Valerio Cozzani

Heat of wood pyrolysis

The heat of pyrolysis of beech and spruce wood was investigated by means of a differential scanning calorimeter. Wide variations were found for the heat of the primary pyrolysis process, depending on the initial sample weight and on the conditions used in the measurements. However, reporting the heat of the primary pyrolysis process versus the final char yield resulted in a linear correlation. This strong dependency of the heat of wood pyrolysis on the final char yield, that is in turn highly sensitive to the experimental conditions, can explain the uncertainty of the data for the heat of wood pyrolysis reported in the literature. A possible explanation for the variability of the heat of wood pyrolysis depending on the final char yield seems to be an exothermic primary char formation process competing with an endothermic volatile formation process.

Devolatilization and pyrolysis of refuse derived fuels : characterization and kinetic modelling by a thermogravimetric and calorimetric approach

Abstract A characterization of pyrolysis behaviour of different refuse derived fuels (RDFs) under heating rates typical of conventional pyrolysis processes is presented. The results of thermogravimetric analysis (TGA) and differential scanning calorimetry (d.s.c.) on different RDFs, and on some materials which have been considered ‘key components’ towards thermal degradation characteristics of RDFs, are reported. The RDF weight loss curve presents two distinct weight loss steps attributable, respectively, to cellulosic materials and plastic degradation. Samples from different plants and different municipal solid waste (MSW) feedstocks show the same qualitative behaviour. In interpretation of the experimental results, the assumption has been made that the pyrolysis rate of thermal degradation of RDF can be considered as the sum of the rates of the main RDF components: paper (cellulose), wood-like materials (cellulose, lignin and hemicellulose), plastics mainly polyethylene (PE); and that each component contributed to the formation of this sum to an extent proportional to its contribution to the composition of the RDF. On the basis of these data and by means of a simple mathematical approach a method is proposed which provides a simple tool for RDF characterization and, in particular, evaluation of plastic content. A kinetic model has been developed, based on the assumption that the RDF degradation rate is the weighed sum of the rates of primary reacting species: cellulose, lignin, hemicellulose, PE. Simplified kinetic pathways were used for the description of the degradation processes of RDF components. The model allows quantitative prediction of RDF weight loss and char yield at least at the heating rates used in present work.

Kinetic modeling of polyethylene and polypropylene thermal degradation

Abstract The actual interest towards thermal degradation of plastics lies in the possibility not only of recovering energy but also of producing useful chemicals. This paper presents a mechanistic kinetic model able to describe the radical chain pyrolysis reactions taking place in the liquid phase. The elementary reaction steps are analyzed and their kinetic parameters are proposed starting from the well known analogous gas phase reactions. On the basis of a very limited number of independent kinetic parameters it is then possible to properly describe this degradation process. The simplifying hypotheses needed to describe this decomposition process in a closed form are carefully discussed. Unfortunately, the enhancing effect of the intermediate formation of unsaturated species forces the use of a numerical approach. As a consequence, the simulation of the pyrolysis process requires the integration of a large system of ordinary differential equations. A few examples of comparisons between model predictions and experimental data confirm the validity of the proposed mechanism for polyethylene and polypropylene degradation. These data refer to very low pressure decomposition experiments as well as to differential thermal analysis at atmospheric pressure. The effect of the molecular weight of the polymer is also discussed.

Domino Effect Analysis Using Bayesian Networks

A new methodology is introduced based on Bayesian network both to model domino effect propagation patterns and to estimate the domino effect probability at different levels. The flexible structure and the unique modeling techniques offered by Bayesian network make it possible to analyze domino effects through a probabilistic framework, considering synergistic effects, noisy probabilities, and common cause failures. Further, the uncertainties and the complex interactions among the domino effect components are captured using Bayesian network. The probabilities of events are updated in the light of new information, and the most probable path of the domino effect is determined on the basis of the new data gathered. This study shows how probability updating helps to update the domino effect model either qualitatively or quantitatively. The methodology is applied to a hypothetical example and also to an earlier-studied case study. These examples accentuate the effectiveness of Bayesian network in modeling domino effects in processing facility.

Industrial accidents triggered by flood events: analysis of past accidents.

Industrial accidents triggered by natural events (NaTech accidents) are a significant category of industrial accidents. Several specific elements that characterize NaTech events still need to be investigated. In particular, the damage mode of equipment and the specific final scenarios that may take place in NaTech accidents are key elements for the assessment of hazard and risk due to these events. In the present study, data on 272 NaTech events triggered by floods were retrieved from some of the major industrial accident databases. Data on final scenarios highlighted the presence of specific events, as those due to substances reacting with water, and the importance of scenarios involving consequences for the environment. This is mainly due to the contamination of floodwater with the hazardous substances released. The analysis of process equipment damage modes allowed the identification of the expected release extents due to different water impact types during floods. The results obtained were used to generate substance-specific event trees for the quantitative assessment of the consequences of accidents triggered by floods.

The development of an inherent safety approach to the prevention of domino accidents

The severity of industrial accidents in which a domino effect takes place is well known in the chemical and process industry. The application of an inherent safety approach for the prevention of escalation events leading to domino accidents was explored in the present study. Reference primary scenarios were analyzed and escalation vectors were defined. Inherent safety distances were defined and proposed as a metric to express the intensity of the escalation vectors. Simple rules of thumb were presented for a preliminary screening of these distances. Swift reference indices for layout screening with respect to escalation hazard were also defined. Two case studies derived from existing layouts of oil refineries were selected to understand the potentialities coming from the application in the methodology. The results evidenced that the approach allows a first comparative assessment of the actual domino hazard in a layout, and the identification of critical primary units with respect to escalation events. The methodology developed also represents a useful screening tool to identify were to dedicate major efforts in the design of add-on measures, optimizing conventional passive and active measures for the prevention of severe domino accidents.

Development of a framework for the risk assessment of Na-Tech accidental events

Abstract Natural events impacting on chemical and process plants may cause severe accidents, triggering the release of relevant quantities of hazardous substances. The present study focused on the development of the tools needed to build up a general framework allowing the extension of quantitative risk assessment procedure to include the analysis of the industrial accidents caused by natural events. Specific methods and models were developed to allow the quantitative assessment of risk caused by two categories of “Na-Tech” accidents: accidents triggered by earthquakes and accidents triggered by floods. The approach allows the identification of the different damage modes expected for process equipment and of the accidental scenarios that may be triggered. The damage models developed allow the calculation of the damage probability of equipment items due to the natural events. A specific methodology was issued to take into account the consequences of the possible contemporary failure of several process units due to the impact of the natural event. The procedure allows the calculation of the overall individual and societal risk indexes including the multiple-failure scenarios caused by the impact of natural events. The overall methodology was applied to the analysis of specific case studies.

Extending the Quantitative Assessment of Industrial Risks to Earthquake Effects

In the general framework of quantitative methods for natural-technological (NaTech) risk analysis, a specific methodology was developed for assessing risks caused by hazardous substances released due to earthquakes. The contribution of accidental scenarios initiated by seismic events to the overall industrial risk was assessed in three case studies derived from the actual plant layout of existing oil refineries. Several specific vulnerability models for different equipment classes were compared and assessed. The effect of differing structural resistances for process equipment on the final risk results was also investigated. The main factors influencing the final risk values resulted from the models for equipment vulnerability and the assumptions for the reference damage states of the process equipment. The analysis of case studies showed that in seismic zones the additional risk deriving from damage caused by earthquakes may be up to more than one order of magnitude higher than that associated to internal failure causes. Critical equipment was determined to be mainly pressurized tanks, even though atmospheric tanks were more vulnerable to containment loss. Failure of minor process equipment having a limited hold-up of hazardous substances (such as pumps) was shown to have limited influence on the final values of the risk increase caused by earthquakes.

Lessons Learned from Toulouse and Buncefield Disasters: From Risk Analysis Failures to the Identification of Atypical Scenarios Through a Better Knowledge Management

The recent occurrence of severe major accidents has brought to light flaws and limitations of hazard identification (HAZID) processes performed for safety reports, as in the accidents at Toulouse (France) and Buncefield (UK), where the accident scenarios that occurred were not captured by HAZID techniques. This study focuses on this type of atypical accident scenario deviating from normal expectations. The main purpose is to analyze the examples of atypical accidents mentioned and to attempt to identify them through the application of a well-known methodology such as the bow-tie analysis. To these aims, the concept of atypical event is accurately defined. Early warnings, causes, consequences, and occurrence mechanisms of the specific events are widely studied and general failures of risk assessment, management, and governance isolated. These activities contribute to outline a set of targeted recommendations, addressing transversal common deficiencies and also demonstrating how a better management of knowledge from the study of past events can support future risk assessment processes in the identification of atypical accident scenarios. Thus, a new methodology is not suggested; rather, a specific approach coordinating a more effective use of experience and available information is described, to suggest that lessons to be learned from past accidents can be effectively translated into actions of prevention.

Industrial Accidents Triggered by Lightning

Natural disasters can cause major accidents in chemical facilities where they can lead to the release of hazardous materials which in turn can result in fires, explosions or toxic dispersion. Lightning strikes are the most frequent cause of major accidents triggered by natural events. In order to contribute towards the development of a quantitative approach for assessing lightning risk at industrial facilities, lightning-triggered accident case histories were retrieved from the major industrial accident databases and analysed to extract information on types of vulnerable equipment, failure dynamics and damage states, as well as on the final consequences of the event. The most vulnerable category of equipment is storage tanks. Lightning damage is incurred by immediate ignition, electrical and electronic systems failure or structural damage with subsequent release. Toxic releases and tank fires tend to be the most common scenarios associated with lightning strikes. Oil, diesel and gasoline are the substances most frequently released during lightning-triggered Natech accidents.