Evaluating bagasse gasification



In most cane sugar factories, the main source of electrical power is the onsite power generation plant which usually uses coal to boil water in order to produce high pressure steam which the turbo-alternators use to generate electricity. This article considers the advantages of using bagasse, the fibrous matter which remains after sugarcane is crushed, as a biofuel instead of coal.

The price of coal increases every year and it has become a major cost concern to the industry.

The cane sugar industry uses a conveniently available source of fuel which is bagasse. Bagasse is the remaining fibre material after the juice has been extracted for processing from the sugar cane stalk. Another advantage of using bagasse besides the fuel cost factor is that the emitted flue gasses are less harmful to the environment than coal [1]. Theoretically, the amount of CO2 produced when burning the bagasse is the same as the amount absorbed in the process of growing the sugar cane producing the bagasse [2].

Table 1: Steam boiler data [7].
Description Value
Bagasse moisture (Ww) 51,41%
Bagasse brix content/pol content (Wrds) 1,49%
Bagasse ash content (Wa) 4,52%
HP steam pressure 31 000 kPa
HP steam temperature 400°C
Exhaust steam pressure 120 kPa
Exhaust steam temperature 130°C
Furnace temperature 1350°C
Feed water temperature 100°C
Ratio of weight of air used for combustion to weight theoretically necessary 1,5
Coefficient of losses due to unburned solids (a) 0,99
Coefficient of losses due to radiation (b) 0,97
Coefficient of losses due to incomplete combustion (h) 0,90

The direct burning of bagasse generates enough thermal energy as required by the sugar making process. Through investigation, it was found that the efficiency of this system is very low. The average bagasse would have moisture content ranging between 48 and 51% [3].

In order for the steam and power generation plants to be efficient, there needs to be an alternative approach. Bagasse gasification needs to be evaluated in order to determine if it can offer greater efficiencies.

Gasification systems

Gasification systems have been studied throughout the energy environment. The main objective in researching new ways or systems in the energy environment, is to find cheaper and more efficient energy generation systems. Most of the work on the gasification systems has been done with coal [4].

It is believed that the success of the bagasse gasification system in the cane sugar industry would result in the energy efficiency of at least 45% more than the current system of direct bagasse combustion [5].

There are, however, some obstacles that need to be overcome before the system can be adopted. One of the problems is the inaccessibility of the experimental equipment to test the benefits of the system.

According to the theory of heat engines developed by William John Rankine (1820 – 1872), an ideal condition of heat energy converted to work will only have an efficiency of between 22 and 26% depending on the thermal conditions and assuming a boiler efficiency of 100% [6].

Comparing the standard back-pressure of condensing turbines used in the sugar industry these days to the gas engines that would be used on the bagasse gasification system, it can be shown that the efficiencies of the back-pressure turbines are 45 to 50% and that of the gas engines are 60 to 85%. The other main limiting factor of the direct combustion system is the entry temperature to the turbines which have an upper limit of 565°C, whereas the gasification plant can be operated at higher temperatures e.g. 900 to 1400°C [6].

Fig. 1: Screen shot of boiler quantities [8].

Current system analysis

The data used for the calculations was actual data, courtesy of ISL: Eston factory data [3].

Current system calculations

The calculations used actual values from a fully operational factory which uses the direct bagasse combustion system. The calculation results are shown in Table 1.

By means of calculation, according to Rein, using specialised software, the higher and lower calorific values of bagasse combustion can be found [7, 9]. The enthalpy of the feed water is taken at 100°C and 50 bar.

Mtunzi shows that about 0,275 t of bagasse is produced per t of crushed cane [10].

Using Lawler’s calculations, the important values for boiler efficiencies can be calculated. The results are shown in Table 2 [11].

The efficiency of the boiler under evaluation is within the theoretical efficiency of 50 to 65%. It can be said that a factory which crushes 6000 t of cane per day would make 750 t of sugar (assuming standard 8:1 cane to sugar ratio), 1650 t of bagasse, and would be capable of producing 3085,5 t steam.

Current system high-pressure steam consumption

The factory under evaluation consists of two turbo-alternators with an installed capacity of 8,5 MW. There are four other steam turbines for mechanical load used in the boilers. There are two boilers in total, one rated at 100 MCR and the other at 50 MCR. There are a total of ten steam turbines throughout the factory which use high-pressure steam to drive mechanical loads and generate exhaust steam for process operations.

In order to obtain the actual steam consumption, the losses are added to the calculated steam consumption [12].

The efficiency of the complete power plant, taking the normal average values, can be calculated.

Table 2: All calculated quantities.
Description Value
Higher calorific value, HCV 8549,51 kJ/kg
Lower calorific value, LCV 6745,88 kJ/kg
Superheated steam enthalpy, Hst 3227,31 kJ/kg
Feedwater enthalpy, Hfw 422,86 kJ/kg
Total sensible heat lost, q 300,39 kcal/kg
Heat unit transferred to the steam, Mv 5231,19 kJ/kg
Overall boiler efficiency, 61,18%
Steam generated per t of bagasse 1,87 t
Bagasse generated per t of crushed cane 0,275 t
Steam generated per t of crushed cane 0,66 t
CO2 emissions under these conditions 16,7% by mass

Efficiency of the turbine:

  • rhom x rhor x rhog x eta = 0,985 x 0,98 x 0,95 x 0,7 = 0,642
  • Losses: rhop = 0,936
  • Efficiency x losses = 0,642 x 0,936 = 0,600

Taking into account auxiliary equipment (feed pump, air heater, draught fan) which takes its power from the turbine, without direct use in the factory, we also included coefficient ra which equals to 0,935.

The useful energy which is contained in the steam:

rhov = efficiency x losses x coefficient of auxiliary equipment

Under normal operations, turbo-alternator number 1 (TA 1) produces approximately 3,5 MW and turbo-alternator number 2 (TA 2) about 2 MW, equating the factory load to 5,5 MW. For simplicity, we will assume that turbo-alternator number 1 is constantly loaded at 75% and turbo-alternator number 2 at 60%, thus:

Average total steam consumed by both TAs = TA 1 steam consumed + TA 2 steam consumed =104,516 TPH.

Steam consumption of turbines driving mechanical load

Table 3: Steam turbines steam consumption.
Steam turbine Steam consumption
Second knives and ID fan number 1 3,43 TPH/turbine
Shredder 4,86 TPH/turbine
Mill numbers 1 and 2 2,03 TPH/turbine
Boiler feedwater pump numbers 1 and 2 0,82 TPH/turbine
ID fan number 2 0,91 TPH/turbine

The formula for calculating the steam consumption of the mechanical load driving turbines is [13]:

M_{s} = frac{P}{H_{hp}-H_{lp}}                    [1]

where

Ms   =   Total steam consumption at operating power

P     =   Operating power

Hhp  =   The specific enthalpy of the steam at the turbine inlet

Hlp     =   The specific enthalpy of the steam at the turbine exhaust

Total steam consumption by turbines driving mechanical load (MST)

Mst   =   18,33 TPH

Therefore, total steam consumed by all turbines onsite (MSTt)

Mstt  =   123 TPH

With the two boilers rated at 100 MCR and 50 MCR respectively, the current steam consumption by the turbines for power generation and mechanical drive as calculated above is 123 TPH. This excludes any other losses not stated in the calculations. The steam plant, therefore, operates at 82% of its capacity. The results show that the two turbo-alternators consume an average of 17,755 kg/kWh of high pressure steam. It must be noted that this is taking into consideration the turbine efficiency, gearbox efficiency, and the alternator efficiency.

Current system efficiency

Procedure

To calculate the energy efficiency of a plant/factory, one needs to understand the amount of steam generated and the amount of steam used for useful work, such as power generation and mechanical load driving.

Calculations

To simplify the calculations of the current system, a simplified Rankine Cycle approach was used to represent the factory steam system.

Results for thermal efficiency

Using the knowledge and information calculated based on the current system, the overall thermal efficiency represented by a simplified Rankine cycle was calculated to be 19,69%.

Table 4: Enthalpy data used.
Description Calculated quantity
Enthalpy into the boiler (h2) 422,86 kJ/kg
Enthalpy out of the boiler to the turbine (h3) 3227,31 kJ/kg
Enthalpy of exhaust (h4) 2675,00 kJ/kg

Gasification system design

Gasifier types

There are four main types of gasifiers available commercially, these are: counter-current fixed bed (up-draft), co-current fixed bed (down-draft), fluidised bed reactor and entrained flow gasifiers. The main difference between the types of gasifiers is the manner in which both the fuel (bagasse) and the gasification agent (steam or oxygen) is introduced into the gasifier and the manner in which the resulting tar is extracted.

Design calculations of downdraft gasifier

The type of a gasifier that was selected was a down-draft gasifier. In this type of a gasifier, the gasification agent flows in a co-current configuration (same direction) with the fuel entering the gasifier. The heat is added to the upper part of the bed, by the combustion of small amounts of the fuel or external heat sources. The tar content of the produced gas is lower than for an updraft gasifier but the particulates content of the gas is high.

Other considerations

Bagasse moisture

Bagasse moisture plays a pivotal role on the overall system thermal efficiency. As seen in the calculation of the HCV, it is evident that minimum moisture will result in maximum HCV if all other quantities remain constant. It is therefore critical to ensure that means of reducing bagasse moisture are explored. This would in turn increase the overall HCV and thus the thermal efficiency of the system. Bagasse moisture of 51,41% was for the calculations of both systems as this was the average value recorded on the current system.

Issues with bagasse handling

Bagasse gasifier designs that are currently available utilise enclosed cylindrical reactors. This calls for a properly controlled feed system into the gasifier.

The current belt conveyors may not necessarily work; instead, tubular conveyors may be required to direct the bagasse into the gasifier. This also requires a very dry bagasse to prevent chokes. The consideration of the bagasse handling system still needs to be investigated further. Also, the sustainability of the bagasse supply needs to be investigated further especially in the case of co-generation since the downtime due to bagasse shortages would not be acceptable and the national grid would not accept constant power dips.

Economic baseline assumption

The economic evaluation used an assumption that labour wages are in the range of between R50 to R160/hour, with the direct cost of bagasse and bagasse handling not considered for the purpose of the analysis. In the current direct bagasse combustion system, it was assumed that the boiler is attended by five personnel at an average cost of R100/hour each.

This, therefore, would result in a labour cost of R500/hour. This would equate to R84 000/week. Reducing the number of operators to three would reduce the overall weekly labour cost by R29 600/week. On the bagasse gasification system one would require the control operator, the gasifier feed attendant, and the output product attendant.

Table 5: Gas engine specifications.
Details of engine Value
GE Jenbacher type 6 twin-turbo engine (Type J624S)
Number of cylinders and arrangement 24/60°
Combustion Lean burn principle
Bore 190 mm
Stroke 185 mm
Speed (rpm) 1500 rpm
Dimensions 11 600 mm (length)
2000 mm (width)
2500 mm (height)
Genset weight 43 000 kg
Volumetric efficiency 80%

Safety

Safety precautions such as ensuring effective insulation and installing air leak early detection system would prevent the risks associated with fire or explosion hazards.

Environmental hazards

Ash does not contain any substance that may be environmentally hazardous. Disposal can therefore be done in a normal but controlled manner.

The properties of exhaust emissions from engines run on producer gas are generally considered to be acceptable, comparable to those of diesel engine, thus there are no significant environmental hazards to the gasification system if all emitted gases are controlled.

Final analysis comparison

The study entailed on this article was meant to give an understanding of the thermal efficiency of the current system which uses direct combustion boilers as compared to a system with similar inputs using a bagasse gasification system.

Summary of final analysis

From the conventional system, one requires 9,549 t of bagasse to produce 1 MWh of electrical power as compared to the 1,89 t of bagasse required to produce the same amount (1 MWh) in a gasification system. This is based on the calculations that showed that 1 t of steam is produced from 0,5347 t of bagasse and that 1,87 t of steam is generated per t of bagasse.

Heat recovery steam generator

If one were to incorporate a bagasse gasification system into an existing system, the factory steam requirements need to be considered. The sugar production process uses mainly exhaust steam in its operation. The system under evaluation requires this steam to remain at an average of 123 TPH at approximately 130 kPa. If a bagasse gasification plant is to be incorporated as an IGCC system, a heat recovery steam generator is critical.

Integration into the current system

The current system that uses the direct combustion boiler already has these main components. The difference in the configuration would be that instead of burning the bagasse in the furnace, the furnace would be replaced by the ducting from the gas outlet stream of the gas engine. This gas will perform a similar function as that performed by the furnace.

Depending on the application required, the ducting can be configured into different streams. If just direct heating is needed, a different stream can be used to direct the gas heat to that area. The heat also can be controlled to work in conjunction with the current direct combustion system. In this case, the furnace would still remain but the internal heat would be supplemented with the gases while the system can still be operated with minimum or no gases through a direct combustion operation in the boiler furnace.

The system can easily be integrated by linking the hot gases to the currently installed forced draft air or secondary air ducts that currently is coming from an air heater at much lower temperatures than the gas from a gas engine gas exit temperature.

System requirements

The system must fulfil the following requirements:

  • Easy change over to the current direct bagasse combustion system as every new system takes time to be fully trusted to work as a standalone system.
  • Be able to run from cold start at the same or quicker time than the current system.
  • Consume less fuel (bagasse) for the same kWh generated.
  • Still ensure the availability of sufficient steam to the process department for sugar processing.
Table 6: Final analysis calculated.
Description Value
Boiler efficiency 61,18%
Steam generated per ton of bagasse 1,87 t
Tons bagasse per ton of crushed cane 0,275 t
Steam generated per ton of cane crushed 0,66 t
Total steam consumption 123 TPH
Total steam consumed by the power generation plant 17,64 TPH/MW
Thermal efficiency 19,69%
Gasification thermal efficiency 61%
Gasifier efficiency 80%
Bagasse consumption by the gasifier 1,89 TPH/MW
Conventional system 9,549 t bagasse per MWh of electrical power produced
Gasification system 1,89 t bagasse per MWh of electrical power produced

Conclusion

The current direct bagasse combustion system thermal efficiency was determined using the Rankine cycle. The system can be summarised as follows:

The water is pumped into a high pressure steam boiler. The water is boiled and converted into HP steam at high temperatures by the addition of heat. This heat is obtained by a direct combustion of bagasse in the furnace. The steam expands in a steam turbine and after work done, it becomes exhaust steam. The exhaust steam is condensed (by the sugar making process), into water and used as the boiler feed water. This completes the Rankine cycle.

The net work done per cycle is the difference between the work produced by the steam as it expands and the work put in externally to pump the water into the boiler. The higher the pressure and temperature of the steam in the boiler, the higher the power output.

Problems with bagasse gasification

Although studies have shown that the bagasse gasification process is more efficient than the current direct combustion process, there are a number of obstacles that need attention before the system can be widely implemented. Some of these obstacles are the feeding and operation of high capacity pressurised gasifiers, the gas cleaning with complete tar cracking, separation of alkali and particles from the gas produced and the modification of current system configuration to incorporate the bagasse gasification system. Another major obstacle is the capital and operational costs.

The gas produced during gasification may contain large quantities of methane and other hazardous gases such as carbon monoxide. There should be an option to convert these compounds into CO and H2 at high temperatures and in the presence of a catalyst such as nickel.

The major concern about this technology is the inaccessibility of documented practical experiments results in South Africa that would attract investment.

References

[1]  N Magasiner: “Boiler Design and Selection in the Cane Sugar Industry”, 1966.
[2]  MV Madurwar: “Use of sugarcane bagasse ash as brick material”, Research Communications, 2014.
[3]  ISL LIMS: “Eston Factory Daily Production Report”, 2014.
[4]  N Ahlawait: “Success story of India’s first coal gasification plant”, New Delhi, MissionEnergy, 2014.
[5]  Granatstein, D: “Gasification vs. Combustion of waste/biomass in Fluidized Bed Reactors”, 2003.
[6]  http://en.wikipedia.org
[7]  Spirax Sarco Limited, 2014, www.spiraxsarco.com
[8]  Eston: Pi ProcessBook, 2014.
[9]  P Rein: “Steam Generation”, in Cane Sugar Engineering, 2007.
[10] B Mtunzi: “Bagassed-based co-generation at Hippo Valley Estates sugar factory in Zimbambwe”, Journal of Energy in Southern Africa, 2012.
[11] W Lawler: “Boilers and Co-Generation”, 2011.
[12] E Hugot: “Handbook of Cane Sugar Engineering”, 1986.
[13] www.sugartech.co.za
[14] R Rajput: “Vapour Power Cycles”, in Engineering Thermodynamics, 2007.
[15] United Nations Forest Products Division: “Wood Gas as Engine Fuel”, 1986.

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