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Ethanol from Lignocellulosic Biomass

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Cellulosic ethanol technology is one of the most commonly discussed second-generation biofuel technologies worldwide. Cellulosic biofuels are derived from the cellulose in plants, some of which are being developed specifically as “energy” crops rather than for food production. These include perennial grasses and trees, such as switchgrass and Miscanthus. Crop residues, in the form of stems and leaves, represent another substantial source of cellulosic biomass.

The largest potential feedstock for ethanol is lignocellulosic biomass, which includes materials such as agricultural residues (corn stover, crop straws, husks and bagasse), herbaceous crops (alfalfa, switchgrass), short rotation woody crops, forestry residues, waste paper and other wastes (municipal and industrial). Bioethanol production from these feedstocks could be an attractive alternative for disposal of these residues. Lignocellulosic feedstocks do not interfere with food security and are important for both rural and urban areas in terms of energy security reason, environmental concern, employment opportunities, agricultural development, foreign exchange saving, socioeconomic issues etc.

Production of Ethanol

The production of ethanol from lignocellulosic biomass can be achieved through two different processing routes. They are:

  • Biochemical – in which enzymes and other micro-organisms are used to convert cellulose and hemicellulose components of the feedstocks to sugars prior to their fermentation to produce ethanol;
  • Thermochemical – where pyrolysis/gasification technologies produce a synthesis gas (CO + H2) from which a wide range of long carbon chain biofuels, such as synthetic diesel or aviation fuel, can be reformed.

Lignocellulosic biomass consists mainly of lignin and the polysaccharides cellulose and hemicellulose. Compared with the production of ethanol from first-generation feedstocks, the use of lignocellulosic biomass is more complicated because the polysaccharides are more stable and the pentose sugars are not readily fermentable by Saccharomyces cerevisiae. In order to convert lignocellulosic biomass to biofuels the polysaccharides must first be hydrolysed, or broken down, into simple sugars using either acid or enzymes. Several biotechnology-based approaches are being used to overcome such problems, including the development of strains of Saccharomyces cerevisiae that can ferment pentose sugars, the use of alternative yeast species that naturally ferment pentose sugars, and the engineering of enzymes that are able to break down cellulose and hemicellulose into simple sugars.

Ethanol from lignocellulosic biomass is produced mainly via biochemical routes. The three major steps involved are pretreatment, enzymatic hydrolysis, and fermentation. Biomass is pretreated to improve the accessibility of enzymes. After pretreatment, biomass undergoes enzymatic hydrolysis for conversion of polysaccharides into monomer sugars, such as glucose and xylose. Subsequently, sugars are fermented to ethanol by the use of different microorganisms.

Pretreated biomass can directly be converted to ethanol by using the process called simultaneous saccharification and cofermentation (SSCF).  Pretreatment is a critical step which enhances the enzymatic hydrolysis of biomass. Basically, it alters the physical and chemical properties of biomass and improves the enzyme access and effectiveness which may also lead to a change in crystallinity and degree of polymerization of cellulose. The internal surface area and pore volume of pretreated biomass are increased which facilitates substantial improvement in accessibility of enzymes. The process also helps in enhancing the rate and yield of monomeric sugars during enzymatic hydrolysis steps.

Pretreatment of Lignocellulosic Biomass

Pretreatment methods can be broadly classified into four groups – physical, chemical, physio-chemical and biological. Physical pretreatment processes employ the mechanical comminution or irradiation processes to change only the physical characteristics of biomass. The physio-chemical process utilizes steam or steam and gases, like SO2 and CO2. The chemical processes employs acids (H2SO4, HCl, organic acids etc) or alkalis (NaOH, Na2CO3, Ca(OH)2, NH3 etc). The acid treatment typically shows the selectivity towards hydrolyzing the hemicelluloses components, whereas alkalis have better selectivity for the lignin. The fractionation of biomass components after such processes help in improving the enzymes accessibility which is also important to the efficient utilization of enzymes.

Conclusions

The major cost components in bioethanol production from lignocellulosic biomass are the pretreatment and the enzymatic hydrolysis steps. In fact, these two process are someway interrelated too where an efficient pretreatment strategy can save substantial enzyme consumption. Pretreatment step can also affect the cost of other operations such as size reduction prior to pretreatment. Therefore, optimization of these two important steps, which collectively contributes about 70% of the total processing cost, are the major challenges in the commercialization of bioethanol from 2nd generation feedstock.

Biomass Energy in Indonesia

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With Indonesia’s recovery from the Asian financial crisis of 1998, energy consumption has grown rapidly in past decade. The priority of the Indonesian energy policy is to reduce oil consumption and to use renewable energy. For power generation, it is important to increase electricity power in order to meet national demand and to change fossil fuel consumption by utilization of biomass wastes. The development of renewable energy is one of priority targets in Indonesia.

It is estimated that Indonesia produces 146.7 million tons of biomass per year, equivalent to about 470 GJ/y. Sources of biomass energy in Indonesia are scattered all over the country, but the biggest potential in concentrated scale can be found in the Island of Kalimantan, Sumatera, Irian Jaya and Sulawesi. Studies estimate the electricity generation potential from the roughly 150 Mt of biomass residues produced per year to be about 50 GW or equivalent to roughly 470 GJ/year. These studies assume that the main source of biomass energy in Indonesia will be rice residues with a technical energy potential of 150 GJ/year. Other potential biomass sources are rubber wood residues (120 GJ/year), sugar mill residues (78 GJ/year), palm oil residues (67 GJ/year), and less than 20 GJ/year in total from plywood and veneer residues, logging residues, sawn timber residues, coconut residues, and other agricultural wastes.

Sustainable and renewable natural resources such as biomass can supply potential raw materials for energy conversion. In Indonesia, they comprise variable-sized wood from forests (i.e. natural forests, plantations and community forests that commonly produce small-diameter logs used as firewood by local people), woody residues from logging and wood industries, oil-palm shell waste from crude palm oil factories, coconut shell wastes from coconut plantations, as well as skimmed coconut oil and straw from rice cultivation.

The major crop residues to be considered for power generation in Indonesia are palm oil sugar processing and rice processing residues. Currently, 67 sugar mills are in operation in Indonesia and eight more are under construction or planned. The mills range in size of milling capacity from less than 1,000 tons of cane per day to 12,000 tons of cane per day. Current sugar processing in Indonesia produces 8 millions MT bagasse and 11.5 millions MT canes top and leaves. There are 39 palm oil plantations and mills currently operating in Indonesia, and at least eight new plantations are under construction. Most palm oil mills generate combined heat and power from fibres and shells, making the operations energy self –efficient. However, the use of palm oil residues can still be optimized in more energy efficient systems.

Other potential source of biomass energy can also come from municipal wastes. The quantity of city or municipal wastes in Indonesia is comparable with other big cities of the world. Most of these wastes are originated from household in the form of organic wastes from the kitchen. At present the wastes are either burned at each household or collected by the municipalities and later to be dumped into a designated dumping ground or landfill. Although the government is providing facilities to collect and clean all these wastes, however, due to the increasing number of populations coupled with inadequate number of waste treatment facilities in addition to inadequate amount of allocated budget for waste management, most of big cities in Indonesia had been suffering from the increasing problem of waste disposals.

The current pressure for cost savings and competitiveness in Indonesia’s most important biomass-based industries, along with the continually growing power demands of the country signal opportunities for increased exploitation of biomass wastes for power generation.

Biomass Energy Potential in Pakistan

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Being an agricultural economy, biomass energy potential in Pakistan is highly promising. Pakistan is experiencing a severe energy crisis these days which is resulting in adverse long term economic and social problems. The electricity and gas shortages have directly impacted the common man, industry and commercial activities.

The high cost of energy mix is the main underlying reason behind the power crisis. The main fuel for the local power industry is natural gas however due to the continued depletion of this source and demands elsewhere the power generation companies are now dependent on furnace oil which is relatively expensive.

The way out of this crisis is to look for fuel sources which are cheap and abundantly available within the country. This description and requirement is fulfilled by biomass resources which have been largely ignored in the past and are also available in sufficient quantities to tackle the energy crisis prevailing in the country.

Biomass Energy in Pakistan

The potential to produce power from biomass resources is very promising in Pakistan. Being an agrarian economy, more than 60% of the population is involved in agricultural activities in the country. As per World Bank statistics, around 26,280,000 hectares of land is under cultivation in Pakistan. The major sources of biomass energy are crop residues, animal manure and municipal solid wastes

Agricultural Residues

Wheat straw, rice husk, rice straw, cane trash, bagasse, cotton sticks are some of the major crop residues in Pakistan. Sugar cane is a major crop in the country and grown on a wide scale throughout Pakistan. During 2010-2011, the area under sugarcane cultivation was 1,029,000 hectares which is 4% of the total cropped area.

Sugarcane trash which constitutes 10% of the sugar cane is currently burned in the fields. During the year 2010-11, around 63,920,000 metric tons of sugarcane was grown in Pakistan which resulted in trash generation of around 5,752,800 metric tons. As per conservation estimates, the bioenergy potential of cane trash is around 9,475 GWh per year.

Cotton is another major cash crop in Pakistan and is the main source of raw material to the local textile industry. Cotton is grown on around 11% of the total cropped area in the country. The major residue from cotton crop is cotton sticks which is he material left after cotton picking and constitute as much as 3 times of the cotton produced.

Majority of the cotton sticks are used as domestic fuel in rural areas so only one-fourth of the total may be considered as biomass energy resource. The production of cotton sticks during 2010-2011 was approximately 1,474,693 metric tons which is equivalent to power generation potential of around 3,071 GWh.

Cotton sticks constitute as much as 3 times of the cotton produced.

Animal Manure

Pakistan is the world’s fourth largest producer of milk. The cattle and dairy population is around 67,294,000 while the animal manure generation is estimated at 368,434,650 metric tons. Biogas generation from animal manure is a very good proposition for Pakistan as the country has the potential to produce electrical energy equivalent to 23,654 GWh

Municipal Solid Waste

The generation or solid wastes in 9 major urban centers is around 7.12 million tons per annum which is increasing by 2.5% per year due to rapid increase in population and high rate of industrialization. The average calorific value of MSW in Pakistan is 6.89 MJ/kg which implies power generation potential of around 13,900 GWh per annum.

Bioenergy in the Middle East

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The Middle East region offers tremendous renewable energy potential in the form of solar, wind and bioenergy which has remained unexplored to a great extent. The major biomass producing Middle East countries are Egypt, Algeria, Yemen, Iraq, Syria and Jordan. Traditionally, biomass energy has been widely used in rural areas for domestic purposes in the Middle East. Since most of the region is arid/semi-arid, the biomass energy potential is mainly contributed by municipal solid wastes, agricultural residues and agro-industrial wastes.

Municipal solid wastes represent the best bioenergy resource in the Middle East. The high rate of population growth, urbanization and economic expansion in the region is not only accelerating consumption rates but also accelerating the generation of municipal waste. Bahrain, Saudi Arabia, UAE, Qatar and Kuwait rank in the top-ten worldwide in terms of per capita solid waste generation. The gross urban waste generation quantity from Middle East countries is estimated at more than 150 million tons annually.

In Middle East countries, huge quantity of sewage sludge is produced on daily basis which presents a serious problem due to its high treatment costs and risk to environment and human health. On an average, the rate of wastewater generation is 80-200 litres per person each day and sewage output is rising by 25 percent every year. According to estimates from the Drainage and Irrigation Department of Dubai Municipality, sewage generation in the Dubai increased from 50,000 m3 per day in 1981 to 400,000 m3 per day in 2006.

The food processing industry in Middle East produces a large number of organic residues and by-products that can be used as source of bioenergy. In recent decades, the fast-growing food and beverage processing industry has remarkably increased in importance in major countries of the Middle East.

Since the early 1990s, the increased agricultural output stimulated an increase in fruit and vegetable canning as well as juice, beverage, and oil processing in countries like Egypt, Syria, Lebanon and Saudi Arabia. There are many technologically-advanced dairy products, bakery and oil processing plants in the region.

date-wastes

Date palm biomass is found in large quantities across the Middle East

Agriculture plays an important role in the economies of most of the countries in the Middle East.  The contribution of the agricultural sector to the overall economy varies significantly among countries in the region, ranging, for example, from about 3.2 percent in Saudi Arabia to 13.4 percent in Egypt. Cotton, dates, olives, wheat are some of the prominent crops in the Middle East

Large quantities of crop residues are produced annually in the region, and are vastly underutilised. Current farming practice is usually to plough these residues back into the soil, or they are burnt, left to decompose, or grazed by cattle. These residues could be processed into liquid fuels or thermochemically processed to produce electricity and heat in rural areas.

Energy crops, such as Jatropha, can be successfully grown in arid regions for biodiesel production. Infact, Jatropha is already grown at limited scale in some Middle East countries and tremendous potential exists for its commercial exploitation.

The Middle Eastern countries have strong animal population. The livestock sector, in particular sheep, goats and camels, plays an important role in the national economy of the Middle East countries. Many millions of live ruminants are imported into the Middle Eastern countries each year from around the world. In addition, the region has witnessed very rapid growth in the poultry sector. The biogas potential of animal manure can be harnessed both at small- and community-scale.

Biogas from Agricultural Wastes

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The main problem with anaerobic digestion of agricultural wastes is that most of the agricultural residues are lignocellulosic with low nitrogen content. To obtain biogas from agricultural wastes, pre-treatment methods like size reduction, electron irradiation, heat treatment, enzymatic action etc are necessary. For optimizing the C/N ratio of agricultural residues, co-digestion with sewage sludge, animal manure or poultry litter is recommended.

Types of Agricultural Wastes

Several organic wastes from plants and animals have been exploited for biogas production as reported in the literature. Plant materials include agricultural crops such as sugar cane, cassava, corn etc, agricultural residues like rice straw, cassava rhizome, corn cobs etc, wood and wood residues (saw dust, pulp wastes, and paper mill waste)

Others include molasses and bagasse from sugar refineries, waste streams such as rice husk from rice mills and residues from palm oil extraction and municipal solid wastes, etc. However, plant materials such as crop residues are more difficult to digest than animal wastes (manures) because of difficulty in achieving hydrolysis of cellulosic and lignocellulosic constituents.

Codigestion of Crop Wastes

Crop residues can be digested either alone or in co-digestion with other materials, employing either wet or dry processes. In the agricultural sector one possible solution to processing crop biomass is co-digested together with animal manures, the largest agricultural waste stream.

In addition to the production of renewable energy, controlled anaerobic digestion of animal manures reduces emissions of greenhouse gases, nitrogen and odour from manure management, and intensifies the recycling of nutrients within agriculture.

In co-digestion of plant material and manures, manures provide buffering capacity and a wide range of nutrients, while the addition of plant material with high carbon content balances the carbon to nitrogen (C/N) ratio of the feedstock, thereby decreasing the risk of ammonia inhibition.

The gas production per digester volume can be increased by operating the digesters at a higher solids concentration. Batch high solids reactors, characterized by lower investment costs than those of continuously fed processes, but with comparable operational costs, are currently applied in the agricultural sector to a limited extent.

Codigestion offers good opportunity to farmers to treat their own waste together with other organic substrates. As a result, farmers can treat their own residues properly and also generate additional revenues by treating and managing organic waste from other sources and by selling and/or using the products viz heat, electrical power and stabilised biofertiliser.

Collection Systems for Agricultural Biomass

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Biomass collection involves operations pertaining to gathering, packaging, and transporting biomass to a nearby site for temporary storage. The amount of biomass resource that can be collected at a given time depends on a variety of factors. In case of agricultural residues, these considerations include the type and sequence of collection operations, the efficiency of collection equipment, tillage and crop management practices, and environmental restrictions, such as the need to control soil erosion, maintain soil productivity, and maintain soil carbon levels.

The most conventional method for collecting biomass is baling which can be either round or square. Some of the important modern biomass collection operations have been discussed below:

Baling

Large square bales are made with tractor pulled balers. A bale accumulator is pulled behind the baler that collects the bales in group of 4 and leaves them on the field. At a later date when available, an automatic bale collector travels through the field and collects the bales.

The automatic bale collector travels to the side of the road and unloads the bales into a stack. If the automatic bale collector is not available bales may be collected using a flat bed truck and a front end bale loader. A loader is needed at the stack yard to unload the truck and stack the bales. The stack is trapped using a forklift and manual labor.

Loafing

When biomass is dry, a loafer picks the biomass from windrow and makes large stacks. The roof of the stacker acts as a press pushing the material down to increase the density of the biomass. Once filled, loafer transports the biomass to storage area and unloads the stack. The top of the stack gets the dome shape of the stacker roof and thus easily sheds water.

Dry Chop

In this system a forage harvester picks up the dry biomass from windrow, chops it into smaller pieces (2.5 – 5.0 cm). The chopped biomass is blown into a forage wagon traveling along side of the forage harvester. Once filled, the forage wagon is pulled to the side of the farm and unloaded. A piler (inclined belt conveyor) is used to pile up the material in the form of a large cone.

Wet Chop

Here a forage harvester picks up the dry or wet biomass from the windrow. The chopped biomass is blown into a forage wagon that travels along side of the harvester. Once filled, the wagon is pulled to a silage pit where biomass is compacted to produce silage.

Whole Crop Harvest

The entire material (grain and biomass) is transferred to a central location where the crop is fractionated into grain and biomass.  The McLeod Harvester developed in Canada fractionates the harvested crop into straw and graff (graff is a mixture of grain and chaff). The straw is left on the field. Grain separation from chaff and other impurities take place in a stationary system at the farmyard.

McLeod Harvester fractionates the harvested crop into straw and graff

For the whole crop baling, the crop is cut and placed in a windrow for field drying. The entire crop is then baled and transported to the processing yard. The bales are unwrapped and fed through a stationary processor that performs all the functions of a normal combine. Subsequently, the straw is re-baled.

Biomass Exchange – Key to Success in Biomass Projects

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Biomass exchange is emerging as a key factor in the progress of biomass energy sector in a particular country. The supply chain management in any biomass project is a big management conundrum. The complexity deepens owing to the large number of stages which encompass the entire biomass value chain. It starts right from biomass resource harvesting and goes on to include biomass collection, processing, storage and eventually its transportation to the point of ultimate utilization.

Owing to the voluminous nature of the resource, its handling becomes a major issue since it requires bigger modes of logistics, employment of a larger number of work-force and a better storage infrastructure, as compared to any other fuel or feedstock. Not only this their lower energy density characteristic, makes it inevitable for the resource to be first processed and then utilized for power generation to make for better economics.

All these problems call for a mechanism to strengthen the value chain. This can be done by considering the following:

  • Assuring a readily available market for the resource providers or the producers
  • Assuring the project developers of a reliable chain and consistent feedstock availability
  • Awareness to the project developer of the resources in closest proximity to the plant site
  • Assurance to the project developer of the resource quality
  • Timely pick-up and drop of resource
  • Proper fuel preparation as per technology requirements
  • Removal of intermediaries involved in the process – to increase value for both, the producers as well as the buyers
  • No need for long term contracts (Not an obligation)
  • Competitive fuel prices
  • Assistance to producers in crop management

The figure below gives a general understanding of how such a model could work, especially in the context of developing nations where the size of land holdings is usually small and the location of resources is scattered, making their procurement a highly uneconomic affair. This model is commonly known as Biomass Exchange

In such a model, the seed, fertilizer shops and other local village level commercial enterprises could be utilized as an outreach or marketing platform for such a service.  Once the producer approves off the initial price estimate, as provided by these agencies, he could send a sample of the feedstock to the pre-deputed warehouses for a quality check.

These warehouses need to be organized at different levels according to the village hierarchy and depending on the size, cultivated area and local logistic options available in that region. On assessing the feedstock sample’s quality, these centers would release a plausible quote to the farmer after approving which, he would be asked to supply the feedstock.

On the other hand, an entity in need of the feedstock would approach the biomass exchange, where it would be appraised of the feedstock available in the region near its utilization point and made aware of the quantity and quality of the feedstock. The entity would then quote a price according to its suitability which would be relayed to the primary producer.

An agreement from both the sides would entail the placement of order and the feedstock’s subsequent processing and transportation to the buyer’s gate. The pricing mechanisms could be numerous ranging from, fixed (according to quality), bid-based or even market-driven.

Roadblocks

The hurdles could be in the form of the initial resource assessment which could in itself be a tedious and time consuming exercise. Another roadblock could be in the form of engaging the resource producers with such a mechanism. Since these would usually involve rural landscapes, things could prove to be a little difficult in terms of implementation of initial capacity building measures and concept marketing.

Benefits

The benefits of  a biomass exchange are enumerated below:

  • Support to the ever increasing power needs of the country
  • Promotion of biomass energy technologies
  • Development of rural infrastructure
  • Increased opportunities for social and micro-entrepreneurship
  • Creation of direct and indirect job opportunities
  • Efficient utilization of biomass wastes
  • Potential of averting millions of tonnes of GHGs emissions

Conclusions

In India alone, there has been several cases where biomass power projects of the scale greater than 5 MW are on sale already, even with their power purchase agreements still in place. Such events necessitate the need to have a mechanism in place which would further seek the promotion of such technologies.

Biomass Exchange is an attractive solution to different problems afflicting biomass projects, at the same time providing the investors and entrepreneurs with a multi-million dollar opportunity. Although such a concept has been in existence in the developed world for a long time now, it has not witnessed many entrepreneurial ventures in developing nations where the need to strengthen the biomass supply chain becomes even more necessary.

However, one needs to be really careful while initiating such a model since it cannot be blindly copied from Western countries owing to entirely different land-ownership patterns, regional socio-political conditions and economic framework. With a strong backup and government support, such an idea could go a long way in strengthening the biomass supply chain, promotion of associated clean energy technologies and in making a significant dent in the present power scenario in the developing world.

Rationale for Biomass Supply Chain

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Biomass resources have been in use for a variety of purposes since ages. The multiple uses of biomass includes usage as a livestock or for meeting domestic and industrial thermal requirements or for the generation of power to fulfill any electrical or mechanical needs. One of the major issues, however, associated with the use of any biomass resources is its supply chain management. The resource being bulky, voluminous and only seasonally available creates serious hurdles in the reliable supply of the feedstock, regardless of its application. The idea is thus to have something which plugs in this gap between the biomass resource availability and its demand.

The Problem

The supply chain management in any biomass based project is nothing less than a big management conundrum. The complexity deepens owing to the large number of stages which encompass the entire biomass value chain. It starts right from the resource harvesting and goes on to include the resource collection, processing, storage and eventually its transportation to the point of ultimate utilization.

Owing to the voluminous nature of the resource, its handling becomes a major issue since it requires bigger modes of logistics, employment of a larger number of work-force and a better storage infrastructure, as compared to any other fuel or feedstock. Not only this their lower energy density characteristic, makes it inevitable for the resource to be first processed and then utilized for power generation to make for better economics.

All these hassles associated with such resources, magnify the issue of their utilization when it comes to their supply chain. The seasonal availability of most of the biomass resources, alternative application options, weather considerations, geographical conditions and numerous other parameters make it difficult for the resource to be made consistently available throughout the year. This results in poor feedstock inputs at the utilization point which ends up generating energy in a highly erratic and unreliable manner.

The Solution

Although most of the problems discussed above, are issues inherently associated with the usage of biomass resources, they can be curtailed to a larger extent by strengthening the most important loophole in such projects – The Biomass Resource Supply Chain.

World over, major emphasis has been laid in researching upon the means to improve the efficiencies of such technologies. However, no significant due diligence has been carried out in fortifying the entire resource chain to assure such plants for a continuous resource supply.

The usual solution to encounter such a problem is to have long term contracts with the resource providers to not only have an assured supply but also guard the project against unrealistic escalations in the fuel costs. Although, this solution has been found to be viable, it becomes difficult to sustain such contracts for longer duration since these resources are also susceptible to numerous externalities which could be in the form of any natural disaster, infection from pests or any other socio-political or geographical disturbances, which eventually lead to an increased burden on the producers.

Biomass Harvesting

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Biomass harvesting and collection is an important step involving gathering and removal of the biomass from field which is dependent on the state of biomass, i.e. grass, woody, or crop residue. The moisture content and the end use of biomass also affect the way biomass is collected. For crop residues, the operations should be organized in sync with the grain harvest as it occupies the centrestage in farming process.

All of other operations such as residue management and collection take place after so-called grain is in the bin. On the other hand, the harvest and collection dedicated crops (grass and woody) can be staged for recovery of the biomass only. In agricultural processing, straw is the stems and leaves of small cereals while chaff is husks and glumes of seed removed during threshing.

Modern combine-harvesters generally deliver straw and chaff together; other threshing equipment separates them. Stover is the field residues of large cereals, such as maize and sorghum. Stubble is the stumps of the reaped crop, left in the field after harvest. Agro-industrial wastes are by-products of the primary processing of crops, including bran, milling offal, press-cakes and molasses. Bran from on-farm husking of cereals and pulses are fed to livestock or foraged directly by backyard fowls.

The proportion of straw, or stover, to grain varies from crop to crop and according to yield level (very low grain yields have a higher proportion of straw) but is usually slightly over half the harvestable biomass. The height of cutting will also affect how much stubble is left in the field: many combine-harvested crops are cut high; crops on small-scale farms where straw is scarce may be cut at ground level by sickle or uprooted by hand.

Modern combine-harvesters generally deliver straw and chaff together

Collection involves operations pertaining to gathering, packaging, and transporting biomass to a nearby site for temporary storage. The amount of a biomass resource that can be collected at a given time depends on a variety of factors. In case of agricultural residues, these considerations include the type and sequence of collection operations, the efficiency of collection equipment, tillage and crop management practices, and environmental restrictions, such as the need to control erosion, maintain soil productivity, and maintain soil carbon levels.

Logistics of a Biopower Plant

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Biomass feedstock logistics encompasses all of the unit operations necessary to move biomass feedstock from the land to the energy plant and to ensure that the delivered feedstock meets the specifications of the conversion process. The packaged biomass can be transported directly from farm or from stacks next to the farm to the processing plant. Biomass may be minimally processed before being shipped to the plant, as in case of biomass supply from the stacks. Generally the biomass is trucked directly from farm to biorefinery if no processing is involved.

Another option is to transfer the biomass to a central location where the material is accumulated and subsequently dispatched to the energy conversion facility. While in depot, the biomass could be pre-processed minimally (ground) or extensively (pelletized). The depot also provides an opportunity to interface with rail transport if that is an available option. The choice of any of the options depends on the economics and cultural practices. For example in irrigated areas, there is always space on the farm (corner of the land) where quantities of biomass can be stacked.  The key components to reduce costs in harvesting, collecting and transportation of biomass can be summarized as:

  • Reduce the number of passes through the field by amalgamating collection operations.
  • Increase the bulk density of biomass
  • Work with minimal moisture content.
  • Granulation/pelletization is the best option, though the existing technology is expensive.
  • Trucking seems to be the most common mode of biomass transportation option but rail and pipeline may become attractive once the capital costs for these transport modes are reduced.

The logistics of transporting, handling and storing the bulky and variable biomass material for delivery to the biopower plant is a key part of the supply chain that is often overlooked by project developers. Whether the biomass comes from forest residues on hill country, straw residues from cereal crops grown on arable land, or the non-edible components of small scale, subsistence farming systems, the relative cost of collection will be considerable. Careful development of a system to minimize machinery use, human effort and energy inputs can have a considerable impact on the cost of the biomass as delivered to the processing plant gate.

The logistics of supplying a biomass power plant with consistent and regular volumes of biomass are complex.

Most of the agricultural biomass resources tend to have a relatively low energy density compared with fossil fuels. This often makes handling, storage and transportation more costly per unit of energy carried. Some crop residues are often not competitive because the biomass resource is dispersed over large areas leading to high collection and transport costs. The costs for long distance haulage of bulky biomass will be minimized if the biomass can be sourced from a location where it is already concentrated, such as sugar mill. It can then be converted in the nearby biomass energy plant to more transportable forms of energy carrier if not to be utilized on-site.

The logistics of supplying a biomass power plant with sufficient volumes of biomass from a number of sources at suitable quality specifications and possibly all year round, are complex. Agricultural residues can be stored on the farm until needed. Then they can be collected and delivered directly to the conversion plant on demand. At times this requires considerable logistics to ensure only a few days of supply are available on-site but that the risk of non-supply at any time is low.

Losses of dry matter, and hence of energy content, commonly occur during the harvest transport and storage process. This can either be from physical losses of the biomass material in the field during the harvest operation or dropping off a truck, or by the reduction of dry matter of biomass material which occurs in storage over time as a result of respiration processes and as the product deteriorates. Dry matter loss is normally reduced over time if the moisture content of the biomass can be lowered or oxygen can be excluded in order to constrain pathological action.

To ensure sufficient and consistent biomass supplies, all agents involved with the production, collection, storage, and transportation of biomass require compensation for their share of costs incurred. In addition, a viable biomass production and distribution system must include producer incentives, encouraging them to sell their post-harvest plant residue.