Along with estimates of minimum energy required by steam explosion pre-treatment of biomass some general problems concerning biomass conversion into chemicals, materials, and fuels are discussed. The energy necessary for processing biomass by steam explosion auto-hydrolysis is compared with the heat content of wood and calculated in terms of the amount of saturated steam consumed per unit mass of the dry content of wood biomass. The fraction of processed biomass available for conversion after steam explosion pre-treatment is presented as function of the amount of steam consumed per unit mass of the dry content of wood.
The estimates based on a simple model of energy flows show the energy required by steam explosion pre-treatment of biomass being within 10% of the heat content of biomass - a realistic amount demonstrating that energy for the process can be supplied from a reasonable proportion of biomass used as the source of energy for steam explosion pre-treatment.
The study concerns sustainable supply of primary energy from biomass and considers the interrelation between the amount of energy captured in biomass by photosynthesis and the total land area under perennial species grown for the purpose. The authors analyse available experimental data statistically relevant to natural growths comprising a large number of individual trees of grey alder (Alnus incana), a well-known fast-growing species broadly spread in Latvia and for centuries being used as firewood. By graphical approximation of the growth-rate data available for growths up to 50 years of age the optimum age for harvesting dependent on the age at which the maximum growth-rate of biomass is reached is shown to be 18 years confirming traditional popular knowledge. With account for long-term sustainable supply of energy under condition of 18-year rotation, the average yield of energy from highest quality sites of the total land area permanently occupied by alder is calculated to be ca. 85 GJ/ha and the required land equivalent - slightly less than 12 ha per TJ of primary energy from photosynthesis.
Potential of Photosynthesis as A Renewable Source of Energy and Materials
Responding to recently published considerations concerning biomass as a renewable substitute for fossil fuels to provide at least part of the necessary total amount of primary energy annually consumed in an economy system the authors estimate capacity of photosynthesis in a case study of the Republic of Latvia (Eastern coast of the Baltic Sea). The calculations are made on the basis of recent inventory data on land use, distribution of forest land between the stands of the main dominating species, and the average level of forest productivity specific to species at felling age. Sustainable annual supply of dry biomass from the present forest area available for economic purposes is estimated being equal to 3.7 million metric tons the energy equivalent of species (aspen and grey alder) traditionally harvested for firewood including logging residues from timber wood comprising ~ 13 thousand GWh, which is equal to ~ 24% of the present annual consumption of primary energy.
Exhausting of world resources, increasing pollution, and climate change are compelling the shift of the world economy from continuous growth to a kind of economy based on integration of technologies into zero emissions production systems. Transition from non-renewable fossil resources to renewable resources provided by solar radiation and the current processes in biosphere is seen in the bio-refinery approach - replacing crude oil refineries by biomass refineries. Biotechnology and nano-technologies are getting accepted as important players along with conventional biomass refinery technologies. Systems design is a significant element in the integration of bio-refinery technologies in clusters. A number of case-studies, steam explosion auto-hydrolysis (SEA) in particular, are reviewed to demonstrate conversion of biomass into value-added chemicals and fuels. Analysis of energy flows is made as part of modelling the SEA processes, the eMergy (energy memory) approach and sustainability indices being applied to assess environmental impacts.
Optimising the Yield of Energy from Biomass by Analytical Models of the Rate of Growth
In the reported study of growth-rates of grey alder (Alnus incana) stands at different quality sites the authors, as a continuation of an earlier study, propose and use analytical models to approximate experimental data of mean annual increments of standing stock. The model equations of growth-rate functions are further used to optimise the cutting age by minimising the total area of stands for sustainable annual supply of biomass. The growth-rate behaviour with the age of natural grey alder stands is described by an exponential function of three parameters defining the initial and the maximum growth-rates, and the age at which the growth-rate maximum is reached. None of the parameters is known from experiment, and they are found by least-square fit of the available experimental mean values appraised at the chosen time intervals into the model. A high correlation between the experimental data and the model function is found. The optimum cutting age of 18 years determined in the earlier study is confirmed. In farmed stands the growth-rate is made to continue increasing at a lower speed, and is well approximated by a linear function, in which case it is shown that the cutting age cannot be optimised with respect to the area minimum existing under the condition of a decreasing growth-rate after passing a maximum. In the case of a constant or slowly growing annual increment the authors suggest considering the ratio between the increment of stock per unit of the total area to the increase in the area. The overall efficiency of using the product of photosynthesis for a 20-year-old grey alder stand is roughly estimated to be 0.3%.
Consumption of wood as a source of energy is discussed with respect to efficiency and restraints to ensure sustainability of the environment on the grounds of a simple analytical model describing dynamics of biomass accumulation in forest stands – a particular case of the well-known empirical Richards’ equation. Amounts of wood harvested under conditions of maximum productivity of forest land are presented in units normalised with respect to the maximum of the mean annual increment and used to determine the limits of CO2-neutrality. The ecological “footprint” defined by the area of growing stands necessary to absorb the excess amount of CO2 annually released from burning biomass is shown to be equal to the land area of a plantation providing sustainable supply of fire-wood.