481 research outputs found
An analysis of the environmental pressure exerted by the eucalyptus-based Kraft pulp industry in Thailand
The study reported here focuses on the environmental pressure exerted by large-scale eucalyptus-based kraft pulp industry in Thailand. The objective of this study was to identify the most important sources of greenhouse gases, acidifying and eutrophying compounds and tropospheric ozone precursors, human toxicity compounds and solid waste associated with the kraft pulp industry. To this end, we performed an environmental systems analysis of the kraft pulp industry system in which we distinguished between two subsystems: the eucalyptus forestry subsystem and the kraft pulp production subsystem. The results indicate that the environmental pressure is caused by the kraft pulp production subsystem rather than by the eucalyptus forestry one. The chemical recovery unit was found to be the most important source of carbon dioxide (CO2) and sulfur dioxide (SO2) and responsible for more than one-half of the emissions of greenhouse gases and acidifying compounds from eucalyptus-based kraft pulp production in Thailand. Biomass combustion in the energy gene ration unit is an important source of nitrogen oxide (NOx) and carbon monoxide (CO) which in turn are responsible for over 50% of the emissions of tropospheric ozone precursors. About 73% of the eutrophication is caused by biological aerobic wastewater treatment emitting phosphorus (P). With respect to the eucalyptus forestry, only fertilizer use in eucalyptus plantations is a relevant source of pollution through the emission of nitrous oxide (N2O) and phosphate (PO4 3-)
Niche emergence as an autocatalytic process in the evolution of ecosystems
The utilisation of the ecospace and the change in diversity through time has been suggested to be due to the effect of niche partitioning, as a global long-term pattern in the fossil record. However, niche partitioning, as a way to coexist, could be a limited means to share the environmental resources and condition during evolutionary time. In fact, a physical limit impedes a high partitioning without a high restriction of the niche's variables. Here, we propose that niche emergence, rather than niche partitioning, is what mostly drives ecological diversity. In particular, we view ecosystems in terms of autocatalytic sets: catalytically closed and self-sustaining reaction (or interaction) networks. We provide some examples of such ecological autocatalytic networks, how this can give rise to an expanding process of niche emergence (both in time and space), and how these networks have evolved over time (so-called evoRAFs). Furthermore, we use the autocatalytic set formalism to show that it can be expected to observe a power-law in the size distribution of extinction events in ecosystems. In short, we elaborate on our earlier argument that new species create new niches, and that biodiversity is therefore an autocatalytic process
Measure-valued differentiation for stationary Markov chains
http://staff.feweb.vu.nl/bheidergot
On RAF Sets and Autocatalytic Cycles in Random Reaction Networks
The emergence of autocatalytic sets of molecules seems to have played an
important role in the origin of life context. Although the possibility to
reproduce this emergence in laboratory has received considerable attention,
this is still far from being achieved. In order to unravel some key properties
enabling the emergence of structures potentially able to sustain their own
existence and growth, in this work we investigate the probability to observe
them in ensembles of random catalytic reaction networks characterized by
different structural properties. From the point of view of network topology, an
autocatalytic set have been defined either in term of strongly connected
components (SCCs) or as reflexively autocatalytic and food-generated sets
(RAFs). We observe that the average level of catalysis differently affects the
probability to observe a SCC or a RAF, highlighting the existence of a region
where the former can be observed, whereas the latter cannot. This parameter
also affects the composition of the RAF, which can be further characterized
into linear structures, autocatalysis or SCCs. Interestingly, we show that the
different network topology (uniform as opposed to power-law catalysis systems)
does not have a significantly divergent impact on SCCs and RAFs appearance,
whereas the proportion between cleavages and condensations seems instead to
play a role. A major factor that limits the probability of RAF appearance and
that may explain some of the difficulties encountered in laboratory seems to be
the presence of molecules which can accumulate without being substrate or
catalyst of any reaction.Comment: pp 113-12
Studying Parallel Evolutionary Algorithms: The cellular Programming Case
Parallel evolutionary algorithms, studied to some extent over the past few years, have proven empirically worthwhile—though there seems to be lacking a better understanding of their workings. In this paper we concentrate on cellular (fine-grained) models, presenting a number of statistical measures, both at the genotypic and phenotypic levels. We demonstrate the application and utility of these measures on a specific example, that of the cellular programming evolutionary algorithm, when used to evolve solutions to a hard problem in the cellular-automata domain, known as synchronization
Phase transition and landscape statistics of the number partitioning problem
The phase transition in the number partitioning problem (NPP), i.e., the
transition from a region in the space of control parameters in which almost all
instances have many solutions to a region in which almost all instances have no
solution, is investigated by examining the energy landscape of this classic
optimization problem. This is achieved by coding the information about the
minimum energy paths connecting pairs of minima into a tree structure, termed a
barrier tree, the leaves and internal nodes of which represent, respectively,
the minima and the lowest energy saddles connecting those minima. Here we apply
several measures of shape (balance and symmetry) as well as of branch lengths
(barrier heights) to the barrier trees that result from the landscape of the
NPP, aiming at identifying traces of the easy/hard transition. We find that it
is not possible to tell the easy regime from the hard one by visual inspection
of the trees or by measuring the barrier heights. Only the {\it difficulty}
measure, given by the maximum value of the ratio between the barrier height and
the energy surplus of local minima, succeeded in detecting traces of the phase
transition in the tree. In adddition, we show that the barrier trees associated
with the NPP are very similar to random trees, contrasting dramatically with
trees associated with the spin-glass and random energy models. We also
examine critically a recent conjecture on the equivalence between the NPP and a
truncated random energy model
Blackwell-Optimal Strategies in Priority Mean-Payoff Games
We examine perfect information stochastic mean-payoff games - a class of
games containing as special sub-classes the usual mean-payoff games and parity
games. We show that deterministic memoryless strategies that are optimal for
discounted games with state-dependent discount factors close to 1 are optimal
for priority mean-payoff games establishing a strong link between these two
classes
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