Energy returned on energy invested (EROEI); the case for gasification as a component of an integrated live stock based farming system

T R Preston and Lylian Rodríguez

TOSOLY, AA #48, Socorro, Santander, Colombia
trpreston@mekarn.org   ;   lylianr@utafoundation.org

Abstract

The aim of the present study was to measure the Energy Return on Energy Invested (EROEI) in an integrated farming system in which: (i)  fibrous crop byproducts (sugar cane bagasse and stems from forage trees) are used as feedstock to produce " producer" gas in a down-draft gasifier; and (ii) all high moisture organic wastes from pigs and the farm family are fed into a biodigester to produce biogas. The hypothesis was that the production of a combustible gas from biomass (gasification), when conducted as part of an integrated farming system involving live stock, would have a much higher EROEI, and be more environmentally friendly,  than the production of other biofuels, especially ethanol production from maize and other "edible" carbohydrates.

In the farming system, sugar cane (1.5ha produces 120 tonnes stalks) supplies the energy (sugar cane juice) to feed a constant population of fattening 40 pigs. Forage trees (1 ha planted with mulberry and Tithonia diversifolia) provides the protein (as leaves) for 20 adult goats and progeny. The residual bagasse (18 tonnes DM/year) from the sugar cane and the stems from the forage trees (6 tonnes DM/year) are the feedstock for the gasifier. Annual outputs are 221,760 MW as producer gas and 40,150 MJ as biogas. Daily production of electricity from an IC gas  engine and alternator  is 54.7 Kwh from the producer gas and  8 KWh from the biogas, the total exceeding six-fold  the daily electricity requirements of the farm.

Annual indirect (embedded) energy costs were estimated to be 33,205 MJ with 34% derived from human muscle power and 30% from purchased animal feeds. The output of 261,910 MJ as combustible gas results in an EROEI of 8: 1.

Key words: Biochar, biogas, EROEI, forage trees, goats, mulberry, pigs. producer gas, sugar cane

Introduction

Renewable energy from biomass and EROEI

The technologies proposed for redirecting energy from the sun into energy to replace that in fossil fuels are many.  The alternatives that  are currently practiced commercially (although in most cases with a high degree of Government subsidy) can be divided into processes that depend on: (i)  the products of photosynthesis (eg:: ethanol produced by fermentation of sugars derived from cereal grains, cassava roots and sugar cane; and biodiesel from soya beans, rapeseed and oil palm); or (ii) that use the physical qualities of solar energy directly (photovoltaic panels, solar water heaters, windmills, tidal barrages and wave motion).

Other technologies that are frequently proposed, but not yet commercially viable,  are described by Rapier (2009) as Renewable Fuel Pretenders. Rapier argues that their proponents believe they have a solution but that it will never develop into a feasible technology because the proponents “have no experience at scaling up technologies”. In this category he lists cellulosic ethanol, hydrogen and diesel oil from algae.

Surprisingly, gasification which is a proven technology for using biomass as a source of fuel, and which was applied widely in several "oil-dependent" countries during World War II, has received little attention from policy makers and the media. Yet, as will be shown in this paper, it appears to hold real prospects of being a sustainable technology, especially when it is  applied as a component in an integrated farming system.

Hall et al (2008a, 2009) have proposed that the most appropriate way to judge the relative merits of different energy technologies is by calculating the ratio between the amount of energy produced and the energy needed to produce it, described as the EROEI (Energy Returned on Energy Invested). The EROEI in its simplest form (EOREI_mm) measures the output energy at the point of production. However, to take account of the final form in which the energy is used  to support the needs of society/civilization, they proposed the term EROEI_ext. Their indicative figures were that an EROEImm for oil  of 3:1 would be sufficient to cover the energy cost of extracting the oil and the associated exploration costs for new discoveries. but that at the level of the end user (eg: to cover the needs of society/civilization), the EROEI (EROEI_ext ) would need to be at least 10:1. By contrast, the EROEI_mm for ethanol derived from maize, was estimated to be at best 1.3:1 (Cleveland et al 2006) and according to some authors (Patzek 2004; Patzek and Pimental 2006; Patzek 2007) ), less than 1:1, implying that maize-based ethanol requires a  "fossil" fuel, as well as a financial, subsidy. 

Gasification as part of an integrated farming system

The concept of gasification being a partner in an integrated farming system was first developed by the senior author in 1980 when research was initiated in Mexico to develop feeding systems for pigs based on sugar juice as an alternative energy source to cereal grains. The feeding system was technically successful (Mena et al 1981), but the constraint to its development was “ how to make productive use of the bagasse that accumulated as a waste product of the extraction of the juice from the sugar cane stalks". Memories of classes in chemistry at secondary school brought to mind the process of gasification, developed in the 19^th century, to produce combustible gas from a range of carbon-rich materials such as coal, charcoal and wood. Knowledge of the use of wood and charcoal-fueled gasifiers to propel cars and trucks in Sweden during World War 2, assisted by personal contacts with staff of the International Foundation of Science (located in Stockholm), led to the opportunity to test the use of sugar cane bagasse from Mexico as fuel in a Scania truck equipped with a wood-burning gasifier. The test took place in the city of Umea in Northern Sweden. With the close cooperation of Arne Lindgren, an engineer skilled in biogas technology, the 20 kg of sugar cane bagasse from Mexico successfully fueled the Scania truck for a 20 minute drive around the city.

It required the onset of the escalation of oil prices, twenty-three years later, to revive interest in gasification as a component of a sustainable farming system. 

Fuel energy as a byproduct of livestock production

Three problems associated with use of biomass as fuel are: (i) its low density (eg: from 97 to 350 kg/m³; Miech Phalla and Preston 2005), resulting in high costs of transport if processed in a centralized utility (as with increasing distance, fossil fuel rather than animal power is needed); (ii) the cost when it is the sole product of the cropping system; and (iii) the potential conflict if land presently devoted to food crops is diverted to production of biomass for fuel. These problems do not arise if the biomass is used at the point of production and it is the byproduct of crops that are grown primarily for human food or animal feed. 

Hypothesis

The hypothesis underlying the present study is that the production of a combustible gas from biomass (gasification), when conducted as part of an integrated farming system involving live stock, will have a much higher EROEI, and be more environmentally friendly,  than other biomass-based technologies and especially ethanol production from maize and other "edible" carbohydrates.

Materials and methods

Location

The study was done in the farm “ TOSOLY , located at 1500msl in the Santander department of Colombia, approximately 250 km north of the capital Bogotá.

The gasifier

A 10KW gasifier (Model WBG-10) (Photo 1) was imported from Ankur Scientific Energy Technology Pvt. Ltd, in India. It was connected to a diesel engine modified to operate in 100% producer gas mode with a 230v, 3-phase alternator to give gross output of about 9 kWe The related accessories, and mode of operation, were described by Miech Phalla and Preston (2005) and Rodríguez and Preston (2009). 

The farming system

The farm (Figure 1) extends to 7 ha of which 1.22 ha are in natural forest, 1.44 ha in Arabica coffee grown under shade from “Guamo” (Inga hayesii Benth) trees, 1.5 ha in sugar cane,  0.50 ha in permanent plantations of forage trees (mainly Mulberry [Morus alba] and Tithonia [Tithonia diversifolia] and 0.30 ha in New Cocoyam (Xanthosoma Sagittarius). The remaining 2 ha are accounted for by areas under citrus, bamboo (Guadua), pasture, fish ponds, roads and buildings.

For the purposes of this study the components of the farm devoted to combined feed and fuel production are sugar cane and the forage trees (Figure 2).

Sources of biomass

The whole sugar cane plant is harvested and the tops (leaves + growing point) separated from the stalk, which is then passed twice through a 3-roll crusher(trapiche). The extracted juice is the dietary energy source for pigs (n=40); the “ tops” are chopped prior to feeding them as the energy source for the goats. The bagasse (the fibrous residue after juice extraction) represents from 35 to 40% of the fresh weight of the cane stalk and contains from 55 to 65% moisture. It is sun-dried during 1 to 2 days to a moisture content of about 15%. The large pieces are presently separated and used as litter for the goats; the remaining smaller particles (1 to 3 cm) being stored for use as fuel in the gasifier (Photo 2).

The Mulberry and Tithonia trees are harvested at 6 to 8 week intervals, removing all the fresh biomass after cutting about 50 cm above soil level. The mixed foliages are fed immediately to the goats, that preferably select the Mulberry of which they eat the leaves and the rind that is completely from the stems (Photo 3). 

Photo 2. Fine bagasse on the left for the gasifier; coarse particles are used as litter for goats and cattle but could be chopped and used in the gasifier	Photo 3.  Mulberry foliage on the left (most of the rind on the stems has been eaten); Tithonia on the right

For the Tithonia only the leaves are eaten; the stems are left with the rind untouched (Photo 3). The stems of both trees that are not eaten by the goats (Photo 4) are collected, passed through a high speed (3500rpm) chopper (driven by a 3KW electric motor which receives power from the gasifier-alternator) and sun-dried to15% moisture for later use in the gasifier.

Photo 4.  Residual stems left by the goats; the white stems (without the rind) are from the Mulberry; the green ones with leaves attached are from the Tithonia

Measurement of the EROEI

The coefficients for the indirect energy cost of inputs to the farming system, such as machinery, steel, cement, polyethylene (for the biodigesters) and animal feed, are taken from several sources  (Table 1). All wastes are recycled. Those of organic origin (excreta from pigs and people; washings from coffee processing, and household activities) are the feedstock for "plug-flow" , tubular polyethylene biodigesters (Photo 5),  All agricultural activities are done by oxen (land preparation) or a horse (transport; Photo 6) or by hand labor (planting, weeding and harvesting). No chemicals are used and fertilizer and organic matter are derived from recycled goat and cattle manure, the effluent from the biodigesters and "biochar" from the gasifier (Rodríguez et al (2009). The only purchased feeds for the animals are rice polishings, fish meal and minerals (calcium carbonate, rock phosphate, salt and sulphur).

Photo 5. Tubular polyethylene biodigester charged with pig manure and water	Photo 6. New Cocoyam leaves+petioles transported by  “Mariscal” in TOSOLY farm

The input-output data from the gasifier (Table 2) were taken from the studies of Miech Phalla and Preston (2005) and Rodríguez and Preston (2009). These were extrapolated to represent the inputs from 1.5 ha of sugar cane (annual yield of 80 tonnes stalk cane) and 1 ha of forage trees (annual yield of 6 tonnes/ha of air-dry stems [15% moisture] and outputs when the bagasse and tree stems were used as feedstock in the gasifier.

EROEI (Energy Return on Energy Invested)

The coefficients used in the calculation of the EROEI are in Table 2. The calculation of the EROEI is in Table 3. If the energy output of the gasifier is based on the calorific value of the producer gas then the EROEI is 63. On the other hand, if the output is measured at the point of usage (eg: as electricity) then the EROEI decreases to 15.

The major components in the fossil energy inputs is the soybean meal. This will eventually be replaced by New Cocoyam silage and yeast-enriched sugar cane juice, produced on the farm.

It is understood that there are some energy costs not accounted for. Apart from the fish meal and rice bran used to supplement the sugar cane juice, fed to the pigs, the animals on the farm are fed almost exclusively on the `products of photosynthesis. The farm workers mostly consume food grown on the farm or in the immediate rural area. There will be additional energy costs in the (small) proportion of the food transported into the area and in the services used by the people providing manual labor (eg: their use of grid electricity, health services, road maintenance and other services).

Results

The combined activities of gasification and biodigestion using 24 tonnes of fibrous byproducts (bagassse + trees stems) from 1.5 ha of sugarcane and 1 ha of forage trees, and the excreta from 40 pigs and a family of two adults and one adolescent, yield a total of 261,910 MJ/year (Table 3; Figure 3). The inputs have an inbuilt energy cost as fossil fuel of 33,205 MJ/year (Figure 4). The resultant EROEI  is 7.9.

Figure 3. Outputs of combustible gas energy (% of total MJ)according to the source	Figure 4. Inputs of inbuilt fossil fuel energy (% of total MJ) according to the source

Discussion

On-farm energy production and EROEI

The results of the study indicate that gasification of fibrous crop residues (from 1.5 ha of sugar cane and 1 ha of forage trees) together with anaerobic biodigestion of excreta from 40 fattening pigs and a family of three persons, can deliver annually 256 thousand MJ of  combustible gas (equivalent to 6.4 tonnes of oil) - with an EROEI of about 8. This is much higher than has been reported for other biofuels derived from biomass (see Hall et al 2009). .

The normal consumption of electricity on the farm is of the order of 5 to 10 KWh daily. The daily consumption of sugar cane juice for the pig unit (n=40) is of the order of 200 litres daily which requires the crushing of 330 kg of cane stalks daily. With an annual yield of 80 tonnes stalks/ha, the requirement is for 120 tonnes of sugar cane stalks to be produced from 1.5ha. The daily production of bagasse is therefore 50 kg (DM basis) sufficient to produce about 41 KWh of electricity daily. From the 1 ha of forage trees needed for the goat unit, the yield of dry stems is estimated at 6000 kg/ha/year, producing a further 13.7 KWh.  - a total of 54.7 KWh per day. This would provide a surplus at the farm of the order of 45 to 50 KWh daily that could be fed into the regional electricity grid or used directly for activities in the local community such as (in the future!) the charging of the batteries of electric vehicles. The gas produced from the biodigesters (about 5 m³ daily) is surplus to the needs for cooking and perhaps 50% could be used for electricity generation which would generate a further 4 KWh of electricity daily.

The EROEI of 8  is more than twice the EROEI (3:1) for oil and six times that for maize-based ethanol (1.3:1) according to data from Hall et al (2008b).  

The major part of the energy inputs not derived from solar energy relate to human muscle power and the purchase of animal feed (Figure 4). Production of soybean meal has a relatively high inbuilt energy cost (5.6MJ/kg). It is planned to replace this protein-rich feed by increasing the area for growing New Cocoyam and producing a high protein supplement on the farm by artisan production of fodder yeast from the cane juice. Replacing the imported animal feed by locally produced alternatives would raise the EROEI to 11. The two people working on the farm will have some embedded fossil fuel attached to their contribution as muscle power. In their calculation of the energy costs of ethanol from maize, Pimental and Patzek (2005) assumed a figure of 8000 litres oil per person in USA working 2000 hours per year. This is the equivalent of 52 barrels of oil per person per year!!. It is suggested that the embodied oil cost of a farm worker in rural Colombia is closer to 1 barrel of oil/year, similar to the average for China. Farm workers in rural Colombia do not have a car, they walk to work, rarely take vacations, consume mostly what is grown locally and have limited access to public services (eg: access roads are unpaved, infrequent or no garbage collection, septic tanks for sewage....). There is an obvious need for detailed analysis of this component, which will be very "location-specific".

The next highest source of indirect energy costs is incurred in the manufacture of the machinery which accounts for 20% of the total fossil fuel inputs. The steel, which is the main component in the machinery being used on the farm, can be recycled at the end of the working life, and as such will have a relatively energy over "new"  as the energy cost of the steel is mostly incurred  in the mining and processing of the ore. Making an allowance for this component would further raise the EROEI.

Carbon sequestration and soil fertility - other benefits from gasification

The gasification of fibrous biomass produces a carbon-mineral residue known as "biochar". The quantities produced appear to depend on the nature of the biomass being gasified and the operating conditions of the gasifier. In the specific case of sugar cane bagasse and tree stems in the gasifier in TOSOLY, the production of biochar was recorded as 8.5 and 11.7% of the input of bagasse and tree stems, respectively (Rodríguez Lylian, 2009, unpublished data). Converting this to a dry matter basis raises the yield to 10 and 14%, which is similar to the values reported by Miech Phalla amd Preston (2005) for a range of fibrous crop residues/byproducts processed in the same type of gasifier.

Biochar has been shown to be an excellent conditioner for the acid soils that predominate in the humid tropics (Rodríguez et al 2009). The growth rate of maize in acid soils from the TOSOLY farm was increased five-fold by application of the equivalent of 50 tonnes/ha of biochar derived from bagasse. It is also expected that most of the carbon in the biochar will be permanently sequestered when incorporated in the soil (Lehmann 2007). From the 50 kg of bagasse derived daily from 330 kg/day of sugar cane stalks and the 16 kg of stem DM from the 130 kg of tree foliage, the daily production of biochar will be 6.6 kg of which 4.4 kg will be carbon. In one year this is 1.6 tonnes of carbon (6 tonnes of CO₂) sequestered annually (3 tonnes CO₂/ha/year). At the same time, each year,  the biochar will act as soil conditioner sufficient to ameliorate 0.24 ha of crop area, assuming an application rate of 20 tonnes/ha (Rodriguez Lylian and Preston T R, Unpublished data). Thus in 6 years, the whole of the sugar cane and tree foliage area could be treated with biochar. The benefits in terms of reduced fertilizer needs have yet to be quantified but appear to be considerable (see Rodriguez et al 2009).

Conclusions

• The EROEI of 8 for the production of a combustible gas from sugar cane bagasse and tree stems appears to be considerably higher than has been reported for other technologies for deriving biofuels from biomass.
  
  
• There are other associated benefits from the technology such as the production of biochar as a soil ameliorator and means of sequestering carbon.
  
  
• The favourable EROEI of the system reflects the minimum use of external sources of energy, which is made possible by the integrated, mixed farming strategy in which most of the required inputs are produced on the farm.

Acknowledgements

The advice and suggestions received from Charles A S Hall and David Pimental are gratefully acknowledged.
 

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Received 4 September 2009; Accepted 29 September 2009; Published 1 November 2009

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