Utilization of Short Rotation Forestry for Fuelwood from an Effluent Disposal Scheme

Hamish T. Lowe², Ralph E.H. Sims³ and Jim A. Cooper^4
2Graduate Assistant, ³Associate Professor, Massey University, Palmerston North, New Zealand
⁴Chief Engineer, Richmond Ltd, Dannevirke, New Zealand

ABSTRACT

At their Oringi abattoir, Richmond Ltd., Meat Packers and Exporters, have established a 100 ha plantation of short rotation Eucalyptus trees grown in combination with a land treatment scheme for the plant's effluent disposal. It is intended that a cyclic renewable system will be created where the biomass grown to treat the irrigated waste water will be used for boiler fuel, thereby substituting for some of the current coal demand.

This abattoir produces approximately 4,000 m³ of meat processing effluent daily, with high nutrient content. In 1986, problems with an irrigation system on to pasture warranted the need to investigate an alternative means of disposal. Based on a pilot study, the chosen option was a furrow irrigation network irrigating Eucalyptus trees grown on a three year coppice short rotation system at a density of 4,000 stems/ha.

A utilization system to harvest, handle, store, dry, comminute and combust the tree crop is now under development. Options for each component are being evaluated and incorporated into a computer model to identify the optimum cost effective system.

Major limitations in harvesting and drying have been identified. Tree specifications and site conditions currently limit the use of harvesting equipment to manual chainsaw operation which is labor intensive. Mechanized systems are therefore being evaluated.

Field trials to determine drying rates of whole-trees cut throughout the year showed rates were influenced by seasonal variations in temperature, rainfall and sunshine hours. These rates will be used, in conjunction with a knowledge of the seasonal fuel demand of the plant, to determine harvest dates and quantities in order to provide a continuous stream of biomass feed-stock for the boiler.

INTRODUCTION

Richmond Ltd, Meat Packers and Exporters, believe that they have invested in the security of their future operations by establishing a sustainable renewable system of land waste treatment of the plant's effluent combined with the production of fuelwood. The concept of combining land treatment with fuelwood production grown under a short rotation intensive culture forestry regime (SRIC) is novel.

Mechanized forestry techniques for harvesting and processing SRIC are not well established. Consequently many private groups and companies are watching the Richmond project with interest to see how the fuelwood utilization phase of the system can be developed successfully.

Why Richmond established a SRIC plantation

Richmond Ltd. have a sheep abattoir (slaughter house) in central New Zealand with the capacity to process up to 10,500 sheep per day. This results in the production of over 5,000 m3 of nitrogen rich liquid wastes daily which require disposal (Sims and Handford, 1993).

In 1986 problems with a pastoral based border dike effluent disposal scheme resulted in contamination of a nearby river. After this incident the company considered alternative disposal methods. These included increasing the land area irrigated, harvesting the pasture instead of using grazing stock, or planting an alternative crop with a higher rate of nutrient and water removal than grass/clover.

A 6 ha trial plantation of fast growing short rotation coppice trees was established in 1987. It was assumed that this would produce large yields of biomass which if removed on a regular basis would remove a significantly large portion of the nutrients applied in the waste water.

The trial plantation included a range of effluent application rates, tree spacings and irrigation methods. Species planted were Eucalyptus botryoides, E. ovata, E. camaldulensis, Acacia dealbata and A. melanoxylon.

Over the four year period of the trial it was established that the effluent could be successfully treated when irrigated on to SRIC. In 1992, a major irrigation scheme was installed and the first 30 ha of a 90 ha plantation was established using various Eucalyptus species. A rotation length of three years, stocking rate of 4,000 stems/ha and a modified form of furrow irrigation were selected for the full scale system.

UTILIZATION OF BIOMASS

The primary objective of the plantation was to create an efficient sustainable effluent treatment system. This requires maximum nutrient removal on a regular basis (Sims and others, 1992). The other important objective was to maximize the boiler fuel potential of the biomass in order to offset the cost of effluent disposal (Sims and Collins, 1993).

Methods and techniques for efficient harvesting, extraction and conversion of the biomass material from a standing tree to a form suitable for combustion need to be clearly defined for this system. During biomass removal there should be minimal damage to the stools and soil structure (Kerruish, 1978; Hytonen, 1985). This will help to maximize plant regrowth, minimize soil compaction, and consequently help to maintain infiltration rates to provide acceptable effluent treatment. A further issue is how to best air dry the biomass from above 60 percent moisture content (wet basis) at harvest to a more acceptable 20 percent for efficient combustion.

Harvesting and drying have been recognized as having a large influence in determining the success or viability of this system. They are areas that are least developed in current utilization systems.

Harvesting

Immediate and long term effects of stump damage, soil and root compaction, and damage to unharvested trees during harvesting operations should be considered. Heavy machinery with high ground contact pressures should be avoided to minimize soil compaction on the wet soils resulting from frequent irrigation (Aust and others 1993). The condition of the site following harvesting can influence the vigor and productivity of coppice regrowth. Damaged stumps can increase the chances of disease build up. All slash should be removed from the site to avoid creating an environment which could encourage disease and physically impede regrowth. Since the site is being used for effluent treatment, all the harvested biomass should be removed to maximize nutrient removal.

For Salix crops in Europe the Claas harvester, Austoft sugar-cane harvester and others have been used for harvesting this SRIC (IEA, 1993). Other prototype specialist machines such as the Canadian FB7 have been developed for harvesting woody biomass crops. However, none of these machines have been tested under New Zealand conditions. Different crop factors such as tree growth forms and stocking rates, and different terrain characteristics make it difficult to estimate the productivity of these machines for the Eucalyptus crops being grown.

Motor manual felling which is commonly used in New Zealand for felling large forestry trees, is relatively inexpensive, simple and versatile (Vaughan and Shula, 1993). Chainsaws, despite often having a lower productivity rate than more mechanized methods, can be cost competitive depending on the size of the trees, area of the plantation, and labor costs. In the absence of other proven equipment for use with SRIC crops, motor manual felling is the only current option.

A potential problem facing the use of some mechanical harvesting machinery is that they operate over a relatively narrow range of stem diameters. The wide variations in stem diameters, together with the large numbers of small stems particularly from coppice regrowth, create design difficulties, reduce efficiencies and increase operating costs of harvesting machines.

Figure 1. Effect of consecutive harvests on a coppicing stool

The first harvest of a typical New Zealand SRIC system occurs at the end of three years when each single stem has a diameter of 75-200 mm. During successive rotations after the tree has been coppiced, multiple stems are produced which must be cut from the original stump. Over this period the stump width increases from a single narrow stool to a wide and often multi-branched stool . Machinery designed for single stem harvesting is not usually able to handle such forms.

It is possible that pruning young regrowth may reduce the widening of the stump, produce a greater piece size, and allow machinery designers to retain a narrower cutting width. Pruning also increases the ratio of wood to leaf material, but since it incurs an added cost, it is not a practical option.

Extraction

The method of extracting the trees from the plantation will be influenced by the harvesting method. The form of the end product, determined by the use of the harvested material, also affects the extraction and transporting method.

It is desirable to avoid fuelwood material becoming contaminated with soil because of difficulties that are encountered during and after combustion so this must be taken into consideration. Continuous one-pass harvest/chip operations which directly chip the material as it is harvested, eliminate soil contamination. However, these machines are often heavy and may create soil compaction problems which is an important concern for irrigated sites. In addition the opportunity for transpirational drying of whole trees is forsaken.

Drying

Allowing felled trees to transpirationally dry on site for several weeks before being extracted and comminuted is considered to be advantageous, the benefits of reducing the moisture content of the biomass offsetting the double handling required.

Trials at both the Richmond site and Massey University have been undertaken to ascertain whether cut trees with foliage left intact dry relatively faster than would comminuted biomass under New Zealand conditions . Initial tree moisture contents of 60 percent wet basis were reduced to at least 35 percent wet basis in four weeks by leaving the cut trees whole.

Storage

Approximately 1,500 oven dry tonnes of biomass can be harvested annually on a sustainable basis from the Richmond site. The establishment of artificial drying facilities (possibly using waste heat from the boiler stack) to further reduce the moisture content of air dried trees for this volume of material could be expensive and needs further investigation.

The young age of these harvested trees means that the ratio of foliage to woody material is high. The large proportion of leaves provides chipped material with a higher nutrient source than chipped stem wood alone. This, together with a smaller particle size from the chipped leaves could provide ideal conditions for the growth of micro-organisms and rapid temperature build up in storage piles resulting in increased dry matter losses (Jirjis, 1988).

It is believed that transpirational drying is a low cost and relatively fast means of obtaining a significant reduction in moisture content at minimal cost, especially at the scale of utilization envisaged.

SEASONAL HARVESTING

The opportunity to harvest trees throughout the year to match the seasonal boiler fuel requirements is being evaluated. With Eucalyptus it appears possible to harvest throughout the year without seriously jeopardizing coppice regrowth or increasing tree mortality rate. Trials comparing the rate of drying at different times of the year have been carried out.

Provisional results show that moisture losses were very rapid in the first 2 to 4 weeks irrespective of weather conditions. The rate of moisture loss then tapers off to a level which was influenced by weather conditions and time of year.

Figure 2. Indicative drying rates for young Eucalyptus trees harvested throughout the year then left as whole trees to transpirationally dry.

A dynamic computer model is being constructed that will enable the tree moisture content to be predicted at any given time after harvest depending on harvest date. If the moisture content required for combustion, and drying rate of unprocessed whole trees is known, it will be possible to predict the optimum times of harvest and the volumes required to meet boiler fuel demand.

SYSTEM SELECTION

To date an optimal solution to suit the utilization process of coppice Eucalyptus production has not been fully defined. The use of direct harvest/chipping/transport machines has been discounted as transpirational drying would be prevented.

For the scale of operation envisaged at the Richmond plant where approximately 120,000 trees will need to be harvested annually a likely mechanization scenario would be;

• chainsaw felling
• extraction by tractor with a grapple mounted on a trailer
• low cost drying of whole trees on a storage pad
• and - chipping prior to feeding into the boiler.

Due to the unavailability of detailed cost and productivity data, this may not be the optimal long term solution to the problem. If the initial cost, life and productivities were available for particular machines, more accurate assessments could be made. Meanwhile sensitivity analyses will need to be conducted to help identify from the limited data available a system that will be practically acceptable to Richmonds.

CONCLUSION

The Richmond company is satisfied with the original objective of efficiently disposing of the plant's effluent on to short rotation Eucalyptus. The environmental impacts of the outputs from their production system (effluent and CO₂) will therefore be lessened.

The successful establishment of an efficient tree harvesting and processing operation will enable the development of a clean renewable energy resource grown as part of an effluent disposal scheme to be developed.

ACKNOWLEDGEMENTS

Richmond Ltd. and Massey University are thanked for financial support towards the attendance of Mr. Lowe at the IEA Conference.

REFERENCES

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2. Hytonen, J. (1985). Effect of cutting season, felling method and stump height on the sprouting ability of energy willows and some other hardwoods. Metsantutkimuslaitoksen Tiedonatoja, Finland 206: pp 440-57.
3. I.E.A. (1993). Status of short rotation forestry mechanization world wide. IEA/BA Task 1 x Activity 1. Workshop and study tour report. Sweden.
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9. Vaughan, L.; and Shula, R.G. (1993). Options for harvesting and processing firewood. The Firewood Venture; planning, execution, evaluation. FRI Bulletin No 137 pp 68-79. Forest Research Institute, Rotorua, New Zealand.

File posted on March 5, 1996; Date Modified: February 21, 1999