What Can We Do With All This. C. Merritt, C&S Engineers. Food Waste?

Transcription

1 What Can We Do With All This C. Merritt, C&S Engineers Food Waste?

2 Here s the Deal How big is this problem- Food is one of the big 4 (water, energy and climate change) Sources of food wastes What are the technologies used on food wastes Follow the economics of a 100ton/day plant through each option Its all about energy conservation and finding a way to convert the waste to a useful product. Sit back and enjoy

3 How big is the problem?

4 Types of food wastes

5 Supply chain waste Food processors Table scraps

6 Waste Food Chemistry Fruits and Vegetables Mostly water Dissolved sugars/ starches in the liquid part Solids have cellulose, hemicellulose, starches, protein and ash not a lot of fats Processing wastes Diverse but generated in mass quantities by type-just walk through a supermarket Higher solids content Can be segregated relatively easy Table scraps Diverse and difficult to separate types

7 Food waste properties The liquid phase of fruits and vegetables is relatively easy to ferment or digest Fruits and vegetables solids do not ferment nor digest easily Processed food waste high in solids does not ferment nor digest easily, but can make good candidates for animal feed or solid fuel Table scrap food wastes are difficult to process into animal feed or solid fuel Table scraps do make good digester feedstock and good compost feed when mixed with yard waste. We tend to lump all three types together and seek to find one technology to handle the three types..hard to do

8 The Hierarchy of Solutions The fact we have strict food safety regs, transport a lot of food and we value appearance will always have food waste

9 The technologies available Landfilling Composting Industrial uses Anaerobic digestion Fermentation Hydrothermal processing Use as animal feed or solid fuel

10 Landfilling: Terrible option for energy recovery

11 Option1: 100 tons taken to the landfill each week 10 dump trucks worth of food waste dumps in a landfill Tipping fee of $40/ton Cost of $40 x 100 = $4000 Benefits Convenient out of sight out of mind Consequences No energy recovery Leachate to deal with CO2 and CH4 escape into the atmosphere

12 Composting

13 What happens in the compost pile. Needs oxygen (>10%) to keep the process aerobic, otherwise the anaerobic bugs take over and you get nasty odors like Hydrogen Sulfide The process creates CO2 and a little CH4 Needs 50-60% moisture for optimal conditions Anywhere from % mass reduction The composting microorganisms produce heat and enzymes that result in organic matter degradation Compost feed stock is measured in Carbon/Nitrogen ratio with 30:1 being optimal C/N ratio goes down with compost time

14 Three stages of biological activity that breakdown the material and digest it. Each stage is governed by the temperature and the bugs that thrive in those temperatures.

15 Option 2: Compost 100 tons food waste per week 10 dump trucks worth of food waste dumps in a composting site Tipping fee of $40/ton Costs are $40 x 100 = $ tons of $10/ton value = $600 return Processing cost $ 5/ton = -$ 500 Net income is then $4,600 $500 = $4100 /100 tons Benefits Cheap fertilizer Mainly CO2 (vs. CH4) produced Better than landfilling Consequences Need land space and method of turning over compost Still have not recovered any energy

16 Anaerobic digestion (industrial process) Biological breakdown of organics in the absence of oxygen. Happens in a land fill, in your septic tank and in your gut Can be duplicated in a controlled environment using tankage Converts organics to methane and carbon dioxide The methane can be captured, purified and used as a fuel source Very good way to convert liquid waste high in organics to energy Not a new technology, be around for decades Waste water treatment plants Large farms- manure pits Becoming an accepted method for treating food wastes

17 Anaerobic Digestion Facts - Sugars and starches digest fast - Fats/ proteins digests a little slower - Cellulose and hemicellulose digest slow - Digestion rates speed up with temperature - Digesters like constant flow and consistent food source

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19 Anaerobic digestion viability Recovered methane is used to fuel CHP units, dryers or boilers Very good way to reduce COD (chemical Oxygen Demand) in waste water from industrial plants Methane is 60-65% of the gas (rest is CO2) and may be contaminated with H2S Has some limitations Bugs need F temps.- challenge in winter months Pretty tight chemistry for digestion- ph, alkalinity, trace metals etc. Long residency times (days for complete digestion) High cost ($600/ton install cost); $20+/ton O&M costs Need consistent loading for good operation Digester size must account for residence time ie 100 tons processing will require a digester capable of storing 1000 tons for a 10 day retention time

20 Anaerobic Digestion Mass Balance Each lb of waste food can create cubic feet of 1.7 cubic feet of CH4 and 1.3 cubic feet of CO2 This equates to about (3,400 cubic ft methane / ton food) Each lb of COD added to a digester will yield 2.4 cubic feet of CH4 Each lb of food digested creates about >0.3lbs lbs of digestate (compost material)

21 Option 3: Digest 100 tons of food waste/week 10 dump trucks worth of food waste dumps in a AD processing site Tipping fee of $40/ton Costs $40 x 100 = $4000 The cost to digest 100 $20/ton is $2000 From 100 tons of digested food we get 340,000 cubic feet of CH4 The amount of heat from CH4 is 3400 therms for a therm = $1700 or The amount of electricity generated form CH4 is 25,000 KWH. At $.08/ KWH = $ 2000 The amount of compost is 30 tons with a value of $10/ton = $300 The net value then is = $ 4300/100 tons waste food

22 Fermentation Fermentation is the process of using yeasts or bacteria to convert sugars to alcohol Methanol, ethanol or butanol We know it best for making beer or spirits, but is used to make fuel grade ethanol. We can only ferment to about 12% ethanol due to its toxicity towards the yeasts/ bacteria. Takes a lot of energy to distill and dry it to 99+% useful fuel. Any waste high in carbohydrates can be fermented fairly easily. Currently, corn is the major commodity used to make ethanol fuel, but we could use waste fruits and vegetables as well.

23 Typical ethanol production plant

24 Fermentation Mass and Energy Balances One (1) lb of sugar will produce about ½ lb of ethanol and ½ lb of CO2 We also know the density of ethanol to be.7893 gm/cc Therefore we can calculate the volume of ethanol produced to be 183cc or.183 liters/ lb sugar Converting.183 liters to gallons =.047 gal ethanol/ lb of sugar We also know for ethanol plants that it takes about 30K btu/gal to distill and dry ethanol from beer We know that fruits and vegetables contain 8-20% sugars We also know that the solid mass left over can be used as animal feed or solid fuel

25 Option 4; Ferment 100 tons food waste each week 10 dump trucks delivers food waste to processing plant Tipping fee of $40/ton Costs $40 x 100 = $4000 The cost to ferment and purify one gallon of ethanol is about $0.65/gal 100 tons of food waste at 12% sugar content will yield 24,000 lbs of fermentable sugars Each lb of sugar will produce liters or gallons of ethanol Therefore the 100 tons waste will produce 1128 gallons of ethanol at a cost of $733. Fuel grade ethanol value vary with gas prices, however at $1.5/gallon this plant could produce $1,692 worth of ethanol You would also get 5 tons of animal feed at a value of $150/ton = $750 Net value is $ $750 +$1692 $733 = $5,709/100 tons

26 Fermentation Limitation Problem is an ethanol plant costs about $200MM to build and there is not a good stencil for a small ethanol plant You would need to located near an ethanol plant Economics do not favor small fuel grade distilleries Waste food is not recognized by DOE/ DOT as a possible transportation fuel source.no credits. Ethanol plant are reluctant to acceptable ethanol from food waste

27 Option 5: Hydrothermal processing Treatment of wastes using moderate heat (750F ) and high pressure (3000psig). Essentially mimicking what take place in the earth underground over millions of years in just a few minutes. Uses steam and a pressure vessel to convert wastes into a oily substance that can be further refined into a fuel source.

28 Hydrothermal Process

29 Could this new technology be viable? Likely not right away. 3000psig vessel in any size > 1000gallons would be very expensive to fabricate. May not lend itself well to continuous flow. If it could work, it would be a very good technology It is to new to even evaluate on a production scale

30 Solid Fuel or Animal Feed Production Animal feed Solids from fermentation process Dry waste fruits and vegetables Lots of nutrient in waste food Craft brewing spent grains, apple pomace, etc Solid fuel Solids from fermentation or anaerobic digestion Dry waste fruits and vegetables Some processing wastes Can be mixed with sawdust or other solid fuels Either way you would likely have to dry to <10% moisture and pelletize (densify)

31 Option 6: Dry 100 tons food waste/week for animal feed 10 dump trucks delivers food waste to processing plant Tipping fee of $40/ton Costs $40 x 100 = $ ,000 lbs at 10% solids = 20,000 lbs of animal feed Dry animal feed is worth up to $200/ ton Total value is 10 tons x $200/ton = $2000 But we have to dry it to make is storable So we need to evaporate the other 178,000 lbs of water To evaporate water from 80F to steam we need about 1100 btu/lb Therefore 178,000lbs x 1100btu/lb = 196,000,000 btu s and this equates to 1,960 therms of natural gas. At $.50 therm we need $980 worth of gas, which does not account for the dryer efficiency losses or the capital costs of the equipment Theoretical revenue is $5120/100 tons of food waste

32 What about making solid fuel from waste food? The value of pelletized solid fuel is less than animal feed ($150 vs. $200/ton) The economics look like this $ 4000 $1500 $ 980 $ 4620/ 100 tons of food waste One advantage this option has is that saw dust of other yard waste could be added to increase volume of solid fuel.

33 What if we could remove some of the water and then dry the food waste to animal feed or solid fuel.. I did some research at home. I took fruits and vegetables and ground then up and pressed the water out. I separated the liquid fraction and analyzed it for COD to determine its value as a anaerobic digester feed stock to produce methane for energy recovery Then I took the wet solids and dried them to see if they would have value as a solid fuel (or animal feed)

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35 Liquid and solid fractions

36 Results I found we could remove about 65 % of the water from the fruits and vegetables and the liquid fraction was full of easily digestible or fermentable organics. Vegetable liquid had 58,000mg/L COD Fruit liquid waste had 140,000 mg/l COD This means drying costs are much lower and the organics in the liquid phase could be digested and the methane produced to provide fuel to a dryer. Could we develop a closed energy cycle while producing a salable product?

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38 Mass balance of the new strategy Start with 200,000 lbs wet food solids Remove 65% liquid 130,000lbs liquid or 15,587 gallon liquid with an average of 100,000 mg/l COD or 388 gm/gallon Left with 70,000lbs of wet cake which contained 60% moisture The 70,000 lbs was 20,000lbs solids and 50,000lbs of water To remove enough water to get to 10% moisture content we need to evaporate 48,000 lbs of water We then need at least 48,000lbs x 1100 btu/lb = 52.8MMbtu or 528 therms of NG Good news: From the 15,587 gallons of liquid waste we have a total of 15,587 gal x 388 gm/ gal COD / 454gm/ lb = 13,321 lbs COD/ 100tons food waste We know digesters will produce 2.4ft3 CH4/ lb COD, so we can generate 31,970 ft3 CH4 which equates to 32 MMBTU

39 Option 7: Economics of making fuel or feed 10 dump trucks delivers food waste to processing plant Tipping fee of $40/ton Costs $40 x 100 = $ ,000lbs of animal feed or fuel from 100tons food waste 11 tons at /ton = $1,650 fuel or $2,200 feed We would need = 21 MM btu or 210 therms to complete drying 210 $.50/ therm = $105 Cost to digest 130,000 lbs $10/ton = $650 Net income would be = $4895 /100 tons food waste turned into solid fuel Net income would be = $5445/100 tons food waste turned into animal feed

40 Summary of economics per 100 tons Landfill..$ 4000 Composting.$ 4100 Anaerobic digestion $ 4300 Fermentation. $ 5709 Hydrothermal Processing -..TBD Animal feed, dried $ 5120 Solid fuel, dried.$ 4610 Solid fuel/ digestion (Fruits and Veg only).. $ 4895 Animal feed/ digestion (Fruits and Veg only)..$ 5445

41 Summary Billions of pounds of waste food each year Currently, landfilling is most used method, but the worst option Composting is better economically and environmentally Several Industrial processes can be used to process wastes, but most have high capital costs and not high returns Waste fruits and vegetables have value as a feed or fuel product.

42 Questions What are the three sources of food waste? Which disposal method produces the most CH4? Composting or Anaerobic Digestion What is the products from fermentation? What are the conditions for hydrothermal processing (Temp. And Pres.) What is the limiting costs for making animal feed or solid fuel from waste food