(381b) 100% Conversion of Waste Brown Grease to Biodiesel and Syngas As Valuable Products for Heat and Power Cogeneration

Hydrocarbon-based biofuels derived from renewable, non-food
carbon sources serve as a promising alternative to petroleum-derived fuels and
are capable of mitigating greenhouse gas emissions. A sustainable biofuel
strategy should utilize widely renewable biomass feedstocks, especially wastes,
for which industries carry the high cost of disposal. Fuel production and power
cogeneration from waste recovery is a sustainable and potentially profitable
option to explore, addressing today's economic challenges of energy
independence and waste management. A potentially interesting candidate in this
scenario is brown grease. Brown grease is a mixture of high-value hydrocarbons,
such as waste vegetable oil, animal fats and grease that is collected in water
treatment plants and requires further treatment before disposal. However, brown
grease has a higher level of contamination, compared to yellow grease, and
cannot be used as animal feedstock or fertilizer. Additionally, brown grease is
responsible for about 40% of all sewer overflows, causing back-ups and damage
to pipe lines and for about 20,000,000 illnesses each year in the USA [1]. Therefore,
special attention must be dedicated to its disposal and the cost associated.
The main goal of this project is to explore the possibility of transforming
100% of the raw brown grease into biodiesel and synthesis gas that can be used
for power generation. This work comprises a multi-disciplinary study of
experimentation and simulation of optimal process options for the conversion of
brown grease to energy and energy carriers.

In this study, heat treatment of the raw brown grease results
in phase separation of the organic mixture. The light product, mainly characterized
by a high free fatty acid (FFA) content (approximately 90%), is extracted and processed
to obtain biodiesel. A two-step esterification process, using 5% (v/v) HCL and
with a 9 to 1 methanol/oil molar ratio, produces liquid products with an acid
value smaller than 0.5% FFA. The triglyceride is then transesterified using 2wt%
KOH and a 6 to 1 of methanol/fats molar ratio, reaching higher ester content in
the biodiesel. Gas Chromatography (Figure 1) shows that the biodiesel from the
extracted brown grease oil not only satisfies the total and free glycerin percentage
requirements imposed by the ASTM specifications, but also presents similar
value in comparison with the biodiesel derived from Yellow Grease (Table 1),
which makes it more competitive as brown grease is more abundant.

Table 1. Comparison
of biodiesel produced from brown grease, yellow grease and ASTM standards in total
and free glycerin

Figure 1. GC analysis of biodiesel produced from brown grease
(Signal Count versus time)

The heavy product consists of a solid mixture, which is extracted
and gasified to obtain synthesis gas. In Figure 2 thermogravimetric analysis (TGA)
of the heavy brown grease product shows that this biosolid is gasifiable in its
entirety. However the minor, almost negligible, residue of gasification contain
inorganic compounds, of undefined environmental impact. Further analysis of the
inorganics and their fate after gasification will be presented and discussed.

Figure 2. Gasification of biosolid residue from biodiesel
production

TGA and gas product analysis (performed in gas
chromatography/mass spectroscopy/Fourier transform Infrared, not shown here)
show that the heavy product of brown grease is rich in hydrogen; thus, a
valuable feedstock. Elemental analysis (Table 2) shows that the carbon to
hydrogen ratio of the brown grease heavy fraction biosolid is lower than pine
sawdust, which confirms its value as a gasification feedstock.

Table 2. 
Elemental analyses of biosolid from biodiesel production, biomass and coal
(mol%)

Due to the relatively low available quantity of the brown
grease heavy fraction (as opposed to the efficiency increase of large scale
gasification), co-gasification of the brown grease residue with fossil fuels,
such as coal, is investigated. Co-gasification of coal and brown grease residues
is capable of increasing the power generation plant throughput; thus, increasing
its overall efficiency [2]. On the other hand, co-gasification reduces the
consumption of coal in power generation plants resulting in economic and
environmental benefits. Experimental results of co-gasification of biosolids
and coal will be compared with the more widely studied co-gasification of woody
biomass and coal [3].

On the basis of experimental results, process simulation of an
integrated brown grease biorefinery is performed in Aspen Plus. The objective
of the simulation study is to identify the cost and profitability of an
integrated brown grease refinery with power co-generation, focusing also on the
benefits of capturing CO2 and the overall reduction of greenhouse gas
emissions. The energy penalties for drying, O2 separation, H2S, SOx, NOx, and
CO2 separation are estimated for various co-gasification scenarios. A brief description
of the process flow sheet is as follows. Raw brown grease is fed to the system
and the first operation consists of heating the mixture in order to evaporate
all the containing water and to phase separate the substance in the two layers.
The oil at the top layer is extracted and sent to the biodiesel production hierarchy,
while the heavier biosolids are taken to the syngas production hierarchy for gasification.
In the biodiesel production hierarchy the liquid animal fat enters the pretreatment
unit where impurities are filtered and removed. Cleaned animal fat enters an esterification
unit where the oil is homogenously mixed at a constant temperature of 60°C with
Methanol (MeOH) and Hydrochloric acid (HCl), which serves as a catalyst. The
reaction reduces the concentration of the FFA. Then the stream is washed and
dried. A second esterification process follows, to convert all the FFA
remaining into Methyl ester. The stream is washed and dried again to remove
remaining methanol and acid catalyst. The processed flow then enters the
Transesterification unit where a base catalyst, Potassium hydroxide (KOH), is
used to convert the triglycerides into biodiesel. This reaction also produces glycerol,
which can be separated and sold as a chemical. Finally, crude biodiesel enters
the post-processing unit where it is refined and tested for quality standards.

In the syngas production hierarchy the dewatered biosolids
enter the preparation unit where they are mixed with coal, which has previously
passed a process of sizing (crushing and screening) and drying. Air is
separated mainly in Nitrogen and Oxygen in the Air Separation Unit (ASU)
through cryogenic distillations. The mixed feedstock, together with the stream
of almost pure Oxygen, enters the gasifier. The melted ash is collected at the
bottom of the reactor and disposed. Then the syngas is cleaned from H2S and CO2
in the Acid Gas Cleaning Unit (Rectisol Process). Captured CO2 is sent to
compression and storage, and the H2S is converted to solid S in the Claus Unit
[3]. Finally the syngas is burned in a combined cycle, where heat and
electricity are produced. The hot gas, after expansion in the gas turbine, pass
through a Heat Recovery Steam Generator (HRSG) where superheated, high pressure
steam is produced which can be used for district heating and/or expanded in a
steam turbine for additional power production.

The experimental analysis integrated with the ASPEN model
allows to examine the feasibility of recovering 100% of the raw brown grease
that would otherwise have to be disposed at a high cost, due to its contamination
as a water treatment waste. The opportunity of transforming it into valuable
energy products opens access to new markets, while the sustainability of the
overall process and the improved carbon life cycles of this process, lead to its
reassessment from an unwanted waste to a precious resource.

Acknowledgment

This work was supported in part by the Faculty Large Grant of
the University of Connecticut and in part by the EAGLES (Engineers as Global
Leaders for Energy Sustainability) Atlantis Fellowship.

References:

[1] Tyson, K. S. Brown Grease Feedstocks for Biodiesel,
National Renewable Energy Laboratory, 2002.

[2] Saw, W.; McKinnon, H.; Gilmour, I.; Pang, S. Fuel. 2012,
93, 473-478.

[3] Valla, L.; Du, S.; Bollas, G. M. 2012 AICHE Annual
Meeting, October 2012, Pittsburgh, PA, USA.