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Anaerobic digestion……show me the data!

  
  
  

AD data checklist resized 600 According to Biogas Info there are currently 146 operational Anaerobic Digestion (AD) plants in the UK. In this blog we spend a few minutes looking at some of the data considerations associated with setting up an (AD) plant. At a micro level this includes performing the initial modelling and cost calculations to see if a project makes economic sense. For large project developers there is the consideration of feedstock availability and surety of supply i.e. where is the waste? There are also operational and legislative considerations that require significant data capture and analysis.

Modelling - Is the project feasible?

Quite a bit of complex modelling is required in order to design and assess the economic viability of an AD project. Factors that need to be considered include:

Feedstock – what type of feedstock is available? What is the moisture content of the feedstock? Sewage and animal wastes have high moisture content whereas municipal solid waste has low moisture content. The biogas yield is also impacted by the type and combination of input feedstock as well as the levels of contamination. What is the volume of feedstock and therefore what plant size is required?

Technology/Process – batch or continuous. Single-stage or multistage, mesophilic or thermophilic. This will primarily be driven by the feedstock type and the available budget. Typically multistage processes are more expensive to build.

Biogas yields – is dependant upon all of the above and is linked to the plant thermal operating conditions and modelling biogas yields is apparently a complex iterative task

Researchers at Strathclyde University have built a modelling tool1 that provides the following outputs:

  • plant sizing including digester sizes and biogas engine size

  • biogas yield output

  • electrical surplus available for export

  • CHP heat surplus available for export

  • process electrical loads

  • process heat loads

  • transport energy and costs

  • overall energy balance

  • greenhouse gas contribution (relative to conventional procedures)

  • overall financial viability calculation, via net present value (NPV) and profitability index methods.

What are the legislative reporting requirements?

Once your plant is up and running, what are the legislative reporting requirements? Operation at an AD facility must comply with the Waste Framework Directive, to ensure waste production as a result of the treatment is avoided, reduced or recovered. As such, AD plants are required to adhere to many of the waste reporting requirements including recording tonnage of waste received and shipped from the facility which is then reported within the quarterly returns report to EA, SEPA or Department of Environment (Northern Ireland).

AD plants are also subject to continuous monitoring within different parts of the process. Scottish Water own and operate the Deerdykes facility2&3 near Cumbernauld in Scotland. The facility processes 30,000 tonne of food waste per annum and produces biogas which is used to power the plant as well as local homes and business. It also generates heat which could be used for district heating purposes. According to Deerdykes plant manager, Fraser Robertson, they are required to continuously monitor the digestate in the pasteurisers to ensure that it is heated to greater than 70 ®C for a period of at least one hour. This is a requirement of the Animal Byproducts legislation and these records are required to be maintained for a minimum period of 2 years. A lot of data! In addition to this AD plants may have other data capture requirements depending upon the configuration of the plant. Fraser points out that at Deerdykes hydrogen sulphide (H2S) levels in the flare stacks are continuously monitored.

At Deerdykes, the Methane (CH4) content in the biogas is monitored to ensure that the plant is operating at or above required efficiency levels. Poor methane yields impacts the profitability of the plant and can be indicative of a bad digester.

Efficiency of AD plants can also be assessed in terms of carbon savings and total level of renewable energy produced.  As previously mentioned, biogas released during the AD process will contain variable amounts of methane depending on the feedstock source and plant specific factors.  The table below (from POSTNOTE4 387, September 2011) shows indicative methane yields for different feedstocks as well as the theoretical annual tonnage available in the UK.

Feedstock

Methane Yielda

(M3/wet t)

Max availability in UK

(wet weight)

in Million Tonnesb

Food and Drink Waste

30 – 154

 

8.3Mt Homes

3.6-5.8Mt Commerce

 

Sewage Sludge

9 -16

 

24-34 MT

71% recycled in 2010

 

Animal Slurry and Manure

12- 23

 

Approx. 76 MT

 

Purpose Grown Crops

20-220

 

10-29 Mt

Could be grown for all energy uses by 2020

 

 

aHigher water contents reduce the total methane yield.  Approx. Water content: 70-85% for food waste; 90-95% sewage sludge, manures and slurry; 60-85% crops.

bThis is the maximum available and does not indicate the feasible quantity for AD.

Where is the waste?

For large scale operators and investors, the availability of desired feedstock and surety of supply is of key importance when assessing viability of projects. Figures provided by WRAP in 2010 outlines potential volumes by source for biodegradable waste arising in the UK.

Wrap Waste Arisings Data 2010

However, getting accurate data on the exact geographical sources of feedstock seems to be a bit more challenging. The governments WasteDataFlow statistics provide some breakdown of waste arisings by local authority region but the exact sources of waste are not disclosed.

So, in summary, it is clear that there are significant data and therefore systems considerations associated with developing, implementing and operating AD plants. Complex modelling of multiple data points is required to assess economic viability. Once plants are operating, strategic monitoring is required to ensure regulatory compliance and operational efficiency. This can be achieved by implementing a suitable waste data software which is integrated with the SCADA plant control system. Finally, the location of feedstock sources is a big data gap that needs to be filled.

References

1 - http://www.esru.strath.ac.uk/EandE/Web_sites/03-04/biomass//calculator.html

2- http://www.scottishwater.co.uk/business/horizons/horizons-environment/anaerobic-digestion

3 -http://intranet.scottishwater.co.uk/portal/page/portal/SWE_PGP_COMMERCIAL/SWE_PGE_COMMERCIAL/SWE_BUS_4_SCOTWST/COM_SWWS_NEWS/COM_SWWS_NEWS_FW

4 - http://www.parliament.uk/briefing-papers/POST-PN-387.pdf

 

 

 

 

 

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