Basic Questions & Answers on Slop/Spent wash fired Boilers

1-What do you mean by Slop or Spent wash?
Slop or Spent wash also known as Vinasse is an acidic effluent produced in Distillery unit. Slop is formed on fermentation of Sugar cane molasses in distillery.
2-Why do you treat Slop as a Hazardous effluent?
Because it is an acidic effluent having pH around 4 to 5
3-What are the standard parameters of Slop or Spent wash?
  • Colour : Brown
  • Odour : Pungent
  • pH : 4 to 5
  • Solids: 9 to 12%
  • BOD : 45000 to 60000
  • COD : 110000 to 1350000
4-What are the various methods of Slop disposal?
  • Disposal methods:
  • Biomethanation
  • Composting
  • Incineration Technology

5-What is the purpose of using incineration technology for Slop disposal?
Incineration technology method leads to Zero Liquid Discharge, where spent wash is concentrated by heating it in specially designed Boilers.
6-What are the benefits of burning Slop in Boilers?

  • Zero liquid discharge (ZLD) or Zero effluent discharge (ZED)
  • Steam generated in Boilers can be used for Distillery process
  • Power generation to fulfil auxiliary power of distillery, hence reduction in production cost.
  • Ash generated is of potash reach which is good manure

7-What are the challenging situations for designing the Spent wash Boilers?
  • Slop & its ash characteristics
  • Design of combustion system for Slop
  • Design of combustion system for supporting fuel
  • Design of pollution control equipments
8-Why it is being challenging task to burn the spent wash?
Because it is a liquid fuel having very low GCV up to 1800 kcal/kg & high ash & moisture content.
9-What are the various supporting fuels used in Incineration Boilers?
  • Spent wash + Coal (Indian/Imported)
  • Spent wash + Rice husk
  • Spent wash + Bagasse
10-What type of Boilers are used in Incineration Technologies?
  • Travelling grate Boiler: Rice husk & Bagasse
  • AFBC Boiler: Indian & Imported coal
11-What are the features of AFBC boilers used in Incineration Technology?
AFBC Boiler:
  • Bottom supported
  • Single drum
  • Over bed feeding
  • 3-pass flue gas construction
  • Combustor is located in pass-1 & Super heater, Evaporator; Economiser & Super heaters are located in Pass-3

12-Briefly explain the combustor & fuel feeding system in AFBC boilers?
Combustor:As discussed above, Combustor is located in Pass -1, fabricated with high temperature refractory material, enclosed by steel casing and supporting structure. Combustor is divided into various compartments & for all compartments ash drain system is provided.
Coal feeding system: consists of coal bunker, VFD screw feeders to feed the coal. Coal is distributed inside the combustor by pneumatic spreading system.
Spent wash feeding system: Concentrated and pre-heated Spent wash from the storage tank is conveyed to boiler through variable speed pump. Spent wash is sprayed over the fluidising bed at first pass through spray guns.

13-Where do the Secondary air & spent wash nozzles are provided in AFBC Boilers?
Secondary air system is provided at 2 to 3 locations at various heights at furnace. And spent wash nozzles provided just below the secondary air nozzles
14-What are the various online cleaning mechanisms are provided for pressure parts?
Mechanical rapping systems for super heaters
Soot blowers for Evaporators
Sonic soot blowers for Economisers
15-How do the spent wash concentration (Brix) & its calorific values are related?
GCV of the spent wash increases with increase in its concentrations
Sl No.
Brix (%)
GCV (Kcal/kg)

16-How do you increase the Brix of Spent wash?
Spent wash Brix can be increased by heating
17-What parameters of Brix are recommended for efficient burning?
  • Brix: 55 to 60%
  • Moisture: 40 to 50%
  • Ash: 18 to 20%
  • GCV: 1550 to 1750 kcal/kg

18-Write down the ultimate analysis parameters of Spent wash
  • Ultimate analysis report:
  • Carbon: 18.5 to 23%
  • Hydrogen: 1.5 to 2.5%
  • Oxygen: 12 to 15%
  • Nitrogen: 1.5 to 2%
  • Sulphur: 0.5 to 0.7%
19-What are the various contents of spent wash?
Ash contents analysis by weight is as follow
  • Silica (SiO2) : 2.3 to 2.8%
  • Iron Oxide (Fe2O3) : 2 to 2.5%
  • Calcium Oxide (Cao) : 12 to 16%
  • Magnesium oxide (MgO) : 6 to 7%
  • Sulphate, SO3: 15 to 17%
  • Phosphate (P2O5) : 8 to 12%
  • Potassium Oxide (K2O) : 45 to 50%
  • Chlorides: 0.8 to 1.2%

20-What is the quantity of steam generated per square meter of heating surface in Spent wash AFBC boilers?
It is around 8 to 10 kg of steam is generated per square meter of heating surface
21-What is the quantity of steam generated per square meter of heating surface in Spent wash Biomass (Bagasse, rice husk etc) fired boilers?
It is around 10 to 12 kg of steam is generated per square meter of heating surface.
Note: For incineration:AFBC Boilers have more heating surface as compared to Biomass fired Boilers
22-Why do the Spent wash Boilers have taller furnaces?
  • To maintain higher residence time in furnace up to 8 seconds
  • To maintain optimum flue gas temperature at super heaters inlet to avoid coils fouling due to alkali ash property and to avoid corrosion of super heater coils due to chlorides (1%) in ash
23-What is the design configuration of APH in spent wash fired Boilers?
In spent wash Boilers APH are designed for air flow through tubes & flue gas outside to avoid choking of tubes
24-What is the flue gas temperature that we get at APH outlet for spent wash fired Boilers?
It is around 180 to 200 Deg C
25-What is the size & density of refractory materials used in Spent wash AFBC Boilers?
Size of refractory materials is around 0.8 to 2.5mm. And density is around 1000 to 1100 kg/m3
26-What are the main compositions of Refractory materials?

Questions & Answers on AFBC Boilers
  • Silica 55 to 60%
  • Alumina 35 to 40% & small traces of Feo, Mno, Cao, Mgo, P2O5 etc
27-What is the optimum temperature of spent wash at the nozzle inlet?
It is around 70 to 80 deg c
28-What is the maximum efficiency that we can get for spent wash & Rice husk fired Boilers?
It is around 72%

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  1. Please let me know what is the use of fly ash obtained from spent wash incinirator in distillaries what is the composition of that ash how we check it

    1. As we know that spent wash ash is potassium rich hence it supply to further processing in which hazardous chemical remove and then uses for farming purposes

  2. Can fire side chemical be effective for spent wash fuel


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