2008 SIT paper
Leipzig, Germany, May 25 – 28, 2008
Zero Discharge Technologies for alcohol distillery with energy recovery
Chung-Chi Chou*, president, Dr Chou Technologies, Inc. New York, USA
Wen-Hong Gao, associate professor, South China University of Technology, China
Hua-Ming Huang, president, Guangdong Zhonneng Alcohol Co. , Ltd. , China
The sugarcane molasses and /or juice-fed alcohol distillery plants produce wastewater effluent, commonly called alcohol distillery waste (ADW), which carries very high loading of both biological and chemical oxygen demand (BOD and COD) and other impurities causing environmental problems. The ADW needs to be disposed in compliance with governmental regulatory discharge standards. The conventional methods of ADW disposal are by composting and /or mixing with other factory waste for agricultural use. Most R&D work has been in the area of BOD and COD removal by aerobic and/or anaerobic digestion without energy recovery and, by bio-methanation and catalytic thermolysis with some degree of energy saving.
In late 1990’s, Zhonneng Alcohol plant of Guangdong, China successfully developed a process to achieve zero emission of ADW by evaporation of ADW to 65 brix followed by combustion of concentrated ADW to produce steam and electricity for use in the plants. The “ADW concentration and combustion” plant was built adjacent to the alcohol factory and put into operation in August 2000 and successfully attains the goal of “zero” discharge of ADW and up-to-standard emission of smoke and flue gas. The plant has passed the comprehensive inspection of the county and municipal Environmental Protection Authorities and granted a science and technology progress award from the local government.
|This paper describes the details of the development approaches and various parameters/data based on the past seven/eight years of plant operational experience.|
(II) Feasibility of the Technology
In my paper on the subject of “Research and Development—Do we have tunnel Vision?” for the Sugar Industry Technologists 66th Annual Technical conference (2007), I have stated that experienced and competent technologists should be able to assess the probability of technical success with significant degree of certainty. Undercurrent competitive business environment in a global economy, I would not undertake any project with probability of technical success of less than 65%.
|Following this approach, the first step for this zero discharge technologies will be to evaluate technical feasibility and the probability of technical success. This typically involves the following:|
|a)||Review and evaluation of literature / published papers based on researchers’ training and education.|
|b)||Available advertised technical brochure|
|c)||Past experience and prior knowledge/information (internal & external)|
|e)||Preliminary laboratory tests|
The literature in the field of ADW disposal by evaporation to 65 Brix and followed by combustion of the concentrated ADW with energy recovery is limited. We practically needed to start the developmental work from the “scratch”
1) Typical composition of alcohol distillery wastes (shown below):
Since the key to the process is the combustion of ADW we need to know two questions: first, is it combustible? If it is, at what brix? and the second, is the process self sustainable with respect to energy? Based on the typical composition of ADW shown below, the answer to the first question is yes because the ash content is only less than 5%. Most other components should be combustible if the ADW is concentrated enough.
|Conductivity (meghohms/cm)||55,000 – 58,000|
|Suspended solid||2.5 -3.5 gram/lit.|
|Total dissolved solid||110-130 g/lit|
|Potassium (K)||11 – 13 g/lit|
|Magnesium (Mg)||2.5-3.0 g/lit|
|Calcium (Ca)||2.5 -3.8 g/lit|
|Sodium (Na)||500 – 750 mg/lit|
|Iron (Fe)||Iron (Fe)|
|Silica (SiO2)||500 – 7000 mg/lit|
|Chloride (Cl)||14 – 16 g/lit|
|Sulfate (SO4)||3.5 -5.0 g/lit|
|Ash||4.0 – 4.5 %|
|Total Nitrogen (kjeldal)||650 – 800 mg/lit|
|COD||125,000 – 135,000 mg/lit|
|BOD||45,000 – 50,000 mg/lit|
2) Heating value of ADW as compared to other common material
Table 1 gives the gross heating values of various commonly used products. It is a surprise to this author to find out the gross heating value of cane bagasse at 50% water content is comparable to that of ADW at 65 brix, both are around 2300 kcal/kg. Cane sugar mill operators are well aware that a well run sugar mill should have about 15% bagasse surplus after generating all the steam and electricity needed for the production of sugar. It could be expected that concentrated ADW, with comparable heating value, should be able to burn as fuel in a boiler since it has only 35% water as compared to 50 % water in case of bagasse. It should be pointed out that the gross heating value includes the heat for evaporation of water in the products. Therefore the net heating value is the available heat after the subtraction of heat of vaporization of water. It appears that, in case of cane bagasse with 50% H2O, more heat is expensed to evaporate its water content.
Gross heating (calorific) values of various products
|Wood||3600 kcal/kg||6480 BTU/lb|
|Alcohol Distillery Waste at 65 Bx (ADW)||2278 kcal/kg||4100 BTU/lb|
|Cane Baggasse at 50% H2O||2300 kcal/kg||4140 BTU/lb|
|Heat of evaporation of water||540 kcal/kg||972 BTU/lb|
|3) Water balance of the ADW zero discharge plant.|
The zero discharge technologies involves the evaporation of around 12 brix ADW to 65 brix followed by combustion of concentrated ADW to produce steam and electricity for use in the plants. We would need to do a water balance in order to calculate how much water is to be evaporated in the evaporation station and how much water is to be evaporated in the boiler during the combustion stage.
|Table 2 water balance: Based on 1,000 Kg. at 12% (brix) solid|
|Wt of solid content: 120 kg|
|Wt of concentrated ADW at 65 Bx:|
|Y x 65% = 120 kg, Y = 120 kg / 0.65 = 184.6 kg|
|Wt of water in 65 Brix ADW: 184.6 kg – 120 kg = 64.6 kg|
|Water to be evaporated: 1000 kg – 184.6 kg = 815.4 kg.|
From the above calculation, for 1,000 Kg of ADW, 815.4 Kg of water is to be evaporated in the evaporator station and 64.6 Kg in the boiler. The next question would be: Dose 1,000 Kg of ADW contain enough heating value for both evaporation processes?
4) Energy balance of the ADW zero discharge plant.
|Table 3 Energy balance based of 1,000 Kg of 12 brix ADW|
|Heat of vaporization of water: 540 kcal/kg|
|Gross heating value of 65 bx ADW: 2278 kcal/kg (by laboratory test)|
|Total heating value of 65 bx ADW:|
|184.6 kg x 2278 kcal/kg = 420,544 kcal|
|Heat needed to vaporize 64.6 kg water:|
|64.6 kg x 540 kcal/kg = 34,884 kcal|
|Net heat available for boiler to generate steam:|
|420,554 – 34,884 = 385,670 kcal|
(Net heating value of 65 Bx ADW: 385,670 kcal / 184.6 kg = 2,089.2 kcal/kg)
|assuming boiler efficiency at 75%|
|Net heat available to generate steam:|
|385,670 kcal x 75% = 289,252 kcal|
|Kg of steam produced and available for evaporator:|
|289,525 kcal / 540 kcal/kg = 535.6 kg|
From Table 3, since a total of 815.4 Kg of water is to be evaporated,
It appears that more than one effect evaporator would be needed.
|Tons of steam produced per ton of 65 Bx ADW|
|535.6 divided by 184.6 = 2.9 tons|
5) Evaporator capacity and steam requirement.
|Table 4 Summary of water and heat balance|
Basis:1,000 kg of 12% (bx) ADW
|Net steam produced: 535.6 kg|
|Water need to be evaporated: 815.4 kg|
|Theoretical number of evaporator (needed) in series:|
|815.4 kg/535.6 kg = 1.52. Therefore, two effect evaporators is sufficient|
|Total steam requirement for evaporation depends on the capacity of the plant. Each one liter of alcohol produced generates 11 liters of 12 Bx ADW. For a 125 KLPD alcohol plant (5.2KL per hour), the quantity 12 Bx ADW produced per hour will be 60 tons/hr (5200liters/hr x 11 x1.05/1,000) and steam available for evaporator per hour will be 32 ton/hr (60 ton of ADW x 535.6/1000).|
|Since a four (4) effect evaporators including one for cleaning and standby is used in the plant, the actual steam used is only about 20 ton/hr.|
|(III) The Zero Discharge Technology plant|
|The plant consists of 1) Evaporation process in which the ADW from the alcohol distillery column is concentrated in a 4 – effect evaporator to a concentration of about 65 % solid content and used as fuel in a boiler to generate both electricity and steam for the plant, 2) burning process in which the concentrated ADW is atomized and sprayed to be burned in the boiler.|
|The schematics of the process flow and layout are shown below followed by the picture of actual plant. The building on the far end of the picture houses the evaporation station. The building on the right is the boiler house. The plant has been in operation since 2000/2001 and meeting the regulatory requirements of the local governmental agencies as described in the print.|
|(IV) Operational details of a Zero Discharge Plant|
Basis: 125 KLPD; 60 tons per hour of ADW
|Treatment of Alcohol Distillery Waste (ADW) by evaporation followed by combustion in a boiler:|
(a ) Concentration of ADW by Evaporation (Evaporation Station)
1 How many effects evaporator?
Four. Three in operation at any given time, one in cleaning or standby.
2 Cleaning frequency (How often)
1st effect evaporator is cleaned after running for 2-3 days.
2nd effect evaporator is cleaned after running for 2 days.
3rd and 4th effect evaporator is cleaned after running for 1 days.
3 Duration (time) needed for cleaning one evaporator
4 The cost of chemicals per cleaning
5 Steam pressure to 1st effect
0.06~0.08MPa (about 9 to 12 psig)
6 Use of 1st effect evaporator’s condensate
7 Use of 2nd, 3rd, 4th effect condensate
Some are recycled and some are transported to temporary tank.
COD over 1000 need to be treated before discharge.
8 Any odor in the condensate? What is the pH? COD or BOD?
a little bit of odor, No tested for pH, COD about 1000.
9 The average brix of Alcohol Distillery Waste (ADW) going into first effect evaporator?
10 The average Brix of concentrated ADW leaving last effect evaporator?
11 Estimate average life of an evaporator due to the corrosiveness of the ADW and cleaning.
A: About 5 years for carbon steel, estimated over15 years for 316 stainless steel
12 Approximately steam needed, in Ton/hr, for the first effect evaporator
A: 15~20 ton per hour for 100-ton-per-day ethanol plant
13 If the steam is not enough, what do you do?
A: Steam is more than enough.
14 Approximately electricity generated in kwh per ton of steam.
A: estimated to be between 50 o 65 kwh per ton steam
15 If the electricity is not enough, how do you make it up?
16 How many operators per shift are needed at the evaporator station?
A: 2~3 operators in China (labor cost is not a major concern)
17 How many minimum space (in Square Meter) are needed for the evaporator station?(Height, Width and Length)
18 How many ton of steam is needed to evaporate one ton of ADW?
A: Less than 400kg in a three effect evaporator
19 Do you need to adjust the pH of condensate of the last effect by lime?
A: Not done. But it is better to adjust the pH.
20 Is there a need for ADW to be pretreated before feeding the 1st effect evaporator?
A: The ADW should be pretreated, but not done yet.
21 Your plant has been in operation for about 7 to 8 years how many percent of the evaporator tube has been replaced?
A: We have been in operation for about 9 years.
All steel tubes (from sugar mills’ old evaporators) have been replaced. All tubes are stainless steel since last year.
22 Is ADW fed to the evaporator co-current or counter-current to the vapor flow?
23 Do you have a typical analysis of the ADW characteristic.
See Table 1.
(b) Boiler Station (Combustion Process)
1 What is the minimum brix of concentrated ADW required for the boiler and its net heating value?
A: Over 60Brix
About 1800-1900kcal/lit. for 65Brix of concentrated liquid ADW
2 If the Brix is too low, for example only 45 Brix, what do you do?
A: If it happens, the liquid re feeds back to 1st effect. There is a large storage tank for the concentrated liquid to continue burning for the boiler during this period
3 How many tons of steam is generated, per ton of concentrated ADW?
A: Two to three tons
4 How many tons of this steam is used for evaporator after electricity generation?
A: There is excess to go to alcohol plant (see table 4)
5 What is the steam pressure from the boiler?
A: 2.4MPa (about 350 psi)
6 What is the steam pressure after turbine generator?
A: Turbine design for 3kg. In practice, much lower.
7 Do you have excess (surplus) steam, how many ton/hr?
A: some (Table 4)
8 Where do you use the surplus steam?
A: Deliver the surplus to alcohol plant.
9 Is the steam system of the zero discharge plant integrated with that of alcohol plants?
10 What is the power generated by the steam from the boiler, in KWH. How much of this power is used by the evaporation station (concentration) and how much is for the boiler itself?
A: 1000-1500kw/hr, For the last two questions: not measured. There is excess for alcohol plant and for sale.
11 How much is excess and used by the alcohol plant?
A: Not measured (question 10)
12 Do you have ESP (Electro static precipitator) installed to removed stack’s ash or do you use water scrubber?
A: No ESP. Yes, we have scrubber.
13 How many ton of ash is removed by the scrubber a day, in Ton/day?
A: No measured.
14 How many ton of ash is removed at the bottom of the boiler?
A: Not measured. Guesstimated 5 tons/day.
15 What is the percent potassium in the ash?
Not analyzed. It may be around 10 to 15 % of potassium
16 How do you dispose of the ash (fertilizer)?
17 Does the boiler emission meet the government limit of stack gas?
A: It has been approved by the government environmental departments.
18 Is your process approved and certified by the government for compliant with environmental air control?
A: It is authorized and certified by the city environmental department.
19 Do you clean ash off the boiler tubes? What do you use (steam or compressed air)? What is the frequency?
A: Yes, for now we clean it with blowing air after operating several weeks.
We are going to apply ultrasound to do it.
20 How often does the boiler need to be shut down for PM (maintenance)?
A: Shut down to do cleaning after operating boiler for 2 months. The tubes are replaced after 4-year-operation.
21 What fuel is used to startup the cold boiler (after shut-down)?
A: Heavy oil was used. Presently “Mixed” alcohol waste is used
22 What is the land requirement for boiler? (space)
23 What is the flue gas temperature?
24 How much water is needed for scrubber (M3/hr)?
A: Pump designs for 30m3/hr
25 How often does the boiler have to be air/steam blow because of ash fouling/plugging?
A: Several weeks to 2 months
26 With respect to corrosion, how many percent of original boiler tube have been replaced after 4 year of operation?
A: The tubes are replaced after 4-year-operation .
27 Is the boiler noise level below 75 dB?
A: Boiler noise level is below 75 dB.
Gas blower’s noise level could be over 75 dB.
28 Is there an economizer in your boiler?
A: Not yet
29 Does the boiler need additional (supplemental) fuel?
30 How often the boiler is shut down for routine maintenance and for how long
A: Once a year for routine maintenance for one month.
31 What is The Chinese Ambient air quality standard in term of: SPM, RSPM, SO2, NOX?
A: Dr. Gao is to look it up from internet.
32 How much does it cost for equipment maintenance per year?.
A: Estimated to be USD$ 75,000 per year (2006).
(c) General Section
1 How long has your alcohol plant been in operation?
A: In routine operation since 2000
2 Term, Condition, and scope of supply
① Do you offer turn-key system?
② Do you offer only design, P&ID, drawing, and start-up assistant?
③ Time of delivery. How many month after signing of contract?
A:15~20 months for turn-key systems
④ What is your term of guarantee?
⑤ Term of payment. How many percent at signing of contract? How many percent after delivery of design and drawing? How many percent after commissioning?
A: Negotiable. The last payment should be below 5%.
⑥ Term of technical/Engineering/operation/construction assistant. Cost per day per person US$
⑦ Design to include boiler, turbine, and evaporator etc.