Exhibit 10.91

OMNIBUS AMENDMENT

THIS OMNIBUS AMENDMENT (this “Amendment”), dated as of August 10, 2010, is by
and among Clean Coal Solutions, LLC, a Colorado limited liability company
(“CCS”), AEC-NM, LLC, a Colorado limited liability company (“AEC-NM”), AEC-TH,
LLC, a Colorado limited liability company (“AEC-TH”), ADA-ES, Inc., a Colorado
corporation (“ADA”) and GS RC INVESTMENTS LLC, a Delaware limited liability
company (“Goldman”). CCS, AEC-NM, AEC-TH, ADA and Goldman are sometimes referred
to herein, collectively, as the “Parties” and, individually, as a “Party”.

WHEREAS, CCS, AEC-NM, AEC-TH and Goldman entered into that certain Agreement to
Lease, dated as of June 29, 2010 (the “Agreement to Lease”);

WHEREAS, each of AEC-NM and AEC-TH entered into an Equipment Lease with Goldman,
dated as of June 29, 2010 (collectively, the “Equipment Leases”);

WHEREAS, CCS, ADA and Goldman entered into a Technology Sublicense Agreement,
dated as of June 29, 2010 (the “Technology Sublicense Agreement”);

NOW, THEREFORE, in consideration of the premises and of the mutual promises,
covenants and agreements contained herein, and for other valuable consideration,
the receipt and sufficiency of which are hereby acknowledged, the Parties agree
as follows:

1. Amendments to Agreement to Lease. CCS, AEC-NM, AEC-TH and Goldman hereby
agree to amend the Agreement to Lease as follows:

(a) Section 9.1(a) of the Agreement to Lease is hereby deleted and the following
is substituted in lieu thereof:

“Each Party shall maintain the terms of this Agreement in confidence and shall
not disclose any information concerning the terms, performance or administration
of this Agreement to any other Person; provided that a Party may disclose such
information: (i) to any of such Party’s Group, (ii) to any prospective member of
such Party’s Group, (iii) to any actual or prospective purchaser of all or a
portion of such Party’s interest in the Facilities and (iv) to any Person
providing or evaluating a proposal to provide financing to the recipient Party
or any direct or indirect owner of such Party; provided in each case that the
recipient Party shall provide to each Person to which disclosure is made a copy
of this Section 9.1 and direct such Person to treat such information
confidentially, and the recipient Party shall be liable for any breach of the
terms of this Section 9.1 by such Persons to which it makes any such disclosure.
The foregoing restrictions will not apply (A) to information that is or becomes
generally available to the public otherwise than as a result of disclosure by
the recipient Party, (B) to information that is already in, or subsequently
comes into, the recipient Party’s possession, provided that the source of such
information was not, to the recipient Party’s knowledge, obligated to keep such
information confidential, (C) to information that is required to be disclosed
pursuant to

 

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Law or stock exchange rules and regulations or is otherwise subject to legal,
judicial, regulatory or self-regulatory requests for information or documents or
(D) subject to Section 9.1(b) below, to the tax structure or tax treatment of
the transaction.”

(b) The following is hereby added to Annex I of the Agreement to Lease where
alphabetically appropriate as a new defined term:

“Group” means, with respect to any Party, such Party and (a) the Affiliates of
such Party; (b) each guarantor of such Party; (c) any other members,
shareholders, partners or other equity owners of such Party or any of its
Affiliates (other than holders of publicly-traded units of such Party or of any
of its Affiliates, except any such holder that controls such Party), and (d) the
respective successors, assigns and Representatives of each Person described in
the foregoing clause (a), (b) or (c), but shall in no event include the other
Parties’ respective Groups.

2. Amendments to Equipment Leases. AEC-NM (as applicable), AEC-TH (as
applicable) and Goldman hereby agree to amend each of the Equipment Leases by
deleting Section 6.1(a) in its entirety and substituting the following in lieu
thereof::

“Each Party shall maintain the terms of this Lease in confidence and shall not
disclose any information concerning the terms, performance or administration of
this Lease to any other Person; provided that a Party may disclose such
information: (i) to any of such Party’s Group, (ii) to any prospective member of
such Party’s Group, (iii) to any actual or prospective purchaser of all or a
portion of such Party’s interest in the Facility and (iv) to any Person
providing or evaluating a proposal to provide financing to the recipient Party
or any direct or indirect owner of such Party; provided in each case that the
recipient Party shall provide to each Person to which disclosure is made a copy
of this Section 6.1 and direct such Person to treat such information
confidentially, and the recipient Party shall be liable for any breach of the
terms of this Section 6.1 by such Persons to which it makes any such disclosure.
The foregoing restrictions will not apply (A) to information that is or becomes
generally available to the public otherwise than as a result of disclosure by
the recipient Party, (B) to information that is already in, or subsequently
comes into, the recipient Party’s possession, provided that the source of such
information was not, to the recipient Party’s knowledge, obligated to keep such
information confidential, (C) to information that is required to be disclosed
pursuant to Law or stock exchange rules and regulations or is otherwise subject
to legal, judicial, regulatory or self-regulatory requests for information or
documents or (D) subject to Section 6.1(b) below, to the tax structure or tax
treatment of the transaction.”

3. Amendment to Technology Sublicense Agreement. ADA, CCS and Goldman hereby
agree to amend the Technology Sublicense Agreement as follows:

(a) Section 9.1 of the Technology Sublicense Agreement is hereby deleted and the
following is substituted in lieu thereof:

“Each Party shall maintain the terms of this Agreement in confidence and shall
not disclose any information concerning the terms, performance or administration
of this

 

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Agreement to any other Person; provided that a Party may disclose such
information: (a) to any of such Party’s Group, (b) to any prospective member of
such Party’s Group, (c) to any actual or prospective purchaser of all or a
portion of such Party’s interest in the Facilities and (d) to any Person
providing or evaluating a proposal to provide financing to the recipient Party
or any direct or indirect owner of such Party; provided in each case that the
recipient Party shall provide to each Person to which disclosure is made a copy
of this Section 9 and direct such Person to treat such information
confidentially, and the recipient Party shall be liable for any breach of the
terms of this Section 9 by such Persons to which it makes any such disclosure.
The foregoing restrictions will not apply (i) to information that is or becomes
generally available to the public otherwise than as a result of disclosure by
the recipient Party, (ii) to information that is already in, or subsequently
comes into, the recipient Party’s possession, provided that the source of such
information was not, to the recipient Party’s knowledge, obligated to keep such
information confidential and the information was not received solely pursuant to
a previous agreement between the Parties , (iii) to information that is required
to be disclosed pursuant to Law or stock exchange rules and regulations or is
otherwise subject to legal, judicial, regulatory or self-regulatory requests for
information or documents, or (iv) subject to Section 9.2 below, to the tax
treatment or tax structure of the transaction.”

(b) The following are hereby added to Article I of the Technology Sublicense
Agreement where alphabetically appropriate as new defined terms:

“Group” means, with respect to any Party, such Party and (a) the Affiliates of
such Party; (b) each guarantor of such Party; (c) any other members,
shareholders, partners or other equity owners of such Party or any of its
Affiliates (other than holders of publicly-traded units of such Party or of any
of its Affiliates, except any such holder that controls such Party), and (d) the
respective successors, assigns and Representatives of each Person described in
the foregoing clause (a), (b) or (c), but shall in no event include the other
Parties’ respective Groups.

“Representative” means, with respect to any Person, each manager, director,
officer, employee, agent, consultant (including consulting engineers), advisor
(including counsel and accountants), and other representative of such Person.

4. The provisions of the Agreement to Lease, the Equipment Leases and the
Technology Sublicense Agreement, each as amended hereby, shall remain in full
force and effect and the applicable Parties hereby ratify, confirm and adopt all
the terms and provisions of each of the Agreement to Lease, the Equipment Leases
and the Technology Sublicense Agreement, as amended hereby.

5. This Amendment shall constitute the entire agreement between the Parties
hereto relating to the amendment of the Agreement to Lease, the Equipment Leases
and the Technology Sublicense Agreement. No modification of this Amendment or
waiver of any provision hereof shall be binding unless the modification or
waiver shall be in writing and signed by the Parties to be bound. This Amendment
expressly supersedes all prior agreements between the Parties relating to the
amendment of the Agreement to Lease, the Equipment Leases and the Technology
Sublicense Agreement.

 

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6. This Amendment may be executed and delivered (including by facsimile
transmission) in one or more counterparts, all of which shall be considered one
and the same agreement and shall become effective when one or more counterparts
have been signed by each of the Parties and delivered to the other Parties, it
being understood that all Parties need not sign the same counterpart.

[SIGNATURE PAGES FOLLOW]

 

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IN WITNESS WHEREOF, this Amendment has been signed by each of the Parties as of
the date first above written.

 

CLEAN COAL SOLUTIONS, LLC By:   /s/ Brian Humphrey Name:   Brian Humphrey Title:
  Manager

 

AEC-NM, LLC By:  

Clean Coal Solutions, LLC,

its managing member

By:  

/s/ Brian Humphrey

Name:  

Brian Humphrey

Title:  

Manager

 

AEC-TH, LLC By:  

Clean Coal Solutions, LLC,

its managing member

By:   /s/ Brian Humphrey Name:   Brian Humphrey Title:   Manager

SIGNATURE PAGE TO AMENDMENT

 

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ADA-ES, INC. By:   /s/ Mark H. McKinnies Name:   Mark H. McKinnies Title:  
Senior Vice President and Chief Financial Officer

 

GS RC INVESTMENTS LLC By:  

GSFS INVESTMENTS I CORP.,

its sole member

By:   /s/Donald Dombrowski Name:   Donald Dombrowski Title:   Authorized
Signatory

SIGNATURE PAGE TO AMENDMENT