Sugar – Manufacture, Co-generation and Sustainability
By Dr. Yashpal Singh
From the Executive Summary of book entitled “Sugarcane and Sugar-Biology, Agriculture, Manufacture, Regulation, Environment and Pollution Control”, 2024 by Dr. Yashpal Singh. Published by The Wealthy Waste School India and U.P. Sugar Mills Association. Available at: https://www.amazon.in/Sugar-Cane-Agriculture-Manufacture-Regulation/dp/936039856X/ref=sr_1_1
Policy
Sugar is an important sector in India because of its impact on the rural livelihood and its potential to provide enormous direct employment opportunities. Small growers, small sugar mills and a short crushing season are detriments to the competitiveness of Sugar cane. Government support for the sector is significant and revolves round providing subsidies in the form of high prices of cane, irrespective of price of sugar, supporting exports, creating buffer stocks, supporting modernization and diversification and also in purchasing sugar from the open market and supplying at lower prices to the poor. Fair and remunerative prices for sugar cane are decided on the basis of the Commission for Agricultural Costs and Prices in consultation with the State Governments, taking into consideration the cost of production of cane, return to growers from alternative crops, general trend of prices, availability of sugar to farmers at a fair price, price at which sugar cane is sold, recovery of sugar, the revenues generated from the sale of other products like molasses, bagasse and press mud and reasonable profit to growers. A FRP of Rs. 305/Qtl. has been fixed for 2022-23 at a recovery rate of 10.25% with incentives for increase in sugar recovery and disincentives for fall in recovery.
The Niti Aayog, Government of India has estimated that the returns from Sugar cane cultivation are nearly 60 to 70% higher than the returns from other crops and with regulations to buy only from cane reservation areas at F.R.P. the interest of the farmers are adequately ensured and despite its problems, India is a Sugar surplus nation. Some states fix their own price (State Advised Price, SAP) over and above the FRP. This raises a pressure on Sugar Mills. The cost of production of Sugar in India is much higher than the international prices. The National Sugar Policy targets a 20% blending percentage for alcohol which has opened up a wide scope for using surplus cane in the production of Ethanol.
The Niti Aayog in 2020, has come up with recommendations on how Government intervention in Sugar Sector and the impacts of the Sugar Sector on the water regime could be minimized. The recommendations also take into consideration the recommendations of the Rangarajan Committee and resolve round recommendations on pricing of Sugar Cane, payment of Sugarcane price to farmers, diversification to less water intensive crops with incentives to farmers who shift, sugar and sugar Development Fund, Ethanol blending program, Trade policy, raising the MSP of Sugar and implementing the reforms suggested by earlier committees including the Rangarajan Committee, expansion of drip irrigation, elimination of buffer stock of sugar, recycling bagasse, promotion of Jaggery, financial assistances to distressed mills, long term pricing formula for ethanol, restructuring of the Sugar Industry.
Export of Sugar is a good outlet for excess sugar but because of cost differences between Indian Sugar and Sugar produced by other countries (Indian Sugar almost twice the cost of global sugar), export does not appear to be a viable option for India.
A sugar buffer subsidy scheme has been started in 2018. The buffer stocks are maintained by the Sugar Industries themselves, as against other crops where the buffer stock is maintained by the Government. The Government reimburses the mills for maintenance of this buffer stock.
Pricing of cane is a major area of concern. The Niti Aayog recommends that in fixing the FRP the domestic supply and demand and the international price of Sugar should be considered. It also suggests that states may desist from fixing SAP over the FRP unless the State absorbs this cost.
The Aayog also suggests that the price of cane should be linked to the price of Sugar and generally be 80% of the revenue realized from sugar or 75% of the revenues realized from Sugar and its bye products. As per the recommendations, mills may be allowed to stagger the payment of cane dues. With a view to support the sugar sector and in the interest of sugar cane farmers the government has fixed the price of ethanol at Rs. 46.66 per liter for C. Heavy molasses, 59.08 Rs. /Liter for B. Heavy molasses and Rs. 59.13 Rs. per liter for units on only juice.
Sustainable Sugar initiatives
Sustainable sugar initiatives in sugarcane farming have introduced methods which ensure a minimal use of seed, water, fertilizer and land and achieve higher returns. The initiative manages row spacings, inter cropping, nutrients after due soil analysis and irrigation water.
World Sugar Production
The USDA estimates for world sugar production indicate a global production of 183.15 million tons which is led by Brazil (38.05 M. ton), India (35.80 M. Ton) and the European Union (16.15 M. Ton).
As against this the global human consumption is estimated to be 176.34 million tons led by India (29.00 M. Ton), European Union (17 M. Ton) and China (15.3 M. Ton).
Brazil (28.20 M. Tons), Thailand (11.0 M. Tons) and India (9.39 M. Tons) are likely to contribute majorly to a total expected Global export of 69.25 million tons. Global stocks are likely to be to the tune of 38.55 M. Tons with India (4.84 M. Tons), China (4.78 M. Tons) and Thailand (3.78 M. Tons) being the principal contributors.
Another estimate indicates that as per October 21, Sugar surplus in India was 14.374 million metric tons and the expected surplus could be 16.5 million metric tons. In order to deal with this surplus, the Government of India has issued directives to all sugar mill to divert more than 35 lakh metric tons of sugar for ethanol production and to endeavor to export about 7.0 million tons in the Sugar season 2021-22. This will solve the problem of surplus and also help mills to pay cane dues to farmers and stabilize the ex-mill price of sugar. The Cabinet Committee on Economic Affairs has fixed FRP for basic recovery rates of 10% with separate rates for recoveries over and below this recovery rate. In 2021, the Government has also cleared subsidies for export of sugar cane. India produced 115.55 Lac tons of sugar by December 2021 which was more than that produced in the previous year by 4.8 Lac tons and against a total requirement of 458 Cr. Liter for 10% Ethanol blending, OMC’s have so far allocated about 366 Cr. Liter in Ethanol Supply Year 2021-22.
The high price of manufacturing sugar in India makes it uncompetitive in the Global markets and dependent on subsidies which are not likely to continue beyond December 2023. In order to meet these exigencies, a Revenue sharing formula has been suggested between farmers and sugar mills and also an increase in the MSP for sugar.
Manufacturing Sugar
Sugar manufacturing involves producing a number of grades of Sugar e.g., Raw Sugar, Centrifugal sugar, white Sugar, Non-Centrifugal sugar and Khandsari Sugar. It also, in the course of manufacture yields by products like Bagasse, Press Mud, Molasses, Sugar Cane Juice, Ethanol, health products, biochar, soil amendments, Bio oil, second generation bio fuels etc.
Ethanol is a major by product and the production and sale of ethanol is a much more profitable enterprise than selling molasses. It has advantages in being a green source of renewable energy with reduced emissions as compared to gasoline. It also has the potential to stabilize the sugar supply demand position and save on foreign exchange. For marketing year 2020-21, The estimated ethanol sales revenue in India are expected to be 15000 crore INR. A blending capacity of 20% is required by 2025. Cogeneration coupled with Ethanol production can give greater revenue (Rs. 531.95) per ton of sugar cane as compared to revenue from other by products (345.75 Rs. Per Ton Sugar cane). The Government of India has permitted the use of ‘B’ Heavy and ‘C’ Heavy molasses, sugar cane juice, Sugar/sugar syrup, surplus rice and maize for the production of Alcohol. Bagasse derived fuel ethanol also holds great potential. Effluents from the Ethanol Industry and alcohol distillation process are problematic and treated through ZLD and non-ZLD based technologies like bio-methanation, bio composting, concentration and incineration. They are rich in micro and macro nutrients and growth promoting substances and could be gainfully utilized in agriculture and substitute for chemical fertilizers. Effluents can also be sustainability used in the reclamation of sodic lands.
With a calorific value of 2100 Kcal/Kg at 50% moisture, bagasse is a valuable fuel and almost 90% of the Bagasse is being consumed in this way with the rest being used by the Paper and other industries. Some major components of sugar cane are sugars, starch, fiber, organic polymers, organic acids and nitrogenous compounds, colour forming compounds and pigments, inorganic compounds, Lipids etc. All these compounds influence the sugar extraction processes and efficiencies.
Cane consists of about 70% water which can be gainfully utilized in meeting the requirements of sugar processing and the excess available water can be used to meet the water requirements in co-generation and distillery units. The Central Pollution Control Board holds that despite this, the sugar factories are drawing about 50 to 100 liters of fresh water per ton of cane crushed from other sources which increases with co-generation and refining units. Sugar mills on the other hand are earnestly trying to lower fresh water consumption and have installed underground reservoirs and cooling towers. Surplus condensates are being reused. Using condensates, recirculating cooling waters, using treated effluents for cooling purposes, using spray pond effluents to create vacuum, preventing fresh waters from cooling in going to the spray ponds and instead taking it to service tanks, using hot water for preparation of seed and mixture, keeping tap connections to a minimum, using hot water for imbibition, adopting closed loop recirculation of hot and cold water adopting dry cleaning of floors, providing for water meters, etc are some suggested ways of conserving water.
Cane should be available to processing units with the least possible delay between cutting and crushing. Any delays result in staling and reduction in recoverable sugar. After receipts in the sugar factory, sugar cane is processed through a number of steps involving cane preparation ; cane crushing; extraction through rollers or diffuser (Milling operations have been modernized to provide almost the same extraction efficiencies as diffusers); clarification including defecation, carbonation, sulphitation and some other processes like the ion exchange process, the Talo Dura process or the Saha Jain process; Evaporation in multi effect evaporators, falling film evaporators, falling film plate type evaporators with the falling film evaporators giving a better steam economy and grain formation and crystallization. During the process of evaporation, salts in solution are thrown out and get deposited on the heating surface and on the inner surfaces of the tube and have to be periodically cleaned during which time the manufacturing operations are shut down. This wash is an important effluent stream form sugar mills and needs to be equalized before treatment through the equalization tanks.
Sucrose losses also occur in the evaporator. Some alternative technologies which aim at reducing the residence time in the evaporator and the juice temperatures help in reducing losses. Increasing pH also helps in minimizing sugar losses from juice. Steam conservation can be achieved by using hot condensates from evaporators and vacuum pans for juice heating, using falling film evaporators instead of Robert evaporators and using continuous pans instead of discontinuous pans.
The syrup from the evaporator contains nearly 4 to 5 times more impurities than in clear juice and has to be treated to remove the impurities. Heated syrup, in the Talo Dura system of treatment, is treated with lime and phosphoric acid and precipitated to remove impurities. Sulphur dioxide and Hydrogen peroxide are also options for syrup treatment and reduction of colour.
The extraction of sucrose from syrup is accomplished through crystallization which influences the quality of sugar and manufacturing losses also. Crystallization is done in stages and the liquor of the final stage is discarded to be used further in the manufacture of other products and by products including rectified spirit and ethanol. Non sucrose impurities in syrups or molasses also exert a negative influence on the rate of crystallization. It is also important that secondary crystals in boiling must be avoided because they retard the passage of molasses through the crystals in centrifugal separation and give rise to poor quality sugar. Conglomerate crystal formation due to poor circulation of massecuites in some pockets should also be avoided. These conglomerates also known as rolled grains may contain impurities from the molasses and also interfere with washing in centrifugal separation. Better circulation in the boiling massecuites to bring better circulation helps in preventing the formation of conglomerate crystals. Many designs for calandria including floating calandria, conical calandria, ring type calandria, horizontal design, rotary horizontal pans and pans with internal condensers have evolved to increase the efficiency in pan boiling. Pan boiling is a very important part of the sugar manufacturing process and has to be very rigidly regulated.
The process of graining involves the establishment of a proper grain footing for boiling. The footing should contain sugar grains of the right size in sufficient number and generally involves ‘waiting’, ‘shock seeding’ or ‘True seeding’. Once the sugar grains in the slurry grow to a certain size, the molasses feed is started and sugar grains in the slurry allowed to develop in meta stable conditions. In white sugar manufacture, grain is made for the ‘B’ and ‘C’ massecuites and ‘A’ Heavy molasses is used as a medium in ‘B’ massecuite boiling. The grain for the ‘C’ massecuites is prepared in either ‘A’ heavy or in a mixture of ‘A’ heavy and ‘B’ Heavy molasses. Concentration of the mother liquors to high super saturation values helps in achieving maximum crystal development in the pans.
After the crystallization process, the various grades of massecuite’s which contain sugar crystals and mother liquor are separated in centrifugals. The sugar separated from the ‘A’ massecuite is steam and air dried and sent for packing and the sugar form the ‘B’ and ‘C’ massecuites is put back in the process as Magma or melt. Batch machines for centrifugal separation were associated with low efficiency because of acceleration, deceleration, motor related speed complexities and less of time in charging and discharging. Continuous centrifugals have evolved as a preferred technology but may present problems of crystal breakage which can be managed by reducing the speed of impact of crystals.
A sugar mill may consume up to 60% of steam on sugar cane in the manufacturing process. Out of this almost 90% is consumed in the boiling house operations. Vapor from the evaporator bodies can be used for juice heating and the entire pan boiling operation can be managed by using vapors from pre evaporator and first body of evaporator. High pressure boilers help in keeping the steam consumption low (say below 40 to 45% on cane). Stoppages may have to be minimized, cane supply to be regular and a high crushing rate maintained to minimize steam consumption. Planned maintenance shut downs should not be more than 16 to 20 hours. It has been suggested that for achieving a better steam economy, only high temperature condensate should be used in the milling sector. Lime and phosphate solution required in clarification could be made using hot condensates, the brix of syrup leaving the evaporator should be monitored meticulously and scale deposits on the heating surfaces could be minimized. Since low brixes increase the volume of massecuites to be boiled, high brix have to be maintained in each strike, recirculation of sugars and non sugars has to be avoided, molasses dilution should be regulated and overheating of molasses should be avoided along with preventing unnecessary use of water in the pans. Use of exhaust steam in pan washings instead of live steam and use of continuous pans has also been recommended for better steam economy. Water use in the centrifugal has to be minimized and the separation of light and heavy molasses has to be efficient. Light molasses from massecuite should be used for preparing magma of sugar purgings from same massecuite.
It is generally felt that steam consumption could be brought down to 40 to 45% on cane through use of appropriate technology.
Condensates from the process operations are generally used as a source of water for generation of steam in the boilers and may comprise of steam condensed in the heat exchangers and the condensates arising out of juice boiling in the evaporator. This has to be monitored and treated in order to be fit for reuse either through chemical processes or ion-exchange processes or chemical processes followed by ion exchange.
Most sugar factories are now converting to the production of refined sugar using the Defeco-melt phosphatation followed by the ion-exchange process.
The best available processes include affination, melting, clarification, secondary decolorization, crystallization and centrifuging and high pressure colour adsorption.
To increase a higher efficiency in the process of production of sugar the sector employs automated processes like bagasse belt conveyor speed control, mill drive speed control, boiler operation control, turbines operation control, raw juice flow control, juice flow stabilization system, process temperature control, juice pH control, chemical dosing systems, vacuum pan feed controls, pumps operation, water flow meters, melting and molasses conditioning etc.
Sustainable Sugar
Sustainability is a continuous effort which aims at achieving growth and development in a frame work where natural resources are maintained over time. Sustainable production includes the adoption of innovative practices which improve the efficiency of the existing systems and prevent resource waste and in being sustainable are socially acceptable to both the urban and rural populations.
Sustainability in the manufacture of sugar involves imparting training to workers, continuously improving the status of water resources and minimizing water use, adopting energy and steam efficient fertilization, planting and irrigation systems, reducing air and water pollutants, implementing the environmental management and corporate social responsibility plans and reviewing performances regularly within the factory and with other factories. In terms of life cycle analysis and global warming potential, the production of only sugar has a much higher environmental impact than scenarios producing both sugar and surplus energy. The alternative use of sugar by products (Trash, press mud and Bagasse) also contributes to negative Green House Gas emissions. The combination of Adsali Sugar cane and very high pressure cogeneration boilers provides the highest environmental benefits.
Sugar crop extraneous matter can be converted into a number of chemicals, bio-energy and bio materials including cellulosic alcohol, electricity and biochar. Predicated energy crops including energy cane, beet and sorghum can be used in the production of second generation bio fuels as well as bio electricity.
Farming technologies have been also innovated for sustainability of cane and include deep ploughing and fine preparation of soil, plastic film mulching for returning soil moisture, prescription fertilization, trash retention in field, drip and other micro irrigation practices, pathogen free seeds, use of spent wash in agriculture, mechanization and integrated nutrient, pest, disease and weed control.
The Central Pollution Control Board has identified some action points for the Sugar Industry subsequent to revisiting the Charter for Corporate Responsibility for Environmental protection and also lists the bare minimum technologies to be followed. These focus on recycle and reuse of effluents, reduction of water use and effluents generated per unit product, cleanliness, housekeeping and hygiene, trapping and treating high concentration wastes, preventing spillages and fugitive discharges, steam and power efficiency, a robust monitoring etc. It has also recommended some practices that could be adopted in the manufacture of plantation white sugar and refined sugar.
Cogeneration
Co-generation which involves simultaneous generation of power and process steam from the same system is a good way of generating environmentally friendly power at optimum efficiencies. Sugar manufacturing has the potential to produce power and sell it to the grid too. Co-generation units are widely used and serve as a major source of revenue generation to the Sugar plants. Cogeneration also reduces the impact of greenhouse gases from fossil fuel burning. Bagasse combustion releases two volumes of particulate. The CO2 emissions are the same as the sugar cane plant absorbs as it grows. CO2 is recycled back through photo synthesis. The replacement of power generation through low pressure boilers, by high pressure multifuel boilers or high-pressure turbo-generators has resulted in an increase in power generation. India has a potential to generate 3500 MW of electricity through Bagasse fired co-generation. Various financial incentives are available to promote bagasse-based cogeneration. About 250-280 kg bagasse per ton cane is generated from the sugar milling process and this can produce 500 to 600 Kg of high pressure steam which is supplied to the crushers and mill turbines (which could be back pressure or extraction condensing turbines or the Biomass Integrated gasifier/gas turbine combined cycle technology also known as BIG/GTCC turbines) where electrical energy is produced. After passing through the turbines, the steam is low pressured and used for running the heaters and pans.
The average energy consumption in an Indian Sugar mill is 38 units/ton of cane crushed. Installing diffusers instead of milling tandems; full automation; use of variable frequency drive driven electric motors at cane carrier, truck tippler, boiler I.D. fan; using planetary drives in boiling house; using hydraulic cane unloaders; using fibrizer compatible with H.T. motors; energy efficient sugar gravity plant; installing triple effect falling film evaporators; cascade type vertical continuous pans; continuous cool crystallization; all molasses heating and conditioning by non-condensable gases; appropriate heat recovery systems; direct contact heaters; tubular/PHE heaters for condensate heat utilization; use of melt concentrator; evaporator bleeding; replacing Robert bodies with falling film evaporators etc. are some ways in which a higher energy efficiency can be achieved.
Particulates, other combustion products (NOX, SOX, CO, CO2) and volatile organic carbon are the primary pollutants from the sugar manufacturing processes and can be controlled through air pollution control systems like cyclones, fabric filters, electrostatic precipitators, wet scrubber. Fugitive dusts can be controlled through effective cleaning and spraying. Sugar mills with co-generation may need to install electrostatic precipitators and sugar mills without co-generation could install wet scrubbers.
Burning a sugar cane field also releases some 2600 to 4500 Kg/Hectare of CO2. Sugar cane fields also act as a very important sink for atmospheric CO2.
The recovery of heat for cogeneration is a circular economy strategy which apart from being a source of increased revenues and lower environmental impacts also adds value to the corporate image.
Air and Water Pollution
The MoEF and CC in 2016, has prescribed the particulate matter emissions from stacks to be restricted to 150 mg per normal cubic meters and in June 2023 prescribed standards for Boilers which prescribe 250 mg/NM3 from bagasse fired boilers of more than 10 TPH steam capacity.
Sugar is included in the red category of Industries and considered grossly polluting by the Central Pollution Control Board. There does appear to be a need to visit this categorization and the assumptions of the CPCB in this regards. Co-products from the sugar industry however may have significant levels of pollution if not utilized carefully. The CPCB has notified effluent standards which prescribe a pH of 5.5 to 8.5, a total suspended solid and BOD3 of 100 mg/liter for disposal on land, a total suspended solid and BOD3 of 30 mg/L respectively for disposal into inland surface waters and an oil and grease concentration of 10 mg/L, total dissolved solids of 2100 mg/L and a final waste water discharge of 200 liters/ton of cane crushed irrespective of final disposal sinks.
Water from cane washings, secondary condensates, water form barometric condensers, boiler blow downs and condensates, water from floor washings, domestic waste water, spillages, scales etc are major sources of water pollution. Based on data provided by the Central Pollution Control Board the characteristics of untreated combined waste waters exhibit a suspended solid concentration of 250-300 mg/L, BOD from 500-800 mg/L, oil and grease from 5-10 mg/L, COD from 1000 to 1600 mg/L and TDS from 1000 to 1200 mg/L. Spray pond overflows may need to be treated for sulphates.
Treatment of sugar industry waste waters is usually achieved through preliminary treatment (Grit, oils and grease) followed by flow and load equalization, clarification for removal of suspended solids, anaerobic biological treatment followed by aerobic biological treatment, biological nutrient removal, chlorination and sludge dewatering. Solid wastes need to be disposed properly.