The Suitability Of Different Feedstocks For Anaerobic Digestion
1. The Suitability of Different Feedstocks for Anaerobic Digestion [email_address] 25 th September 2007
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7. The Fractional Multiple ROC System (Source: DEFRA, 2007) 2.0 ROCs/MWh Wave, tidal, advanced conversion techniques, (gasification, pyrolysis and AD), dedicated regular biomass with CHP, solar, geothermal Emerging 1.5 ROCs/MWh Offshore wind, dedicated regular biomass Post Demonstration 1.0 ROCs/MWh Onshore wind, hydro-electric, co-firing of energy crops, energy from waste with CHP Reference 0.25 ROCs/MWh Sewage gas, landfill gas, co-firing of non-energy crop biomass Established
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Notes de l'éditeur
We’ll have a quite a detailed look at 1 of the key drivers (Renewable Obligations) for AD before moving on to the process requirements to ensure that AD will work by considering potential contaminants, what the impact of different wastes will have on the methane yields by analysing waste characteristics and wrapping up by achieving a nice balance in your anaerobic reactor to achieve stable plant conditions. First off then, RO’s…
Renewable obligations contribute to higher energy prices, 4% to electricity and 3% to gas currently, but expected to rise to 5% by 2015. Often referred to as distributed energy which is energy that is local and low in carbon but is often more expensive to generate. ROC value is currently £40. Target of 7.9% for 2007/8, 10% by 2010, 15.4% by 2015 then static until 2027 when it will end, but headroom of 20% if the market is there. 1 ROC = 1MWh, sold to electricity supplier. Buy out price is linked to RPI and does vary depending upon whether or not energy suppliers are meeting their obligations. Supply and demand, so you can take a risk and may get £60 ROC if you play the game well. However, change is afoot. There is currently a flat rate of 1 ROC per MWh from eligible sources, so what are eligible sources.
Here are some of the eligible sources And as we can see some sources appear to be more stable than others, which is important as this has been used to assess the future potential of markets to increase or which are likely to remain static like sewage gas.
Asses the stability and potential of the different renewable energy markets, levelised costs reflect the electricity revenue per MWh, net of PPA (Power Purchase Agreement) discounts, which is needed throughout the life of the technology to make the respective technology commercially viable. The sewage gas market has been assumed to be saturated. What is the timescale for other Biomass technologies, according to 2020 levelised costs unlikely to be saturated by then. What is the technology and cost of co-digestion? Is there an argument for Biomass ROC’s to be based upon inputs rather than technology and thus open the window to co-digestion? Or are gates fees sufficient alone? DEFRA Waste strategy 2006 highlights the need for a ‘joined up approach to waste’, this surely lends itself to co-digestion. Also waste should be viewed as a resource. Digestate standards needed for non-source as well as source segregated wastes to be truly considered a resource. Is a 20 year arrangement enough given that LA’s often have 25 year contracts.
The new system, Implemented from 2009 is the Fractional / Multiple ROC system and accounts for levelised costs and the market potential for eligible sources. Different bands are based upon a market assessment and fixed until 2013 (reviews expected every 3-5 years). Based upon ROC’s we understand that it is the methane content that is critical and not the just the biogas volume. Depending upon the components of your feedstock you can realistically expect a methane content ranging anywhere from 50% to 75% methane, so in a way we can list the different wastes available to us and rank them in terms of what we would like to receive at the AD facility.
Quite clearly the quality of the biogas in terms of percentage is just as important as the quantity produced. A biogas with 50% CH4 and 50% CO2 will yield less ROC’s and less energy than a biogas with 75% CH4 and 25% CO2. There’s also some materials we need to watch out for that may cause us some problems and reduce the yield, so we need a clean feed stock. ‘Feedstock – any substrate that can be converted into methane by anaerobic bacteria.’ And certain contaminants can also cause us problems when we come to dispose of the final product, which ideally we want to spread to land and not send to landfill. So firstly we have inorganic sources of contaminants…
Heavy metals. Of course the less sorting of your waste the more likely these are to appear. And organic contaminants….
But hopefully by sorting your waste source well the impact of many of these inorganic and organic contaminants can be neutralised.
And here’s where we come on to the impact your feedstock can have on your methane yield. And fairly easily we can develop a hierarchy of energy sources. Fats digest well and rapidly and will yield a high quality and large quantity of biogas, about 2.4 kg COD/kg VS. Proteins - the basic bond is the peptide bond, which when hydrolysed releases amino acids, which are then rapidly de-aminated (which means that ammonia is released) to short chain fatty acids, ammonia and reduced sulphur compounds. Some proteins are very complex and difficult to hydrolyse and as these are proteins there is the potential for high levels of ammonia to be released (blood / meat waste). Proteins have a specific COD of ~1.4 kg COD/kg VS with ~60% biogas as methane. Carbohydrates – sugars, starches, cellulose, some are simple and some are complex. Lignin is manufactured by plants and is therefore a major component of wood, straw and plants. Lignin combines with cellulose to give lignocellulose, which is very difficult to hydrolyse and virtually no breakdown occurs. Wood/straw has ~29% lignin, paper and card ~22% and office paper ~18%. Therefore wastes with varying paper and green waste contents will significantly affect your potential yield.
pH changes can directly affect the activity of the organisms, but more than this we see some indirect effects as the toxicity of different compounds changes with pH, one example is ammonia and ammonia is becoming more and more important with more stringent discharge consent levels resulting in higher levels present within the final sludge product to dispose of. Combined with this some of the benefits of the advancements in technologies have been to feed a digester at 10-12% dry solids, compared to 4-6% traditionally, which is all well and good but there’s a doubling of the ammonia concentration. Although nitrogen is an important nutrient for cell growth, so some ammonium uptake by cells can be expected. However, in organic rich wastes a large excess of nitrogen can become present in the reactor, as in this case. The potential toxicity of unionised ammonia is dependent upon the prevailing pH, being greater at higher pH’s (more alkaline) and we have been observing in some tests using predominantly domestic sewage sludges and particularly high in SAS pH’s reaching 8.5.
When we talk about achieving a balance there are a number of parameters to consider. Go through each column Look at this briefly. It’s more reference material. And these factors all combine to impact upon the energy balance.
By considering the different elements of your feedstock and the different potential sources we can see that there are a range of potential pitfalls and that you need to achieve the right balance: Retention time – shorter with more readily biodegradable material C:N – balance too low and there’s insufficient nutrients for bacteria to perform their role, too high and ammonia can become toxic as we have seen. Volatile solids – the higher this is the more potential in the waste reduce the final disposal volume COD: VSS – the higher this is the more methane potential. Methane content – the higher the better, more power, more ROC’s Organic loading rate Feedstock contaminants Inhibitors
But where does this start, well we come again full circle back to knowing your waste and it is actually relatively straightforward to assess the potential of a waste stream for anaerobic digestion, as this is based on simple tests such as chemical oxygen demand (COD), volatile organic solids and total Kjeldahl nitrogen (TKN). Where waste streams are found to be deficient in key components there is always the potential for co-digestion with an alternative waste in order to create an admixture with the ideal properties. However, bench studies will always be needed, not only to ensure the waste stream is amenable to anaerobic digestion but also to provide the important quantitative data to undertake a meaningful cost benefit analysis. Whereas such trials are not complex some patience is necessary as a period of acclimatisation will generally be required. The potential rewards from application of anaerobic digestion are high not only for the client but also for the environment. Anaerobic digestion has waited a long time but it is one technology whose time has finally arrived.
There are enormous variations reported in the literature on the potential methane yields and for this reason it is important to understand how your waste is processed and transported to minimise energy loss from the food source. The mode of waste generation, the arrangement of waste collection, transport and occasionally required pretreatment, strongly influence the overall process course. Long transport distances, as well as high required storage capacities have detrimental effects on the overall process economics. Numerous wastes require effective pre-treatment strategies prior to digestion. Common procedures include removal of non-degradable components (wood, plastics, sand, metals, glass etc.), grinding or cutting of bulky material and finally homogenizing of the waste organic matter. Co substrates, particularly ones such as biogenic wastes, garden wastes or kitchen and restaurant waste, often require expensive pre-treatment procedures.
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