required.  This means that CHP efficiencies and overall site efficiency is reduced resulting in higher OPEX.

8 CHP For Advanced Treatment  CHP Plant for AAD applications is much more complex and expensive.  The CHP needs to be integrated into the overall site energy infrastructure.  Particular attention has to be paid to the control systems to accommodate the rapidly varying steam demands from the THP plant.  Steam is critical to the process so standby fuel supplies are needed to maintain steam production during engine outages.  Efficiency can be improved if there is a use for low grade heat, e.g. building heating, without this up to 50% of the energy in the fuel can be wasted.

9 WwTW with AAD - CHP Installation

10 CHP Heat Recovery  Heat recovery is in the form of steam at 12 bar g.  This requires complex boilers capable of being supplementary fired.  Steam systems fall under Pressure Safety Regulations. This is unfamiliar to water utilities.  No condensate recovery so all of the steam energy is absorbed into the process.  Efficient design of the entire steam cycle to maximise use of low grade heat is key.

11 CHP- Economics  CHP plants commissioned before April 2009 receive 1 ROC per MW electricity generated  CHP plants commissioned after this receive ½ ROC per MW  After 2017 ROCS will not be available for new plant, the incentives will be replaced by Contracts for Difference.  Current proposals for the Sewage Gas strike price is £75 / MWhr.  At a reference price of £56.09 this gives an incentive of £18.91 i.e approx 0.45 ROC/MWhr Generally it is anticipated that available revenues from incentives will fall. With increasing complexity and costs, lower efficiency and reduced incentives CHP may not be the optimum solution for THP installations.

12 Alternatives to CHP  Biogas upgrading to biomethane has recently been commissioned by Severn Trent at Minworth and is under construction on Wessex Water, Northumbrian Water and Welsh Water sites.  Biomethane can be injected into the grid, used for vehicle fuel or as feedstock for advanced generation equipment such as fuel cells.  Currently injection to the grid is supported by high levels of incentives through the RHI but these will fall from  Upgrading fits well with THP as the heat required by the THP process is not deducted from RHI payments.  Up to 95% of the energy in the biogas can be recovered and sold.  Existing CHP can be converted to dual fuel to provide heat & power using injected biomethane.  Additional income can be made from CO 2 sale

13 CHP v G 2 G At high level - For a new build project at current incentive rates:  A stream of 1000 Nm 3 /hr of biogas contains 60% Methane  The energy content is equivalent to a continuous power of 6 MWt  Electrical output from a typical CHP 40% h would be 2.4 Mwe  New CHP receives 0.5 ROCs / Mwe i.e. £21 / MWe  Generation offsets bought in grid power at £84 / MWe  Total income = 2.4 x ( ) = £252 per 1000 Nm 3 /hr biogas  Equivalent energy output from a gas to grid 97% h would be 5.8 MWt  RHI income is £71 per MWt  Gas Sale value is £15 per MWt  Total income = 5.8 x ( ) = £499 per 1000 Nm 3 /hr biogas  Gas to Grid can give twice the returns of CHP for new plant

14 CHP v G 2 G Detail financial evaluation is required to determine the business case for each application This needs to include:  CHP  Benefits from LECs, CRC and value of recovered heat  Costs of standby or redundant plant  Maintenance costs  Gas to Grid  Costs of providing heat to the THP process  Costs of imported power  Costs of pipeline connection to the gas network  Potential CO 2 sale revenues Typically gas to grid projects can give IRRs of >20% on WwTW.

Integration of CHP Into Waste Water Treatment Processes Thank You Peter Bense