PROCESSES: BASICS OF PROCESS DESIGN - Biomass as a Sustainable Energy Source for the Future - page 188

Chapter 1. Within each category, many different processing options are available.
The selection of the most suitable process route(s) for a given task, based on prede-
fined criteria, is one of the major goals of process design.
7.2.3 Broad Product Spectrum
Biomass-based processes can produce a number of different products: heat (often in
the form of steam), electricity, fuels (energy-carrying chemicals), and other chemicals.
Many processes deliver more than one product. The term cogeneration is typically
used when both electricity and useful heat are produced; it is also referred to as com-
bined heat and power (CHP). The term polygeneration is used when three or more
products are delivered, e.g., heat (or cold), electricity, and synthesis gas. Synthesis
gas or syngas, a mixture of CO and H 2 , is used as the feedstock for many chemical
processes.
7.2.4 Batch versus Continuous Processing
Another important question when designing biomass conversion processes is whether
the process will be operating in batch mode or in a continuous way. In batch proces-
sing, all feed is put into a vessel, and multiple steps are carried out one after the other in
order to step-by-step change the feed into the end product. This approach is used for
certain small-volume, high-value products, e.g., pharmaceuticals, foods, and inte-
grated electronic circuits. Continuous processing is more widely employed in the
high-volume, bulk chemical process industry. Continuous processing works like an
assembly line: a continuous flow of feed enters the process, typically consisting of
multiple steps positioned one after another, all continuously operating. At the end
of the process, a constant outflow of product is obtained. Continuous processing is
especially more economical when handling large streams, which is typically the case
in energy conversion processes. Therefore, we will focus on continuous processes in
this chapter. Design of batch units and processes is covered in Douglas (1988), Smith
(2005), and Biegler et al. (1997). Processing of feeds with bulk solids generates par-
ticulates that induce more fouling by deposition of solids than fluid feeds. Conse-
quently, in the design of such processes, one must account for cleaning and
regeneration operations in addition to regular production. Here, we only consider pro-
cesses in steady-state operation: start-up and shutdown effects will not be taken into
account.
Figure 7.1 shows a typical example of a biomass conversion process, presented as a
sequence of processing units. The feedstock is wood, and the products are electricity
and liquid biofuels. After pretreatment of the wood by chopping, grinding, and drying
(see Chapter 8), it is converted into raw syngas by gasification with an oxygen carrier
(air, pure oxygen, steam, or a mixture of air or oxygen with steam). The raw syngas is
cleaned from components harmful to further processing, such as solids (carbonaceous
fly ash) and gaseous components such as H 2 S. The clean syngas may undergo one or
more gas processing steps, after which it is converted into liquid hydrocarbon fuel