Combined Heat & Aerobic Composting Ticks Most Boxes

07 July 2010

Combined Heat & Aerobic Composting ‘ticks most boxes’

You will be aware of the UK drivers to recover value from food, biosolids and other bio-wastes and to deliver renewable energy to replace fossil fuels. You will also know of the previous administration’s preference and double ROCs for AD, and the new Secretary of State at DEFRA has announced the intention of to review waste policy and its commitment to “work towards a zero waste economy and encourage councils to pay people to recycle and work to reduce littering” She and SofS DECC also speak of measures to promote a huge increase in energy from waste through anaerobic digestion.” So why did Christophe Cros of SITA, France urge caution about AD, as do several UK experts who have knowledge of AD?

Also you may not be aware that combined heat and aerobic composting (CHAC) ‘ticks all of the following boxes’:

Renewable Energy

Biomass as fuel

Renewable Heat Incentives

Efficiencies

Scaleability

Bio-Collection economies

Waste licensing

Management efficiency

Environmental benefits

Competitive Costs

The accompanying ORBIT paper 205.pdf describes how CHAC works and the CHAC Model for DECC.xls forecasts outputs and costs.

Invitation: Alpheco is seeking one or more waste management, water and sewage, energy or manufacturing companies that might be interested in bringing CHAC properly to market for UK, EU and beyond

Our in-vessel composting has been proved commercially but the delivery of renewable heat still need demonstration, so a pilot plant is probably the next step to optimise, measure, demonstrate and validate the cost forecasts for CHAC.

Alpheco would be pleased to discuss such technology transfer. Full sets of design drawings and manuals plus ‘know-how’ documents will be made available subject to an appropriate confidentiality agreement. Alpheco’s patent can be reassigned.

Yours sincerely,

Neil Winship

Explanatory Notes

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Renewable energy

The heat inevitably released by composting is about 30% more than the total heat + electricity from AD or four times the net electricity from AD. The heat from CHAC can be delivered via the road network to within metres of customers obviating the need for costly heat distribution pipe networks.

The heat from CHAC is, inevitably, low grade at about 40 Celsius, which may suit some modern buildings and applications such as greenhouses and grain drying. Older buildings with hot water radiators need heat at over 60 Celsius to which CHAC heat can be raised by integrating a heatpump.

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Biomass as fuel

The EU includes biowastes and hence food wastesand biosolids under biomass, thus they are classed as renewable. (Biowastes are in reality inevitable compared to renewables that are specially grown such as wood chips.) Treating biowastes earns income by avoiding the escalating cost of landfill and its tax. That income may be higher for difficult-to-treat biowaste such as meat-included catering wastes so they may command a higher income than fruit, vegetable and ‘green’ wastes.

Thus CHAC can fulfil the Merton Rule and similar planning policy targets.

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Renewable Heat Incentives (RHI)

UK Government has proposedan incentive of £0.065 per kWh for heat from a medium sized biomass facility from April 2011. CHAC’s forecast income from heat plus RHIs offers good scope for competitive gate fees. The latter could offset the cost of source-separated collection of biowastes. If those putrescibles bio-collections areweekly, they should lead to high capturerates of biowastes that are relatively uncontaminated.

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Efficiencies of Alpheco’s CHAC systems:

• Aergestorvessels are specially designed to maximise payload on the highways. They are automatically filled to optimum levels and initiate individual load records on an enterprises computer
• Aeration sub-system is separate from Aergestors to avoid wear and tear of frequent movement and unnecessary impact on payload on the highway;
• Minimal electricity is used to blow the moist air that is key to heat recovery
• Residence time is minimised by downward or upward aeration and superior time/temperature control in seven phases. Skilled operators can adjust from default values to suit particular feedstocks and so achieve the Quality Compost Protocol threshold from controlled waste to product in the shortest possible time. Residence time dominates capital and operating costs.
• Logistically, Aergestor movement by standard RollOnOff trucks is more cost-efficient than mechanical shovels for distances over about 50 m and than bulk carriers for distances under about 100 kms. RollOnOffs can deliver compost to the headlands offields where bulk carrier lorries dare not venture.
• Seasonal redeployment of systems between urban and rural consumers demanding cheap heat enables more usage than if systems were fixed. That should earn extra income affording the CHAC enterprise more flexibility in competitive contract pricing.

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‘Scaleability’

CHAC systems can have from two to ten Aergestors per group with its Aerator-cum-heat unit and biofilter. Each macerator-mixer-AlphLoaderline with a single operator can ‘serve’ about 20 Aergestors assuming a 37.5-hour working week and ten-day residence time, thus processing about 10,500 tonnes/year. That modularity allows flexibility in the deployment and site-layout of Aergestor groups; modularity also allows a CHAC enterprise to grow with the flow of biowastes and local demand for heat and compost.

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Bio-Collection economies:

The Quality Compost Protocol and PAS100 demand source separation but UK waste collection authorities are cautious of the cost of dedicated weekly biowaste rounds. As CHAC offers full cost-effectiveness by 10,500 tonnes/year, so BioColl vehicles can save time in urban traffic or about 40%of tonne-milesin rural districts by delivering to a local facility rather than commute to a 30,000 tonnes/year or larger plant. BioColl vehicles can co-collect municipal and commercial biowastes but with computerised weighing and thus separate billing. Such dedicated bio-collection costs are bound to be higher in £ per tonne than mixed waste collection but can be offset by the other savings. Waste authorities may also recognise that dedicated weekly collections encourage high capture rates thus reducing other collection and treatment or disposal costs.

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Waste licensing

Licensing by the Environment Agency (EA) of a composting site, for which planning permission is a pre-requisite, is a justified but possibly fairly expensive requirement. The concept of deploying groups of two to ten Aergestors in order to deliver the renewable heat thus appears quite onerous. The Environment Protection Act 1990 however, made a forward-looking provision for mobile equipment to operate at several places but under a single waste management licence. The EA’s Innovations Panel has offered to work with a CHAC pilot project to confirm that this provision of the EPA will apply. Clearly the proper containment of aerosols, odours and leachate will be an important matter for the EA to examine.

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Management efficiency

Economies of scale usually associated with large plants can still be enjoyed with CHAC because monitoring, accounts, sales and support can all be provided for several 10,500 t/yr facilities from a single management centre via the Internet. Each 15-tonne load of composting matter gets a unique ID so it can be traced from biowastes ingredients all the way to where the compost will be spread and BioColl enables computerised billing.

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Environmental benefits in addition to renewable heat & efficiencies

• Odour control is vital; lack of it has caused more plants to be closed than any other single reason. CHAC systems are hermetically sealed at all times except:
• A small proportion of the very smelly process gases are exhausted to atmosphere via an enhanced biofilter in order to be replaced by fresh air that is sucked in to maintain the oxygen level to ensure aerobic composting.
• The two aeration pipe connection spigots on each Aergestor are momentarily open while connecting/disconnecting to its aeration valve. As the Aergestor’s load is at atmospheric pressure and the dis/connect only takes about 10 seconds, this does lead to objectionable odour escape

Aerosol control is similarly effective. Odour and aerosol emissions when loads are tipped out of Aergestors, typically in rural locations, are relatively weak and unobjectionable because the compost will be stable albeit immature.

• Bio-fertiliserrich, soil-improving compost is produced. In contrast to some digestates, it needs not further treatment and unlike the ‘compost like output’ of MBT, quality compost is intrinsically valuable
• CO2 emissions from AD and composting are unavoidable, but CHAC minimises these by recirculating most of the process gases and maximising the soil organic matter output.
• Plant-nutrients’ (N, P & K) quantities are similar to those from anaerobic digestion of the same amount of biowastes. Composting has the advantage that the soluble nutrients remain in the compost rather than being diluted in liquid digestate that may be costly to transport or dispose of.
• The soil organic matter, which should at least be of comparable quantity to the digestate from AD, has benefits to cropping that are far above its proportion in top soil would suggest; those benefits include improved germination and yields, less need for pesticides and herbicides as well as synthesised fertilisers and reduced irrigation and run-off of rain.
• Alpheco has studied the energy and carbon balances from BioColl all the way through CHAC and delivery to where the compost matures before spreading; those positive balances challenge the efficacy of simple in-vessel composting and the view that AD is always the best way to deal with food wastes. That paper was presented at ORBIT’s 6th International Conference and is available on request.

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Competitive Costs:

Alpheco has built a detailed model for a CHAC enterprise with default values for a town of 125,000 people generating some 20,000 tonnes of household and commercial biowaste per year. The model, whose data extrapolates from Alpheco experiences supplying Anglian Water and Aberdeenshire Council with systems, assumes two reception-preparation-loading facilities and five heat-customer sites. The assumptions may be altered to suit other situations in order to forecast costs and a copy of the model will be supplied on request.

• CAPEX: Those default assumptions forecast a capital cost of £1.94M, which includes the BioColl bins and vehicles as well as the civil engineering and CHAC equipment. We believe it covers nearly every capital expense except for the real estate.
• OPEX: Data was drawn mainly from Alpheco’s own operating experience1997- 2001 co-composting green wastes with food wastes from five Sainsburys, four McDonalds, the Ipswich Hospital and Suffolk District Council’s Kesgrave Ploughback trial collection of household biowastes. Prices and rates have been updated to 2010.
• Collection is forecast to be £1.6M or £77/tonne including depreciation;
• Composting including the delivery of heat and of the stabilised yet immature compost is forecast to be £0.57M or £28.11 per tonne.

CAVEAT on costs

Those forecasts necessarily depend on numerous assumptions, the most sensitive one being residence time. The UK Government’s preference from 2007 for AD to treat food wastes, which has been supported by the subsidy of double ROCs for electricity from AD, prevented Alpheco obtaining orders, grants or private investment that could have substantiated or adjusted those forecasts. C’est la vie!

Comparison with AD.

While highly subjective, Alpheco contends that CHAC CAPEX will be approximately half that of an AD with CHP plant processing the same quantity of food wastes. Our model suggests CHAC will be significantly cheaper in operating cost too.

CHAC may also be useful for the aerobic composting of digestates from AD.

Given the forecast Renewable Heat Incentives, it appears ‘AD with CHP vs. CHAC playing field’ will be re-levelled so each technology will probably earn its special niche and enjoy similar returns on investment.

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Additional Notes

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Energy from AD

Alpheco’s approximate comparisons of energy from AD and CHAC drew on:

1. An AD mass balance chart from a presentation by Dr Charles Banks, Southampton University about the Greenfinch technology.
2. The Official Information Portal on Anaerobic Digestion by National Non-Food Crops Centre
3. Andersons AD cost calculator for NNFCC
4. The practical Handbook on Compost Engineering by Dr R .T. Haug
5. ‘Theoretical and Experimental Determination of Key Operating Parameters for Composting Systems’ thesis by Dr D Notton 2005. (Available on request.)
6. Alpheco’s ‘CHAC Model for DECC ver 3.xls on April 2010. (Accompanying)

The reason aerobic composting releases some 30% more total energy than AD is probably due to decomposition of fibrous matter by fungi, none of which are anaerobes.

Electricity from AD is usually via combustion of biogas (primarily methane) in a combined heat and power (CHP) engine, which inevitably releases about as much heat as electrical energy. Very, very few AD plants in UK and Europe have yet been built where the majority of that heat can be delivered to replace fossil fuel.

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Heat distribution networks

Heat is most commonly distributed by pumping water or steam via hot-out and cool-return pipes, both of which are lagged. Particulars of temperature, viscosity etc. set the maximum distance between heat source and point of use before the energy for pumping plus transmission losses negate the value of the heat. That distance is usually tens of meters rather than the hundreds of kilometres for electricity.

Laying heat-pipe networks in urban areas typically costs over £750 per metre.

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Heat Pump

New buildings with under-floor or space heating and farmers wanting to dry crops or biomass will usually opt to use the low-grade heat at the optimum composting (i.e. heat-flux[1]) temperature of 35 – 45 Celsius. Older buildings with conventional hot-water radiators will probably want to raise the renewable heat’s temperature to about 60 Celsius. Heat-pump manufacturers predict a coefficient of performance (CoP) of over 4:1. As electricity generation in UK consumes on average 2.83 units of primary energy (gas, oil or coal) for every unit of electricity consumed – the balance being heat that is often lost – a heat pump with a CoP of under 2.83 is perverse. Typical ground and air source heat pump installations usually have CoPs of 3.5 - 4.25. CHAC should do well by its source heat being over 35 Celsius.

Ground and air source heat pumps have been well developed and many of their components should be immediately suitable for integration with CHAC. The main exception will be the evaporator that will need to be in the CHAC aerator’s in-line expansion chamber-cum-moisture trap. Previously Alpheco has used conventional 0.8 x 0.8 m, aluminium finned, copper pipe, two-pass heat exchangers in this position. No major problems are foreseen in procuring stainless steel evaporators to withstand the higher pressure and be more corrosion proof. They will be of similar size but three or even four ‘pass’ because heat recovery has become a key objective.

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Food waste

Under the European Animal By-Products Regulation (ABPR) EC 1777/2002 (as amended), food waste from kitchens is classed as catering waste and falls within Category 3 of animal by-products. That European Regulation automatically applied within member states like UK without needing transposition into national law. It required that Category 3 matter must be reduced to 12 mm particle thickness and then composted for at least one hour at 70 Celsius or more to satisfy ABPR.

EC 177/2002 permitted member states to regulate catering wastes, but not other Category 3 ABP under national rules. The UK introduced national ABPRs (note plural) for composting catering waste, with meat-included and meat-excluded sub-categories and particle thickness-time-temperature regimes that it felt might be cheaper.

While CHAC-equipped enterprises could adopt the English, Scottish, Welsh or Northern Ireland ABPRs, Alpheco recommends implementing the EU ABPR because:

Other Category 3 ABP like blood, feathers, fur, guts and - most importantly - pre-kitchen ‘ former food stuffs of animal origin’ from distributors and retailers can then be co-composted. Thus a CHAC enterprise can earn extra income by providing a comprehensive bio-waste collection and composting service.

Loads in individual Aergestors can satisfy the 70 C for 1 hour requirement relatively quickly (1 – 2 days) to ‘pass’ ABPR and then be moved to their ‘heat customer location for about 8 –12 days intensive composting and after that directly to the compost customer. That avoids the inefficiency of returning to the reception-preparation-loading site.