Towards Sustainability: An Optimisation Framework for the Australian Hardwood Plantation Mid-Thinning Management Using Life Cycle Approach
Embargoed until: 2019-02-05
Hanandeh, Ali El
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Australian hardwood plantations cover more than 1 million hectare, which is almost half of the total national plantations area. Hardwood plantations are commonly slow to mature, with a long rotation time potentially reaching 35 years. Pruning and thinning processes are required during the early stage of the plantation to produce high-quality logs for commercial purpose. Approximately, 50% of the trees are typically cut from the third year during the first thinning, with further 30% removed during the second thinning (10 to 15 years). It is estimated that more than 2 million cubic metres of logs are generated annually during the plantation mid-rotation thinning operation in Australia. However, these logs are considered as low commercial value product because of high defect ratio and poor mechanical quality. In order to ensure the continued expansion of the Australian hardwood plantation sector, higher value products need to be developed to maximise the utility of the available resources and to reduce wastage. The use of the low commercial value thinned logs may offer an opportunity to improve the environmental and economic performance of the hardwood forestry sector. Therefore, this study focused on the potential utilisation pathways for logs produced during the second thinning operation. The study used the case of South-east Queensland to demonstrate the framework, and it was conducted following the standardised methods for Life Cycle Assessment (LCA) (ISO14040:2006) and Life Cycle Costing (LCC) (AS/NZ4536:1999 R2014). To provide an equitable comparison among the different alternatives assessed in this study, the functional unit was defined as the treatment of 1 Mg of green timber logs from the second thinning at the plantation floor. Both the LCA and LCC analysis were conducted based on a 60-year timeframe. The OpenLCA 1.4.1 and SimaPro v.220.127.116.11 software were used in the simulation. Primary data were used whenever possible to calculate the life cycle inventory. Eco-invent 3.3 and AusLCI databases were used to model the background processes and in cases when the primary data were missing. The lifecycle impact assessment was processed followed the best practice guide for conducting LCA studies in Australia and included five different environmental impact categories: Global Warming (GWP); Eutrophication (EP); Acidification (AP); Fossil depletion (FDP) and Human Toxicity (HTP). Excel spreadsheet was used to conduct the LCC analysis and calculate the present value of the future costs. A constrained stepwise, multi-objective linear programming (LP) model was then constructed to identify the optimal solution for the plantation thinned logs utilisation. The LP problem was solved using LINDO software package over a 60-year period. The time intervals used was ten years. The constraints of the model were classified into three groups: the total feedstocks availability; final product requirements; and environmental and economic targets. A number of pathways may be followed to valorise the thinning logs including the production of engineered wood; solid fuel and biomass to liquid fuels (BTL). This study examined several products under each utilisation pathway in an attempt to identify the optimal combination to maximise the environmental and economic benefits from a lifecycle perspective. Under the engineered wood options, two possible utilisations were assessed: (1) veneer based composite (VBC) utility poles and (2) structural building frame using laminated veneer lumber (LVL). The LCA results indicated that engineered wood products manufactured from the second thinning could offer tangible environmental benefits compared to the conventional construction materials such as concrete and steel. The study highlighted that resin consumptions during engineered wood manufacturing, and use of preventatives for wood treatment were the major contributors to the environmental impacts of engineered wood. Furthermore, for the end of life treatment, incineration with energy recovery rather than landfilling was identified as the most favourable waste management option to reduce the environmental impacts. Additionally, using LVL in multi-level building structural frame presents greater environmental benefits than the VBC utility poles case. The longer durability and higher material efficiency of the LVL option compared to the VBC option contributed significantly to the better environmental performance. Under the energy utilisation pathway, six options were assessed: woodchip gasification in combined heat and power plant (WCG); wood pellets gasification in a combined heat and power plant (WPG); wood pellet combustion for domestic water and space heating (WPC); pyrolysis for power generation (PyEl); pyrolysis with bio-oil upgrading to transportation fuels (PyLT) and ethanol production for transportation fuel mix (EthP). All the bioenergy conversion options had noticeable environmental benefits, particularly on the GWP impact. The WCG option was identified as the best performer followed by the WPG and then WPC. The carbon offsets due to the displaced fossil fuel from electricity and heat generation were the main reason for lowering the GWP impact of these options. Although wood pellets have higher energy density than woodchips, they required additional manufacturing processes, extra energy consumption and additional transportation leading to higher environmental impacts which could not be offset by the improved energy density. The study highlighted that use of the biomass as solid fuel with the least processing requirement had higher environmental benefits than BTL options. Lastly, the environmental benefits gained are dependent on the energy being displaced by the final product. From a life-cycle cost perspective, the results showed that per functional unit, utilising thinned logs to produce engineered wood products was more likely to result in higher LCC than energy production. Overall, the VBC utility pole option had the highest LCC followed by the LVL building frame option. The shorter lifespan and lower material efficiency of the VBC compared to LVL option was a significant contributor to the higher LCC of the VBC option. In regard to the energy conversion, the WCG had the least LCC per functional unit among all options, followed by the EthP and PyEl options. However, it is important to note the shortcoming of the functional unit considered here was based on input (treatment of 1 Mg of thinned logs). To overcome this limitation, the output should be considered. For the energy pathway, the levelised cost per megajoule energy was calculated, which showed that the WCG had the best performance followed by the WPG and then WPC. This is in line with the LCA results. Nevertheless, the energy output does not offer a valid comparison for the engineered wood pathway. Therefore, the costs of the displaced products by the output were considered, in line with the LCA analysis, to provide a more equitable comparison. The concept of ‘substituted values’ (SV) was then introduced in this study to overcome this limitation. The ‘SV’ borrows the concept of ‘displaced emissions’ from the LCA. The SV was calculated as the difference between the LCC of the final product (alternative) and the displaced (substituted) product. When the substituted materials were considered, the engineered wood products presented the highest economic savings while the energy options continued to pose a cost. Nonetheless, the WCG option continued to have the best economic performance among the energy options. The LCA and LCC analysis showed conflicting results regarding the best option to follow in order to maximise the economic and environmental utility of the plantation thinning. Based on equal weighting of the economic and environmental objectives, the multi-objective optimisation (MOO) program solution indicated that the LVL and WCG were the dominating options. However, the percentage allocated to each option varied in different periods. For the first 30 years, the solution favoured energy production with 85% allocation to the WCG in the first 10 years period. Nevertheless, the percentage allocated to energy production declined progressively over the subsequent periods. The LVL option became more dominant starting from the 30th year of the simulation. The share allocated to the LVL option reached 100% of the biomass during the last 10 years period. However, when the environmental weight exceeded 70%, energy production (particularly the WCG option) became the dominant solution while the LVL option became less favourable with only 2% allocation of the biomass. On the other hand, the VBC option did not feature in the solution until the weight assigned to the economic objective exceeded 80%. This research study is relevant nationally and internationally as it presents a novel method to integrate LCA with LCC analysis in a multi-objective optimisation framework to identify the optimal utilisation of forestry products including multiple utilisation pathways. This study also presents a novel method to simplify the complex optimisation problem by converting the MOO question to a single objective optimisation (SOO) problem. The framework introduced three novelties: (a) introduce normalisation factors for better representation of the Australian situation; (b) incorporate the potential economic credits from alternative substitutions into the LCC; this is in line with the accepted accounting methods of LCA to allow credits for offset emissions, and (c) formulate and solve the problem as a stepwise constraint LP to avoid over/under allocation issues resulting from averaging over the timescale. Although the study focused on the South-east Queensland case, the developed method can also be applied in different industry sectors and locations. The outcomes of this research have major implications on Australian forestry sector, particularly on hardwood plantation management. The developed optimisation management strategy can enhance the current economic profitability of the forestry sector by developing new markets for the low value thinned logs from timber plantation while increasing the utilisation rate of wood waste hence to satisfy the global rising timber demand and accumulating global carbon. The results of the study are also relevant to other timber residues and low-grade products from the softwood and pulp-wood plantations. In addition, results of this study can be potentially used by decision-makers to make informed choices to lower the environmental impacts and lifecycle cost of utility infrastructures systems and buildings. Furthermore, this study has implications for the bio-energy generation sector. This study confirms the feasibility of using forestry residue as feedstock to substitute fossil fuel energy while mitigating negative environmental impacts and achieving sustainable development strategy. Globally, this study is a small yet significant contribution towards the achievement of the United Nations Sustainable Development Goals, specifically under the Affordable and Clean Energy; Climate Action and Responsible Consumption and Production.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Eng & Built Enviroment
The author owns the copyright in this thesis, unless stated otherwise.
Australian Hardwood Plantation
Life Cycle Approach