Sugarcane Trash as Biomass Resource

cane-trashSugarcane trash (or cane trash) is an excellent biomass resource in sugar-producing countries worldwide. The amount of cane trash produced depends on the plant variety, age of the crop at harvest and soil and weather conditions. Typically it represents about 15% of the total above ground biomass at harvest which is equivalent to about 10-15 tons per hectare of dry matter. During the harvesting operation around 70-80% of the cane trash is left in the field with 20-30% taken to the mill together with the sugarcane stalks as extraneous matter.

Cane trash’s calorific value is similar to that of bagasse but has an advantage of having lower moisture content, and hence dries more quickly. Nowadays only a small quantity of this biomass is used as fuel, mixed with bagasse or by itself, at the sugar mill. The rest is burned in the vicinity of the dry cleaning installation, creating a pollution problem in sugar-producing nations.

Cane trash and bagasse are produced during the harvesting and milling process of sugarcane which normally lasts between 6 to 7 months. Cane trash can potentially be converted into heat and electrical energy. However, most of the trash is burned in the field due to its bulky nature and high cost incurred in collection and transportation.

Cane trash could be used as an off-season fuel for year-round power generation at sugar mills. There is also a high demand for biomass as a boiler fuel during the sugar-milling season. Sugarcane trash can also converted in biomass pellets and used in dedicated biomass power stations or co-fired with coal in power plants and cement kilns.

Burning of cane trash creates pollution in sugar-producing countries

Burning of cane trash creates pollution in sugar-producing countries

Currently, a significant percentage of energy used for boilers in sugarcane processing is provided by imported bunker oil. Overall, the economic, environmental, and social implications of utilizing cane trash in the final crop year as a substitute for bunker oil appears promising. It represents an opportunity for developing biomass energy use in the Sugarcane industry as well as for industries / communities in the vicinity.

Positive socio-economic impacts include the provision of large-scale rural employment and the minimization of oil imports. It can also develop the expertise necessary to create a reliable biomass supply for year-round power generation.

Recovery of Cane Trash

Recovery of cane trash implies a change from traditional harvesting methods; which normally consists of destroying the trash by setting huge areas of sugarcane fields ablaze prior to the harvest.  There are a number of major technical and economic issues that need to be overcome to utilize cane trash as a renewable energy resource. For example, its recovery from the field and transportation to the mill, are major issues.

Alternatives include the current situation where the cane is separated from the trash by the harvester and the two are transported to the mill separately, to the harvesting of the whole crop with separation of the cane and the trash carried out at the mill. Where the trash is collected from the field it maybe baled incurring a range of costs associated with bale handling, transportation and storage. Baling also leaves about 10-20% (1-2 tons per hectare) of the recoverable trash in the field.

A second alternative is for the cane trash to be shredded and collected separately from the cane during the harvesting process. The development of such a harvester-mounted cane trash shredder and collection system has been achieved but the economics of this approach require evaluation. A third alternative is to harvest the sugarcane crop completely which would require an adequate collection, transport and storage system in addition to a mill based cleaning plant to separate the cane from the trash .

A widespread method for cane trash recovery is to cut the cane, chop into pieces and then it is blown in two stages in the harvester to remove the trash. The amount of trash that goes along with the cane is a function of the cleaning efficiency of the harvester. The blowers are adjusted to get adequate cleaning with a bearable cane loss.

On the average 68 % of the trash is blown out of the harvester, and stays on the ground, and 32 % is taken to the mill together with the cane as extraneous matter. The technique used to recover the trash staying on the ground is baling. Several baling machines have been tested with small, large, round and square bales. Cane trash can be considered as a viable fuel supplementary to bagasse to permit year-round power generation in sugar mills.

Thus, recovery of cane trash in developing nations of Asia, Africa and Latin America implies a change from traditional harvesting methods, which normally consists of destroying the trash by setting huge areas of cane fields ablaze prior to the harvest. To recover the trash, a new so-called “green mechanical harvesting” scheme will have to be introduced. By recovering the trash in this manner, the production of local air pollutants, as well as greenhouse gases contributing to adverse climatic change, from the fires are avoided and cane trash could be used as a means of regional sustainable development.

Cane Trash Recovery in Cuba

The sugarcane harvesting system in Cuba is unique among cane-producing countries in two important respects. First, an estimated 70 % of the sugarcane crop is harvested by machine without prior burning, which is far higher than for any other country. The second unique feature of Cuban harvesting practice is the long-standing commercial use of “dry cleaning stations” to remove trash from the cane stalks before the stalks are transported to the crushing mills.

Cuba has over 900 cleaning stations to serve its 156 sugar mills. The cleaning stations are generally not adjacent to the mills, but are connected to mills by a low-cost cane delivery system – a dedicated rail network with more than 7000 km of track. The cleaning stations take in green machine-cut or manually cut cane. Trash is removed from the stalk and blown out into a storage area. The stalks travel along a conveyor to waiting rail cars. The predominant practice today is to incinerate the trash at the cleaning station to reduce the “waste” volume.

Properties and Uses of POME

POMEPalm Oil processing gives rise to highly polluting waste-water, known as Palm Oil Mill Effluent (POME), which is often discarded in disposal ponds, resulting in the leaching of contaminants that pollute the groundwater and soil, and in the release of methane gas into the atmosphere. POME is an oily wastewater generated by palm oil processing mills and consists of various suspended components. This liquid waste combined with the wastes from steriliser condensate and cooling water is called palm oil mill effluent.

On average, for each ton of FFB (fresh fruit bunches) processed, a standard palm oil mill generate about 1 tonne of liquid waste with biochemical oxygen demand 27 kg, chemical oxygen demand 62 kg, suspended solids (SS) 35 kg and oil and grease 6 kg. POME has a very high BOD and COD, which is 100 times more than the municipal sewage.

POME is a non-toxic waste, as no chemical is added during the oil extraction process, but will pose environmental issues due to large oxygen depleting capability in aquatic system due to organic and nutrient contents. The high organic matter is due to the presence of different sugars such as arabinose, xylose, glucose, galactose and manose. The suspended solids in the POME are mainly oil-bearing cellulosic materials from the fruits. Since the POME is non-toxic as no chemical is added in the oil extraction process, it is a good source of nutrients for microorganisms.

Biogas Potential of POME

POME is always regarded as a highly polluting wastewater generated from palm oil mills. However, reutilization of POME to generate renewable energies in commercial scale has great potential. Anaerobic digestion is widely adopted in the industry as a primary treatment for POME. Biogas is produced in the process in the amount of 20 mper ton FFB. This effluent could be used for biogas production through anaerobic digestion. At many palm oil mills this process is already in place to meet water quality standards for industrial effluent. The gas, however, is flared off.

Palm oil mills, being one of the largest industries in Malaysia and Indonesia, effluents from these mills can be anaerobically converted into biogas which in turn can be used to generate power through CHP systems such as gas turbines or gas-fired engines. A cost effective way to recover biogas from POME is to replace the existing ponding/lagoon system with a closed digester system which can be achieved by installing floating plastic membranes on the open ponds.

As per conservative estimates, potential POME produced from all Palm Oil Mills in Indonesia and Malaysia is more than 50 million m3 each year which is equivalent to power generation capacity of more than 800 GW.

New Trends

Recovery of organic-based product is a new approach in managing POME which is aimed at getting by-products such as volatile fatty acid, biogas and poly-hydroxyalkanoates to promote sustainability of the palm oil industry.  It is envisaged that POME can be sustainably reused as a fermentation substrate in production of various metabolites through biotechnological advances. In addition, POME consists of high organic acids and is suitable to be used as a carbon source.

POME has emerged as an alternative option as a chemical remediation to grow microalgae for biomass production and simultaneously act as part of wastewater treatment process. POME contains hemicelluloses and lignocelluloses material (complex carbohydrate polymers) which result in high COD value (15,000–100,000 mg/L).

POME-Biogas

Utilizing POME as nutrients source to culture microalgae is not a new scenario, especially in Malaysia. Most palm oil millers favor the culture of microalgae as a tertiary treatment before POME is discharged due to practically low cost and high efficiency. Therefore, most of the nutrients such as nitrate and ortho-phosphate that are not removed during anaerobic digestion will be further treated in a microalgae pond. Consequently, the cultured microalgae will be used as a diet supplement for live feed culture.

In recent years, POME is also gaining prominence as a feedstock for biodiesel production, especially in the European Union. The use of POME as a feedstock in biodiesel plants requires that the plant has an esterification unit in the back-end to prepare the feedstock and to breakdown the FFA. In recent years, biomethane production from POME is also getting traction in Indonesia and Malaysia.

Global Trends in the Biomass Sector

There has been a flurry of activity in the biomass energy and waste-to-energy sector in recent year, with many new projects and initiatives being given the green light across the globe. This movement has been on both a regional and local level; thanks to the increased efficiency of green energy generators and a slight lowering in implementation costs, more businesses and even some homeowners are converting waste-to-energy systems or by installing biomass energy units.

Latest from the United Kingdom

Our first notable example of this comes from Cornwall in the UK. As of this week, a small hotel has entirely replaced its previous oil-based heating system with biomass boilers. Fuelled from wood wastes brought in from a neighboring forest, the BudockVean hotel has so far been successful in keeping the entire establishment warm on two small boilers despite it being the height of British winter – and when warmer weather arrives, plans to install solar panels on the building’s roof is to follow.

Similar projects have been undertaken across small businesses in Britain, including the south-coast city of Plymouth that has just been announced to house a 10MW biomass power plant (alongside a 20MW plant already in construction). These developments arein part thanks to the UK government’s Renewable Heat Incentive which was launched back in 2011. The scheme only provides funding to non-domestic properties currently, but a domestic scheme is in the works this year to help homeowners also move away from fossil fuels.

Initiatives (and Setbacks) in the US

Back across the pond, and the state of New York is also launching a similar scheme. The short-term plan is to increase public education on low-emission heating and persuade a number of large business to make the switch; in the longer term, $800m will be used to install advanced biomass systems in large, state-owned buildings.

A further $40m will be used as part of a competition to help create a series of standalone energy grids in small towns and rural areas, which is a scheme that could hopefully see adopted beyond New York if all goes well.


Unfortunately, the move away from fossil fuels hasn’t been totally plain sailing across the US. Georgia suffered a blow this week as plans to convert a 155MW coal plant to biomass have been abandoned, citing large overheads and low projected returns. The company behind the project have met similar difficulties at other sites, but as of this week are moving ahead with further plans to convert over 2000MW of oil and coal energy generation in the coming years.

Elsewhere in the US, a company has conducted a similar study as to whether biomass plant building will be feasible in both Florida and Louisiana. Surveying has only just been completed, but if things go better than the recent developments in Georgia, the plants will go a long way to converting biomass to fertilizer for widespread use in agriculture in both states.

Far East Leading the Way

One country that is performing particularly well in biomass energy investment market is Japan. Biomass is being increasingly used in power plants in Japan as a source of fuel, particularly after the tragic accident at Fukushima nuclear power plant in 2011.  Palm kernel shell (PKS) has emerged as a favorite choice of biomass-based power plants in the country. Most of these biomass power plants use PKS as their energy source, and only a few operate with wood pellets. Interestingly, most of the biomass power plants in Japan have been built after 2015..

On the contrary, the US and Europe saw a fairly big fall in financing during this period; it should be noted, however, that this relates to the green energy investment market as a whole as opposed to biomass-specific funding. The increase seen in Japan has been attributed to an uptake in solar paneling, and if we look specifically to things such as the global demand for biomass pellets, we see that the most recent figures paint the overall market in a much more favorable light for the rest of the world.

Brighter Times Ahead

All in all, it’s an exciting time for the biomass industry despite the set backs which are being experienced in some regions.  On the whole, legislators and businesses are working remarkably well together in order to pave the way forward – being a fairly new market (from a commercially viable sense at least), it has taken a little while to get the ball rolling, but expect to see it blossom quickly now that the idea of biomass is starting to take hold.

Energy Potential of Bagasse

Sugarcane is one of the most promising agricultural sources of biomass energy in the world. Sugarcane produces mainly two types of biomass, Cane Trash and Bagasse. Cane Trash is the field residue remaining after harvesting the Cane stalk while bagasse is the fibrous residue left over after milling of the Cane, with 45-50% moisture content and consisting of a mixture of hard fibre, with soft and smooth parenchymatous (pith) tissue with high hygroscopic property. Bagasse contains mainly cellulose, hemi cellulose, pentosans, lignin, Sugars, wax, and minerals. The quantity obtained varies from 22 to 36% on Cane and is mainly due to the fibre portion in Cane and the cleanliness of Cane supplied, which, in turn, depends on harvesting practices.

The composition of Bagasse depends on the variety and maturity of Sugarcane as well as harvesting methods applied and efficiency of the Sugar processing. Bagasse is usually combusted in furnaces to produce steam for power generation. Bagasse is also emerging as an attractive feedstock for bioethanol production. It is also utilized as the raw material for production of paper and as feedstock for cattle. The value of Bagasse as a fuel depends largely on its calorific value, which in turn is affected by its composition, especially with respect to its water content and to the calorific value of the Sugarcane crop, which depends mainly on its sucrose content.

Moisture contents is the main determinant of calorific value i.e. the lower the moisture content, the higher the calorific value. A good milling process will result in low moisture of 45% whereas 52% moisture would indicate poor milling efficiency. Most mills produce Bagasse of 48% moisture content, and most boilers are designed to burn Bagasse at around 50% moisture. Bagasse also contains approximately equal proportion of fibre (cellulose), the components of which are carbon, hydrogen and oxygen, some sucrose (1-2 %), and ash originating from extraneous matter. Extraneous matter content is higher with mechanical harvesting and subsequently results in lower calorific value.

For every 100 tons of Sugarcane crushed, a Sugar factory produces nearly 30 tons of wet Bagasse. Bagasse is often used as a primary fuel source for Sugar mills; when burned in quantity, it produces sufficient heat and electrical energy to supply all the needs of a typical Sugar mill, with energy to spare. The resulting CO2 emissions are equal to the amount of CO2 that the Sugarcane plant absorbed from the atmosphere during its growing phase, which makes the process of cogeneration greenhouse gas-neutral.

35MW Bagasse and Coal CHP Plant in Mauritius

Cogeneration of Bagasse is one of the most attractive and successful energy projects that have already been demonstrated in many Sugarcane producing countries such as Mauritius, Reunion Island, India and Brazil. Combined heat and power from Sugarcane in the form of power generation offers renewable energy options that promote sustainable development, take advantage of domestic resources, increase profitability and competitiveness in the industry, and cost-effectively address climate mitigation and other environmental goals.

Cogeneration of Bagasse

Cogeneration of bagasse is one of the most attractive and successful energy projects that have already been demonstrated in many sugarcane producing countries such as Mauritius, Reunion Island, India and Brazil. Combined heat and power from sugarcane in the form of power generation offers renewable energy options that promote sustainable development, take advantage of domestic resources, increase profitability and competitiveness in the industry, and cost-effectively address climate mitigation and other environmental goals.

According to World Alliance for Decentralized Energy (WADE) report on Bagasse Cogeneration, bagasse-based cogeneration could deliver up to 25% of current power demand requirements in the world’s main cane producing countries. The overall potential share in the world’s major developing country producers exceeds 7%. There is abundant opportunity for the wider use of bagasse-based cogeneration in sugarcane-producing countries. It is especially great in the world’s main cane producing countries like Brazil, India, Thailand, Pakistan, Mexico, Cuba, Colombia, Philippines and Vietnam. Yet this potential remains by and large unexploited.

Using bagasse to generate power represents an opportunity to generate significant revenue through the sale of electricity and carbon credits. Additionally, cogeneration of heat and power allows sugar producers to meet their internal energy requirements and drastically reduce their operational costs, in many cases by as much as 25%. Burning bagasse also removes a waste product through its use as a feedstock for the electrical generators and steam turbines.

Most sugarcane mills around the globe have achieved energy self-sufficiency for the manufacture of raw sugar and can also generate a small amount of exportable electricity. However, using traditional equipment such as low-pressure boilers and counter-pressure turbo alternators, the level and reliability of electricity production is not sufficient to change the energy balance and attract interest for export to the electric power grid.

On the other hand, revamping the boiler house of sugar mills with high pressure boilers and condensing extraction steam turbine can substantially increase the level of exportable electricity. This experience has been witnessed in Mauritius, where, following major changes in the processing configurations, the exportable electricity from its sugar factory increased from around 30-40 kWh to around 100–140 kWh per ton cane crushed. In Brazil, the world’s largest cane producer, most of the sugar mills are upgrading their boiler configurations to 42 bars or even higher pressure of up to 67 bars.

Technology Options

The prime technology for sugar mill cogeneration is the conventional steam-Rankine cycle design for conversion of fuel into electricity. A combination of stored and fresh bagasse is usually fed to a specially designed furnace to generate steam in a boiler at typical pressures and temperatures of usually more than 40 bars and 440°C respectively. The high pressure steam is then expanded either in a back pressure or single extraction back pressure or single extraction condensing or double extraction cum condensing type turbo generator operating at similar inlet steam conditions.

Due to high pressure and temperature, as well as extraction and condensing modes of the turbine, higher quantum of power gets generated in the turbine–generator set, over and above the power required for sugar process, other by-products, and cogeneration plant auxiliaries. The excess power generated in the turbine generator set is then stepped up to extra high voltage of 66/110/220 kV, depending on the nearby substation configuration and fed into the nearby utility grid. As the sugar industry operates seasonally, the boilers are normally designed for multi-fuel operations, so as to utilize mill bagasse, procured Bagasse/biomass, coal and fossil fuel, so as to ensure year round operation of the power plant for export to the grid.

Latest Trends

Modern power plants use higher pressures, up to 87 bars or more. The higher pressure normally generates more power with the same quantity of Bagasse or biomass fuel. Thus, a higher pressure and temperature configuration is a key in increasing exportable surplus electricity.

In general, 67 bars pressure and 495°C temperature configurations for sugar mill cogeneration plants are well-established in many sugar mills in India. Extra high pressure at 87 bars and 510°C, configuration comparable to those in Mauritius, is the current trend and there are about several projects commissioned and operating in India and Brazil. The average increase of power export from 40 bars to 60 bars to 80 bars stages is usually in the range of 7-10%.

A promising alternative to steam turbines are gas turbines fuelled by gas produced by thermochemical conversion of biomass. The exhaust is used to raise steam in heat recovery systems used in any of the following ways: heating process needs in a cogeneration system, for injecting back into gas turbine to raise power output and efficiency in a steam-injected gas turbine cycle (STIG) or expanding through a steam turbine to boost power output and efficiency in a gas turbine/steam turbine combined cycle (GTCC). Gas turbines, unlike steam turbines, are characterized by lower unit capital costs at modest scale, and the most efficient cycles are considerably more efficient than comparably sized steam turbines.

Energy Potential of Palm Kernel Shells

palm-kernel-shellsThe Palm Oil industry in Southeast Asia and Africa generates large quantity of biomass wastes whose disposal is a challenging task. Palm kernel shells (or PKS) are the shell fractions left after the nut has been removed after crushing in the Palm Oil mill. Kernel shells are a fibrous material and can be easily handled in bulk directly from the product line to the end use. Large and small shell fractions are mixed with dust-like fractions and small fibres. Moisture content in kernel shells is low compared to other biomass residues with different sources suggesting values between 11% and 13%.

Palm kernel shells contain residues of Palm Oil, which accounts for its slightly higher heating value than average lignocellulosic biomass. Compared to other residues from the industry, it is a good quality biomass fuel with uniform size distribution, easy handling, easy crushing, and limited biological activity due to low moisture content. PKS can be readily co-fired with coal in grate fired -and fluidized bed boilers as well as cement kilns in order to diversify the fuel mix.

The primary use of palm kernel shells is as a boiler fuel supplementing the fibre which is used as primary fuel. In recent years kernel shells are sold as alternative fuel around the world. Besides selling shells in bulk, there are companies that produce fuel briquettes from shells which may include partial carbonisation of the material to improve the combustion characteristics. As a raw material for fuel briquettes, palm shells are reported to have the same calorific characteristics as coconut shells. The relatively smaller size makes it easier to carbonise for mass production, and its resulting palm shell charcoal can be pressed into a heat efficient biomass briquette.

Palm kernel shells have been traditionally used as solid fuels for steam boilers in palm oil mills across Southeast Asia. The steam generated is used to run turbines for electricity production. These two solid fuels alone are able to generate more than enough energy to meet the energy demands of a palm oil mill. Most palm oil mills in the region are self-sufficient in terms of energy by making use of kernel shells and mesocarp fibers in cogeneration. In recent years, the demand for palm kernel shells has increased considerably in Europe, Asia-Pacific, China etc resulting in price close to that of coal. Nowadays, cement industries and power producers are increasingly using palm kernel shells to replace coal. In grate-fired boiler systems, fluidized-bed boiler systems and cement kilns, palm kernel shells are an excellent fuel.

Cofiring of PKS yields added value for power plants and cement kilns, because the fuel significantly reduces carbon emissions – this added value can be expressed in the form of renewable energy certificates, carbon credits, etc. However, there is a great scope for introduction of high-efficiency cogeneration systems in the industry which will result in substantial supply of excess power to the public grid and supply of surplus PKS to other nations. Palm kernel shell is already extensively in demand domestically by local industries for meeting process heating requirements, thus creating supply shortages in the market.

Palm oil mills around the world may seize an opportunity to supply electricity for its surrounding plantation areas using palm kernel shells, empty fruit branches and palm oil mill effluent which have not been fully exploited yet. This new business will be beneficial for all parties, increase the profitability for palm oil industry, reduce greenhouse gas emissions and increase the electrification ratio in surrounding plantation regions.

Salient Features of Sugar Industry in Mauritius

Sugar industry has always occupied a prominent position in the Mauritian economy since the introduction of sugarcane around three centuries ago. Mauritius has been a world pioneer in establishing sales of bagasse-based energy to the public grid, and is currently viewed as a model for other sugarcane producing countries, especially the developing ones.

Sugar factories in Mauritius produce about 600,000 tons of sugar from around 5.8 million tons of sugarcane which is cultivated on an agricultural area of about 72,000 hectares. Of the total sugarcane production, around 35 percent is contributed by nearly 30,000 small growers. There are more than 11 sugar factories presently operating in Mauritius having crushing capacities ranging from 75 to 310 tons cane per hour.

During the sugar extraction process, about 1.8 million tons of Bagasse is produced as a by-product, or about one third of the sugarcane weight. Traditionally, 50 percent of the dry matter is harvested as cane stalk to recover the sugar with the fibrous fraction, i.e. Bagasse being burned to power the process in cogeneration plant. Most factories in Mauritius have been upgraded and now export electricity to the grid during crop season, with some using coal to extend production during the intercrop season.

Surplus electricity is generated in almost all the sugar mills. The total installed capacity within the sugar industry is 243 MW out of which 140 MW is from firm power producers. Around 1.6 – 1.8 million tons of bagasse (wet basis) is generated on an annually renewable basis and an average of around 60 kWh per ton sugarcane is generated for the grid throughout the island.

The surplus exportable electricity in Mauritian power plants has been based on a fibre content ranging from 13- 16% of sugarcane, 48% moisture content in Bagasse, process steam consumption of 350–450 kg steam per ton sugarcane and a power consumption of 27-32 kWh per ton sugarcane.

In Mauritius, the sugarcane industry is gradually increasing its competitiveness in electricity generation. It has revamped its boiler houses by installing high pressure boilers and condensing extraction steam turbine. All the power plants are privately owned, and the programme has been a landmark to show how all the stakeholders (government, corporate and small planters) can co-operate. The approach is being recommended to other sugarcane producing countries worldwide to harness the untapped renewable energy potential of biomass wastes from the sugar industry.

Biomass Gasification Process

Biomass gasification involves burning of biomass in a limited supply of air to give a combustible gas consisting of carbon monoxide, carbon dioxide, hydrogen, methane, water, nitrogen, along with contaminants like small char particles, ash and tars. The gas is cleaned to make it suitable for use in boilers, engines and turbines to produce heat and power (CHP).

Biomass gasification provides a means of deriving more diverse forms of energy from the thermochemical conversion of biomass than conventional combustion. The basic gasification process involves devolatization, combustion and reduction.

During devolatization, methane and other hydrocarbons are produced from the biomass by the action of heat which leaves a reactive char.

During combustion, the volatiles and char are partially burned in air or oxygen to generate heat and carbon dioxide. In the reduction phase, carbon dioxide absorbs heat and reacts with the remaining char to produce carbon monoxide (producer gas). The presence of water vapour in a gasifier results in the production of hydrogen as a secondary fuel component.

There are two main types of gasifier that can be used to carry out this conversion, fixed bed gasifiers and fluidized bed gasifiers. The conversion of biomass into a combustible gas involves a two-stage process. The first, which is called pyrolysis, takes place below 600°C, when volatile components contained within the biomass are released. These may include organic compounds, hydrogen, carbon monoxide, tars and water vapour.

Pyrolysis leaves a solid residue called char. In the second stage of the gasification process, this char is reacted with steam or burnt in a restricted quantity of air or oxygen to produce further combustible gas. Depending on the precise design of gasifier chosen, the product gas may have a heating value of 6 – 19 MJ/Nm3.

Layout of a Typical Biomass Gasification Plant

The products of gasification are a mixture of carbon monoxide, carbon dioxide, methane, hydrogen and various hydrocarbons, which can then be used directly in gas turbines, and boilers, or used as precursors for synthesising a wide range of other chemicals.

In addition there are a number of methods that can be used to produce higher quality product gases, including indirect heating, oxygen blowing, and pressurisation. After appropriate treatment, the resulting gases can be burned directly for cooking or heat supply, or used in secondary conversion devices, such as internal combustion engines or gas turbines, for producing electricity or shaft power (where it also has the potential for CHP applications).

 

See some of our favorite inspirational quotes

Bagasse-Based Cogeneration in Pakistan: Challenges and Opportunities

Considering the fact that Pakistan is among the world’s top-10 sugarcane producers, the potential of generating electricity from bagasse is huge.  Almost all the sugar mills in Pakistan have in-house plants for cogeneration but they are inefficient in the consumption of bagasse. If instead, high pressure boilers are installed then the production capacity can be significantly improved with more efficient utilization of bagasse.

However, due to several reasons; mostly due to financing issues, the sugar mill owners were not able to set up these plants. Only recently, after financial incentives have been offered and a tariff rate agreed upon between the government and mill owners, are these projects moving ahead.

The sugar mill owners are more than willing to supply excess electricity generated form the in-house power plants to the national grid but were not able to before, because they couldn’t reach an agreement with the government over tariff. The demand for higher tariff was justified because of large investments in setting up new boilers. It would also have saved precious foreign exchange which is spent on imported oil.

By estimating the CDM potential of cogeneration (or CHP) projects based on biofuels, getting financing for these projects would be easier. Renewable energy projects can be developed through Carbon Development Mechanism or any other carbon credit scheme for additional revenue.

Since bagasse is a clean fuel which emits very little carbon emissions it can be financed through Carbon Development Mechanism. One of the reasons high cogeneration power plants are difficult to implement is because of the high amount of costs associated. The payback period for the power plants is unknown which makes the investors reluctant to invest in the high cogeneration project. CDM financing can help improve the rate of return of the project.

Bagasse power plants generate Carbon Emission Reductions in 2 ways; one by replacing electricity produced from fossil fuels.  Secondly if not used as a fuel, it would be otherwise disposed off in an unsafe manner and the methane emissions present in biomass would pollute the environment far more than CO2 does.

Currently there are around 83 sugar mills in Pakistan producing about 3.5 million metric tons of sugar per annum with total crushing capacity 597900 TCD, which can produce approximately 3000 MW during crop season Although it may seem far-fetched at the moment, if the government starts to give more attention to  sugar industry biomass rather than coal, Pakistan can fulfill its energy needs without negative repercussions or damage to the environment.

However some sugar mills are opting to use coal as a secondary fuel since the crushing period of sugarcane lasts only 4 months in Pakistan. The plants would be using coal as the main fuel during the non-crushing season. The CDM effect is reduced with the use of coal. If a high cogeneration plant is using even 80% bagasse and 20% of coal then the CERs are almost nullified. If more than 20% coal is used then the CDM potential is completely lost because the emissions are increased. However some sugar mills are not moving ahead with coal as a secondary fuel because separate tariff rates have to be obtained for electricity generation if coal is being used in the mix which is not easily obtained.

Pakistan has huge untapped potential for bagasse-based power generation

One of the incentives being offered by the State Bank of Pakistan is that if a project qualifies as a renewable project it is eligible to get loan at 6% instead of 12%. However ones drawback is that, in order to qualify as a renewable project, CDM registration of a project is not taken into account.

Although Pakistan is on the right track by setting up high cogeneration power plants, the use of coal as a secondary fuel remains debatable.  The issue that remains to be addressed is that with such huge amounts of investment on these plants, how to use these plants efficiently during non-crushing period when bagasse is not available. It seems almost counter-productive to use coal on plants which are supposed to be based on biofuels.

Conclusion

With the demand for energy in Pakistan growing, the country is finally exploring alternatives to expand its power production. Pakistan has to rely largely on fossils for their energy needs since electricity generation from biomass energy sources is considered to be an expensive option despite abundance of natural resources. However by focusing on growing its alternate energy options such as bagasse-based cogeneration, the country will not only mitigate climate change but also tap the unharnessed energy potential of sugar industry biomass.

Summary of Biomass Combustion Technologies

Direct combustion is the best established and most commonly used technology for converting biomass to heat. During combustion, biomass fuel is burnt in excess air to produce heat. The first stage of combustion involves the evolution of combustible vapours from the biomass, which burn as flames. The residual material, in the form of charcoal, is burnt in a forced air supply to give more heat. The hot combustion gases are sometimes used directly for product drying, but more usually they are passed through a heat exchanger to produce hot air, hot water or steam.

The combustion efficiency depends primarily on good contact between the oxygen in the air and the biomass fuel. The main products of efficient biomass combustion are carbon dioxide and water vapor, however tars, smoke and alkaline ash particles are also emitted. Minimization of these emissions and accommodation of their possible effects are important concerns in the design of environmentally acceptable biomass combustion systems.

Biomass combustion systems, based on a range of furnace designs, can be very efficient at producing hot gases, hot air, hot water or steam, typically recovering 65-90% of the energy contained in the fuel. Lower efficiencies are generally associated with wetter fuels. To cope with a diversity of fuel characteristics and combustion requirements, a number of designs of combustion furnaces or combustors are routinely utilized around the world

Underfeed Stokers

Biomass is fed into the combustion zone from underneath a firing grate. These stoker designs are only suitable for small scale systems up to a nominal boiler capacity of 6 MWth and for biomass fuels with low ash content, such as wood chips and sawdust. High ash content fuels such as bark, straw and cereals need more efficient ash removal systems. Sintered or molten ash particles covering the upper surface of the fuel bed can cause problems in underfeed stokers due to unstable combustion conditions when the fuel and the air are breaking through the ash covered surface.

Grate Stokers

The most common type of biomass boiler is based on a grate to support a bed of fuel and to mix a controlled amount of combustion air, which often enters from beneath the grate. Biomass fuel is added at one end of the grate and is burned in a fuel bed which moves progressively down the grate, either via gravity or with mechanical assistance, to an ash removal system at the other end. In more sophisticated designs this allows the overall combustion process to be separated into its three main activities:

  • Initial fuel drying
  • Ignition and combustion of volatile constituents
  • Burning out of the char.

Grate stokers are well proven and reliable and can tolerate wide variations in fuel quality (i.e. variations in moisture content and particle size) as well as fuels with high ash content. They are also controllable and efficient.

Fluidized Bed Boilers

The basis for a fluidized bed combustion system is a bed of an inert mineral such as sand or limestone through which air is blown from below. The air is pumped through the bed in sufficient volume and at a high enough pressure to entrain the small particles of the bed material so that they behave much like a fluid.

The combustion chamber of a fluidized bed plant is shaped so that above a certain height the air velocity drops below that necessary to entrain the particles. This helps retain the bulk of the entrained bed material towards the bottom of the chamber. Once the bed becomes hot, combustible material introduced into it will burn, generating heat as in a more conventional furnace. The proportion of combustible material such as biomass within the bed is normally only around 5%. The primary driving force for development of fluidized bed combustion is reduced SO2 and NOx emissions from coal combustion.

Bubbling fluidized bed (BFB) combustors are of interest for plants with a nominal boiler capacity greater than 10 MWth. Circulating fluidized bed (CFB) combustors are more suitable for plants larger than 30 MWth. The minimum plant size below which CFB and BFB technologies are not economically competitive is considered to be around 5-10 MWe.