Renewable Energy

1. Renewable Energy Biogas 1 (Biology and Workings)

2. Definition of what we are talking about. • Biogas is a mixture of different gases like methane, carbon dioxide, hydrogen sulfide, nitrogen etc. which is part of the decay process. Only the methane portion is valued as a fuel. The other gases are not flammable or exist in such small quantities as to make them of no consequence.

3. Who, What, Why, When, Where, How???? • Why am I building it. • What are the digestible materials avail.? • What kind of digester is appropriate? • How big, what capacity? • Who will operate it? • What will the fuel be used for? • Where will it be located?

4. Digester Biogas Sludge Gas Cycle Nutrient Cycle Water Burners Algae pond Boiler, Heat Engine Crops Fish pond Algae pond Food Heat, Animal Feed Light Closed Nutrient System of a Complete Digester Operation

5. Types of Decay • 1- Aerobic • with oxygen • 2- Anaerobic • without oxygen • All OM can be decomposed either way but the end product and process is different.

6. Organic Wastes ANAEROBIC AEROBIC Natural Artificial Natural Artificial Dead plants, animals, manure Compost Piles Decay under water Guts of Animals Air-tight digester Humus Biogas NH3& CO2 NH3 & CO2 Manure Sludge Peat

7. History • Anaerobic decay started …… “In the beginning” One of the Earth’s oldest processes. • Louis Pasteur once discussed possibility of methane production from manure. • China, India, and Africa have used for a long time

8. History cont. • During WW II, fuel shortages in Germany lead to methane production plants in rural areas. Gas used as tractor fuel • Previous digester projects in the U.S. for animal waste control were not very successful. • Can’t ignore the system, need skills and time • Incompatible systems applied to situation

9. History cont. • “Cookie cutter” designs don’t work. Of 30 plug flow digesters built, 19 by one designer, 90% failed to function. • Incorrect sizing of equipment or noncorrosion resistant pipes and fittings • Inadequate training • Insufficient financial returns on investment

10. Biology of Anaerobic Digestion • Biological process • Oxygen destroys bacterial activity • Initially, oxygen in fresh batch is used up by aerobic bacteria which produce CO2. • Only after all oxygen is used up can digestion begin. • Different kinds of bacteria are prevalent at different stages of digestion.

11. Biology cont. • Bacteria feed off by-products of previous group. • Stage 1- Acid producing bacteria break fats, proteins and most starches into simpler compounds. They rapidly reproduce, are tolerant of environmental changes, excrete enzymes that work on volatile acids.

12. Importance of Volatile Acids • Low molecular weight organic acids. Important example is acetic acid (vinegar), which is a by-product of all fat, starch, and protein digestion. About 70% of methane produced during fermentation comes from acetic acid.

13. Methane Producing Bacteria • Next step in digestion • Convert volatile acids into methane gas. • Reproduce slowly • Sensitive to environmental changes

14. Digesters • Vessels or ponds that hasten anaerobic digestion process. • Three main types • 1. Covered Lagoons • 2. Complete Mix Digester (Batch Load) • 3. Plug Flow (Continuous Load)

15. Covered Lagoons • Treats liquid manure with less than 2% solids. • Large volume, preferable >12 ft deep • Covered

16. Complete Mix (Batch Load) • Heated tanks above or below ground • 3-10% solids • Tank is filled and left alone until gas production stops

18. Plug Flow (Continuous Load) • Daily load added on one end which displaces previous material. • 11-13% solids • Not suitable process for swine manure due to lack of fiber • Digestion occurs as progresses through vessel with bubbles of gas formed along way. • Near end, little gas produced. • Residues begin to stratify.

19. Phase Useable Resource Biogas Scum Supernatant Digested Sludge (spent slurry) Inorganic Solids GAS COMBUSTABLE GAS FERTILIZER, INSULATOR BIOLOGICALLY ACTIVE FERTILIZER LIQUID SOLID TRASH, DISCARD

20. What do you feed a digester? • Slurry! • A mixture of water, manure, plant material, or vegetative matter

21. Sludge- spent solids, volume reduced to about 40% of raw volume • Supernatant- spent liquids of original slurry. Great fertilizer value. • Scum- coarse, fibrous material, released from raw manure, gas, and liquid. Removal is big problem because it can inactivate digester in large amounts.

22. pH and buffering • Important for maintaining balance between acid forming and methane forming bacteria. • pH range for digesters is 7.5-8.5 • At start of digestion, pH may drop below 6 for a couple of weeks then rise over next couple of months while volatile acids and nitrogen compounds are digested. Ammonia formed during this stage.

23. Methane production starts at about pH 7. • Mixture is well buffered at 7.5-8.5 • Small amounts of fresh slurry can be added after pH 7.5 • Batch load (plug flow) digesters require continuous feeding or else enzymes will build up, organic solids become exhausted and methane production ceases.

24. pH 8-8.5 is sign of stable digestion • Check pH with • meter (most accurate) • pH dip strips • litmus paper (least accurate) • blue paper--> red in acid 1-7 • red paper--> blue in base 7-14

25. Problems with pH Condition Reason Solution Too acid 1) adding raw materials too fast reduce feeding (pH <6) rate 2) wide temperature fluctuations stabilize temp. 3) toxic substances 4) build-up of scum remove scum Too alkaline 1) Initial raw material too alkaline be patient (pH >9) never add acid

26. More solutions to pH problems • Fresh effluent added back into inlet of continuous load digester will raise pH. A little addition of ammonia may also help. • If the solution becomes too alkaline, a lot of CO2 will be produced which will self-correct because CO2 promotes acidity. • Adding acid will create hydrogen sulfide. Don’t Do It!

27. Temperature • Ideal is 95 deg F (36 deg C) • Suitable ranges 85-105 and 120-140 deg F • At higher range, thermophillic species operate • Difficult to maintain high temp, produce poor sludge quality, bacteria sensitive to changes, most materials will digest well at lower temperatures anyway.

28. Raw Organic Wastes Inorganic Portion Organic portion Indigestible Digestible Acid producing Simple comp. Bacteria Volatile solids Methane Producing Bacteria Mineral Fiber, Methane CO2 Water & Gas Compounds Lignin

29. This material is based upon work supported by the National Science Foundation under award DUE-0434405

31. Renewable Energy Biogas 2 (C:N and Feed Quality)

32. Factors Affecting Digestibility • Moisture- measured by drying at 220 deg F • Total Solids (dry weight)- Weight of dry material remaining after drying. Note: sun dried materials will still contain about 30% moisture • Volatile Solids- weight of organic solids burned off when dry material is ignited. This is what the bacteria actually can use.

33. Factors cont. • Fixed solids (ash content)- weight of material remaining after ignition. Bugs can’t utilize it.

34. How much manure resource is available? • Tables showing output are very general for three main reasons. • 1- Size (age) of livestock • ratio manure to live animal changes with time • see next slide • 2- Degree of confinement • 3- Kind of manure collected

35. Hog manure changes by age Total manure Hog weightlbs/dayFecesUrineRatio manure: hog wt. 40-80 5.6 2.7 2.9 1:11 80-120 11.5 5.4 6.1 1:9 120-160 14.6 6.5 8.1 1:10 160-200 17.6 8.5 9.1 1:10

36. Degree of Confinement • Confined easier to collect from than “free range” • Quantity collectable will vary too

37. Kind of manure collected • Feces + urine • All excrement + bedding • Wet feces only • Dry feces only

38. Volatile solids/day Equivalent Animal (80% of TS, 85% for swine) Animal units 1000 lb cow 8.0 1 850 lb horse 5.5 1.5 160 lb swine 1.3 6.2 67 lb sheep 0.4 20 150 lb human .25 32 Broiler chicken .06 133 Only feces is figured in total for cow, horse, swine, sheep because of difficulty in collecting urine too.

39. Carbon:Nitrogen (C:N) Ratio • Bacteria utilize carbon in the form of carbohydrates for energy and nitrogen in the forms of ammonia, protein, nitrates, etc. for cell structure. This is their food. Only the form is different depending on slurry (raw ingredients). • C is used about 30 times faster than N

40. Both C and N required for digestion. • If the ratio was 15 times more carbon than nitrogen, it would be written 15:1, 15/1, or 15. • If the ratio is unbalanced, the process is hindered.

41. Tables showing C:N ratios vary because • 1. Lab data is not the same as real-life. Some of the C digested in lab can’t be digested by bugs. • 2. Wide variation in plant development stages, animal diet, confinement etc, so fluctuations in C/N are common.

42. Nitrogen • Forms in slurry • TKN, NO3, NH3, protein • Varies by age of plant or manure “ the lot or batch is variable” • Body excretes excess N in urine. Is urine included in your sample? • Fowl excrete urine and feces together • Ruminant’s bacteria use much of the N in digestion so fecal N is lowest of animals.

43. Bacteria can use most forms of N so best way to test and report is as Total N (% of dry weight)

44. Carbon • Exists in many forms but not all are bacteria-usable. • Lignin is unusable • Best to test and report as non-lignin carbon content

45. Approx. % and form of N found in different kinds of manure Organic Organic Ammonia NH3 Pig Ammonia Chicken Cow Ammonia Organic

46. C:N Handout Contact Instructor for copy

47. Example C/N Ratio Calculation • You want to compost 50 lbs horse manure and 50 lbs dry wheat straw. Will the C/N ratio be ok? • First- from table find the C/N ratios for each and the percent N by dry weight. • Manure C:N is 25:1 and N% is 0.023 • Straw C:N is 150:1 and N% is 0.005 • Second- Multiply % N by weight of product • 0.023 x 50 lbs = 1.15 lbs actual N in manure • 0.005 x 50 lbs = 0.25 lbs actual N in straw

48. Third- Multiply carbon value by lbs N in problem • 25 x 1.15 lbs = 28.75 for manure • 150 x .25 lbs = 37.5 for straw • Fourth- Add the carbon contribution from the straw and manure into total pounds. Repeat for the nitrogen. (see next page)

49. Manure Straw Total Carbon 28.75 37.5 66.25 lbs Nitrogen 1.2 .25 1.45 lbs Last- Calculate the overall carbon:nitrogen ratio for the mix 66.25/1.45 = 45.69 Ratio is a little high but probably ok. Around 30 is better.

50. You try this C/N problem • Calculate the C/N ratio of 8 pounds grass clippings (ratio = 12) mixed with 2 lbs chicken manure (ratio = 15). • Answer: • Grass C:N is 12:1 and N% is 0.04 • Chicken manure C:N is 15:1 and N% is 0.063 • 0.04 x 8 lbs = .32 lbs actual N in grass clippings • 0.63 x 2 lbs = 0.13 lbs actual N in chicken manure