Pyrolysis in Waste to Energy Conversion (WEC)

Abstract

Solid waste (SW), mostly now wasted biomass, could fuel approximately ten times more of USA’s increasing energy needs than it currently does. At the same time it would create good non-exportable jobs, and local industries. Twenty four examples of wasted or under-utilized solids that contain appreciable organic matter are listed. Estimates of their sustainable tonnage lead to a total SW exceeding 2 billion dry tons. Now usually disposal problems, most of these SW’s, can be pyrolyzed into substitutes for or supplements to expensive natural gas. The large proportion of biomass (carbon dioxide neutral plant matter) in the list reduces Greenhouse problems. Pyrolysis converts such solid waste into a medium heating value gaseous fuel usually with a small energy expenditure. With advanced gas cleaning technologies the pyrogas can be used in high efficiency gas turbines or fuel cells systems. This approach has important environmental and efficiency advantages with respect to direct combustion in boilers and even air blown or oxygen blown partial combustion gasifiers. Since pyrolysis is still not a predictive science the CCTL has used an analytical semi-empirical model (ASEM) to organize experimental measurements of the yields of various product {CaHbOc} yields vs temperature (T) for r dry ash, nitrogen and sulfur free (DANSF) feedstock having various weight % of oxygen [O] and hydrogen [H]. With this ASEM each product is assigned 5 parameters (W, T0, D, p, q) in a robust analytical Y(T) expression to represent yields vs. temperature of any specific product from any specified feedstock. Patterns in the dependence of these parameters upon [O], [H], a, b, and c suggest that there is some order in pyrolysis yields that might be useful in optimize the throughput of particular pyrolysis systems used for waste to energy conversion (WEC). An analytical cost estimation (ACE) model is used to calculate the cost of electricity (COE) vs the cost of fuel (COF) for a SW pyrogas fired combined cycle (CC) system for comparison with the COE vs COF for a natural gas fired CC system. It shows that high natural gas prices solid waste can be changed from a disposal cost item to a valuable asset. Comparing COEs when using other SW capable technologies are also facilitated by the ACE method. Implications of this work for programs that combine conservation with waste to energy conversion in efforts to reach Zero Waste are discussed.